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I'm trying to add an endpoint to an existing application that sends Server Sent Events. There often may be no event for ~5 minutes. I'm hoping to configure that endpoint to not cut off my server even when the response has not been completed in ~1min, but all other endpoints to timeout if the server fails to respond.
Is there an easy way to support server sent events in HAProxy?
| Here is my suggestion for HAProxy and SSE: you have plenty of custom timeout options in HAProxy, and there is 2 interesting options for you.
The timeout tunnel specifies timeout for tunnel connection - used for Websockets, SSE or CONNECT. Bypass both server and client timeout.
The timeout client handles the situation where a client looses their connection (network loss, disappear before the ACK of ending session, etc...)
In your haproxy.cfg, this is what you should do, first in your defaults section :
# Set the max time to wait for a connection attempt to a server to succeed
timeout connect 30s
# Set the max allowed time to wait for a complete HTTP request
timeout client 50s
# Set the maximum inactivity time on the server side
timeout server 50s
Nothing special until there.
Now, still in the defaults section :
# handle the situation where a client suddenly disappears from the net
timeout client-fin 30s
Next, jump to your backend definition and add this:
timeout tunnel 10h
I suggest a high value, 10 hours seems ok.
You should also avoid using the default http-keep-alive option, SSE does not use it. Instead, use http-server-close.
| HAProxy | 21,419,859 | 10 |
I want to use haproxy as a proxy and load balancer for thousands of backends. So a request needs to be proxied to the correct backend depending on hostname and then load balanced within the backend. I am using haproxy-1.5dev21.
The config file looks like this :
frontend public
bind :80
mode http
acl host1 hdr_reg(host) host1.com
use_backend be_host1 if host1
acl host4000 hdr_reg(host) host4000.com
use_backend be_host4000 if host4000
backend be_host[n]
server hostn_1
server hostn_2
Problem is that I get an added latency of 30ms per request if there are 5000 hosts. And for 20k backends, haproxy takes long time to load, not to mention the disaster on latency per request.
Can I do something better than sequential acl rules?
I have not found an example for the new map feature - the release notes say it can be used for massive redirect rules.
I tried this :
use_backend %[hdr(host), map(host_to_backend_map.file)]
Something obviously stupid above in using maps, but any guidance would be helpful.
Thanks!
| Some shortcomings were removed from the config file after expert input, and I list them here in case anyone else may find it useful.
Use hdr(Host) instead of hdr_reg(). This vastly improves the time consumed to evaluate the ACLs. Even better, avoid acl and use the inline evaluation e.g.
use_backend host1 if { req.fhdr(host,1) -m str host1.domain.com }
Use nbproc>1. In case of concurrent connections, this helps. Though it makes it difficult to debug.
For the backends, use IP address directly instead of 'server hostn_1 dns_of_server:port_number'
Put 'fullconn 1000' in the defaults section. This improves the load time immensely.
Finally, use the latest haproxy git checkout and observe the enhancement in load time. It has come down considerably. Its now order of seconds compared to minutes before.
Also, regarding the 'map' feature, a new dynamic use_backend scheme is in the works that should remove the need to write as many ACLs.
| HAProxy | 22,025,412 | 10 |
I am trying to achieve this:
http://front-end --> http://back-end/app-1
http://front-end/app-2 --> http://back-end/app-2-another-path
So that requests will be handled this way:
http://front-end/do-this --> http://back-end/app-1/do-this
http://front-end/app-2/do-that --> http://back-end/app-2-another-path/do-that
How can I do this? Thank you.
| You can achieve this "http://front-end/app-2/do-that --> http://back-end/app-2-another-path/do-that" with the following configuration:
frontend http
#match url ending with /xxxxx/do-that
acl do-that path_end -i /app-2/do-that
use_backend server1 if do-that
backend server1
reqirep ^([^\ :]*)\ /app-2/(.*) \1\ /app-2-another-path/\2
server server 168.192.X.X
Here is more information on reqirep.
| HAProxy | 22,219,479 | 10 |
I am currently refactoring a haproxy configuration that we use on our production servers to forward TCP traffic from a central server. The goal is to get everything working with docker containers to help with deployment reliability.
Everything has gone well so far, but now I have a couple of "listen" proxies using "mode tcp" that don't seem to be forwarding their traffic. I think the issue is either in the SSL certificate verification or in the forwarding to the secondary server itself.
In an attempt to debug the issue, I have turned on all syslog debugging ('debug' level) and have used the -d flag to run haproxy in debug mode. This provides extensive debugging information for all incoming http traffic, but doesn't seem to give me anything for TCP.
The lack of debug output for TCP forwarding is something I have run into in the past and have not found any way to get more details.
Is there some set of magic flags, configuration, or compile options I can use to see the full details of the TCP connection processing? (ex: accept, handshake, SSL cert verification, forwarding, timeouts, etc)
| If you are using a TCP mode proxy, you have to specify option tcplog in your frontend's definition. This enables tcp mode logging. There's extensive documentation about this in the haproxy manual, for example here for haproxy 1.5: http://cbonte.github.io/haproxy-dconv/configuration-1.5.html#8.2.2
| HAProxy | 22,391,876 | 10 |
I am using HAProxy listening on 80 to send requests to a node server (port:3000) or php server (4000); I also have CSF installed which have ports 3000 and 80 available.
It works okay when I browse a page at http://example.com/forum/1/page.php, but sometimes when I accidentally enter example.com/forum/1 (no trailing slash), it keeps loading and eventually leads to an error page ERR_CONNECTION_TIMED_OUT. The address bar shows that it has redirected to http://example.com:4000/forum/1/.
Since I don't have the 4000 port open, when I keep going to example.com/forum/1 a multiple times, it would trigger a block by CSF. My question is, can I redirect all the requests that are pointing to an actual folder example.com/forum/1 (no trailing slash) to an 404 page? I have tried adding a rewrite rule to append a trailing slash to every request, but that would break all of my relative paths (see my issue at this post).
So what I want to know is, why was I redirected to http://example.com:4000/forum/1/ from http://example.com/forum/1? Was it caused by HAproxy?
Some HAproxy config:
frontend all 0.0.0.0:80
timeout client 1h
# use apache2 as default webserver for incoming traffic
default_backend apache2
backend apache2
balance roundrobin
option forwardfor
server apache2 myIpAddress:4000 weight 1 maxconn 1024 check
# server must be contacted within 5 seconds
timeout connect 5s
# all headers must arrive within 3 seconds
timeout http-request 3s
# server must respond within 25 seconds. should equal client timeout
timeout server 25s
Here's my rewrite rules:
RewriteCond %{REQUEST_FILENAME} -d [OR]
RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -l
RewriteRule ^ - [L]
RewriteCond %{REQUEST_URI} !^.*\.(jpg|css|js|gif|png)$ [NC]
RewriteCond %{REQUEST_FILENAME}.php -f
RewriteRule !.*\.php$ %{REQUEST_FILENAME}.php [QSA,L]
RewriteCond %{THE_REQUEST} /index\.php [NC]
RewriteRule ^([^\.]+)$ $1.php [NC,L]
| Since /forum/1 is a valid physical directory you get redirected to /forum/1/ due to this setting:
DirectorySlash On
which is used by a module called mod_dir that adds a trailing slash after directories if it is missing.
You can of course turn this flag off by using:
DirectorySlash Off
but be aware of security implications.
To be more secure you also need:
Options -Indexes
to turn off directory listing.
Security Warning (copied from linked manual)
Turning off the trailing slash redirect may result in an information disclosure. Consider a situation where mod_autoindex is active (Options +Indexes) and DirectoryIndex is set to a valid resource (say, index.html) and there's no other special handler defined for that URL. In this case a request with a trailing slash would show the index.html file. But a request without trailing slash would list the directory contents.
Update: To answer this part of the question:
My question is, can I redirect all the requests that are pointing to an actual folder example.com/forum/1 (no trailing slash) to an 404 page?
You can use this code in /forum/1/.htaccess:
DirectorySlash On
RewriteEngine On
RewriteRule ^$ - [L,R=404]
This will force a trailing slash so that rewrite rule can used to send 404 error.
| HAProxy | 30,594,199 | 10 |
Consider the following HAProxy Config:
frontend front
default_backend default
backend default
balance roundrobin
http-response set-header X-RGN us-east-1
server app-1a app.us-east-1a.example.com:443 ssl verify none check
server app-1c app.us-east-1c.example.com:443 ssl verify none check
server app-1b app.us-east-1b.example.com:443 ssl verify none check
I would like to return a response header the indicates the server that was chosen. For example, if the frontend receives a request, it will balance roundrobin and forward the request to a backend server, when it responds, I would like to see in my browser which server was used.
The config might look something like this:
frontend front
default_backend default
backend default
balance roundrobin
http-response set-header X-RGN us-east-1
server app-1a app.us-east-1a.example.com:443 ssl verify none check
server app-1c app.us-east-1c.example.com:443 ssl verify none check
server app-1b app.us-east-1b.example.com:443 ssl verify none check
http-response set-header X-Server app-1a if server -i app-1a
http-response set-header X-Server app-1b if server -i app-1b
http-response set-header X-Server app-1c if server -i app-1c
Has anyone tried this before?
| Assuming HAProxy 1.5 or later:
http-response set-header X-Server %s
| HAProxy | 43,105,840 | 10 |
I need to integrate several web applications on-premise and off-site under a common internally hosted URL. The on-premise applications are in the same data center as the haproxy, but the off-site applications can only be reached via a http proxy because the server on which haproxy is running has no direct Internet access. Therefore I have to use a http Internet proxy, SOCKS might be an option too.
How can I tell haproxy that a backend can only be reached via proxy ?
I would rather not use an additional component like socksify / proxifier / proxychains / tsocks / ... because this introduces additional overhead.
This picture shows the components involved in the setup:
When I run this on a machine with direct Internet connection I can use this config and it works just fine:
frontend main
bind *:8000
acl is_extweb1 path_beg -i /policies
acl is_extweb2 path_beg -i /produkte
use_backend externalweb1 if is_extweb1
use_backend externalweb2 if is_extweb2
backend externalweb1
server static www.google.com:80 check
backend externalweb2
server static www.gmx.net:80 check
(Obviously these are not the URLs I am talking to, this is just an example)
Haproxy is able to check the external applications and routes traffic to them:
In the safe environment of the company I work at I have to use a proxy and haproxy is unable to connect to the external applications.
How can I enable haproxy to use those external web application servers behind a http proxy (no authentication needed) while providing access to them through a common http page / via browser ?
| How about to use delegate ( http://delegate.org/documents/ ) for this, just as an idea.
haproxy -> delegate -f -vv -P127.0.0.1:8081 PROXY=<your-proxy>
http://delegate9.org/delegate/Manual.shtml?PROXY
I know it's not that elegant but it could work.
I have tested this setup with a local squid and this curl call
echo 'GET http://www.php.net/' |curl -v telnet://127.0.0.1:8081
The curl call simluates the haproxy tcp call.
| HAProxy | 47,605,766 | 10 |
Is it possible to split configuration arguments (in haproxy.cfg) onto multiple lines?
Example
Current
frontend
https-in bind :443 ssl strict-sni crt </path/to/cert1.pem> crt </path/to/cert2.pem> crt </path/to/cert3.pem> ...
Ideal
frontend
https-in bind :443 ssl strict-sni
crt </path/to/cert1.pem>
crt </path/to/cert2.pem>
crt </path/to/cert3.pem>
...
Error when trying ideal
$ /usr/sbin/haproxy -c -V -f /etc/haproxy/haproxy.cfg
[ALERT] 343/210133 (25646) : parsing [/etc/haproxy/haproxy.cfg:45] : unknown keyword 'crt' in 'frontend' section
[ALERT] 343/210133 (25646) : Error(s) found in configuration file : /etc/haproxy/haproxy.cfg
[ALERT] 343/210133 (25646) : Fatal errors found in configuration.
| You can't do multiline syntax in the haproxy.cfg.
Take a look at the file format documentation: https://cbonte.github.io/haproxy-dconv/1.8/configuration.html#2.1
Update:
Thanks to the comment from Venky I see that there is also the option to use crt-list which does provide an option for multi line pem file references. https://cbonte.github.io/haproxy-dconv/1.8/configuration.html#5.1-crt-list
the improved config will be:
frontend
https-in bind :443 ssl strict-sni
crt-list </path/to/list.txt>
...
the list.txt:
</path/to/cert1.pem>
</path/to/cert2.pem>
</path/to/cert3.pem>
| HAProxy | 53,713,425 | 10 |
I have some Lua code, which I use in my openresty nginx.conf file. This Lua code contains such lines:
...
local secret = os.getenv("PATH")
assert(secret ~= nil, "Environment variable PATH not set")
...
Just for testing reasons I tried to check if PATH variable is set and for some reason the assert statement does not pass. I see in the console:
Environment variable PATH not set
However, when I run this
$ echo $PATH
I see, that this variable indeed has some value. So, what is wrong with that and how can I fix it?
| You need to tell nginx to make environment variables available. From the docs for the env directive: "By default, nginx removes all environment variables inherited from its parent process except the TZ variable. This directive allows preserving some of the inherited variables, changing their values, or creating new environment variables."
So, in your case you'd need to specify env PATH; in nginx.conf.
| OpenResty | 41,800,071 | 23 |
I have a question regarding NGINX rate limiting.
Is it possible to do rate limiting based on the decoded value of JWT token? I cannot find any information like this in the docs.
Or even if there is a way of doing rate limiting by creating pure custom variable (using LuaJIT) which will be assigned with a value from my decoded JWT - will also do the job.
The thing is that the limit_req module seems to execute way before the request reaches the luaJIT stage so its already too late!
A solution will be appreciated.
| As you may know that rate limit is applied through unique ip address for best result you should use unique jwt value or token to rate limit.
You can follow any of these 3 methods
Method
You can directly use jwt token in limit_req_zone.
http {
...
limit_req_zone $http_authorization zone=req_zone:10m rate=5r/s;
}
conf.d/default.conf
server {
listen 80;
listen [::]:80;
server_name localhost;
#charset koi8-r;
#access_log /var/log/nginx/host.access.log main;
if ($http_authorization = "") {
return 403;
}
location /jwt {
limit_req zone=req_zone burst=10 nodelay;
return 200 $http_authorization;
}
...
}
Method
You can send decoded jwt value from frontend in reqest header like http_x_jwt_decode_value and then you can use that in limit_req_zone.
http {
...
limit_req_zone $http_x_jwt_decode_value zone=req_zone:10m rate=5r/s;
}
conf.d/default.conf
server {
listen 80;
listen [::]:80;
server_name localhost;
#charset koi8-r;
#access_log /var/log/nginx/host.access.log main;
if ($http_x_jwt_decode_value = "") {
return 403;
}
location /jwt {
limit_req zone=req_zone burst=10 nodelay;
return 200 $http_x_jwt_decode_value;
}
...
}
Method
You can decode jwt token in nginx though njs javascript module or perl module or lua module and assign it to variable then use that to rate limit.
Description: here i just decoded jwt value and checked if its not empty you can use it to work with and jwt decoded value.
jwt_example.js
function jwt(data) {
var parts = data.split('.').slice(0,2)
.map(v=>String.bytesFrom(v, 'base64url'))
.map(JSON.parse);
return { headers:parts[0], payload: parts[1] };
}
function jwt_payload_sub(r) {
return jwt(r.headersIn.Authorization.slice(7)).payload.sub;
}
export default {jwt_payload_sub}
nginx.conf
# njs module
load_module modules/ngx_http_js_module.so;
http {
...
include /etc/nginx/conf.d/*.conf;
js_import main from jwt_example.js;
js_set $jwt_payload_sub main.jwt_payload_sub;
limit_req_zone $jwt_payload_sub zone=req_zone:10m rate=5r/s;
}
conf.d/default.conf
server {
listen 80;
listen [::]:80;
server_name localhost;
#charset koi8-r;
#access_log /var/log/nginx/host.access.log main;
if ($jwt_payload_sub = "") {
return 403;
}
location /jwt {
limit_req zone=req_zone burst=10 nodelay;
return 200 $jwt_payload_sub;
}
...
}
| OpenResty | 64,263,895 | 12 |
I'm pretty new to C++ so I tend to design with a lot of Java-isms while I'm learning. Anyway, in Java, if I had class with a 'search' method that would return an object T from a Collection< T > that matched a specific parameter, I would return that object and if the object was not found in the collection, I would return null. Then in my calling function I would just check if(tResult != null) { ... }
In C++, I'm finding out that I can't return a null value if the object doesn't exist. I just want to return an 'indicator' of type T that notifies the calling function that no object has been found. I don't want to throw an exception because it's not really an exceptional circumstance.
This is what my code looks like right now:
class Node {
Attr& getAttribute(const string& attribute_name) const {
//search collection
//if found at i
return attributes[i];
//if not found
return NULL; // what should this be?
}
private:
vector<Attr> attributes;
}
How can I change it so I can give that kind of marker?
| In C++, references can't be null. If you want to optionally return null if nothing is found, you need to return a pointer, not a reference:
Attr *getAttribute(const string& attribute_name) const {
//search collection
//if found at i
return &attributes[i];
//if not found
return nullptr;
}
Otherwise, if you insist on returning by reference, then you should throw an exception if the attribute isn't found.
(By the way, I'm a little worried about your method being const and returning a non-const attribute. For philosophical reasons, I'd suggest returning const Attr *. If you also may want to modify this attribute, you can overload with a non-const method returning a non-const attribute as well.)
| Sentinel | 2,639,255 | 101 |
While reading Eric Niebler's range proposal,
I've come across the term sentinel as replacement for the end iterator.
I'm having a difficult time understanding the benefits of sentinel over an end iterator.
Could someone provide a clear example of what sentintel brings to the table that cannot be done with standard iterator pairs?
"A sentinel is an abstraction of a past-the-end iterator. Sentinels are
Regular types that can be used to denote the end of a range. A
sentinel and an iterator denoting a range shall be EqualityComparable.
A sentinel denotes an element when an iterator i compares equal to the
sentinel, and i points to that element." -- N4382
I think sentinels work as functions in determining the end of a range, instead of just the position?
| Sentinel simply allows the end iterator to have a different type.
The allowed operations on a past-the-end iterator are limited, but this is not reflected in its type. It is not ok to * a .end() iterator, but the compiler will let you.
A sentinel does not have unary dereference, or ++, among other things. It is generally as restricted as the weakest iterators one past the end iterator, but enforced at compile time.
There is a payoff. Often detecting the end state is easier than finding it. With a sentinel, == can dispatch to "detect if the other argument is past the end" at compile time, instead of run time.
The result is that some code that used to be slower than the C equivalent now compiles down to C level speed, such as copying a null terminated string using std::copy. Without sentinels, you either had to scan to find the end before the copy, or pass in iterators with a bool flag saying "I am the end sentinel" (or equivalent), and check it on ==.
There are other similar advantages when working with counting based ranges. In addition, some things like zip ranges1 become easier to express (the end zip sentinel could hold both source sentinels, and return equal if either sentinel does: zip iterators either only compare the first iterator, or compare both).
Another way of thinking about it is that algorithms tend to not use the full richness of the iterator concept on the parameter passed as the past the end iterator, and that iterator is handled way differently in practice. Sentinel means that the caller can exploit that fact, which in turn lets the compiler exploit it easier.
1 A zip range is what you get when you start with 2 or more ranges, and "zip" them together like a zipper. The range is now over tuples of the individual range elements. Advancing a zip iterator advances each of the "contained" iterators, and ditto for dereferencing and comparing.
| Sentinel | 32,900,557 | 24 |
We're currently using Redis 2.8.4 and StackExchange.Redis (and loving it) but don't have any sort of protection against hardware failures etc at the moment. I'm trying to get the solution working whereby we have master/slaves and sentinel monitoring but can't quite get there and I'm unable to find any real pointers after searching.
So currently we have got this far:
We have 3 redis servers and sentinel on each node (setup by the Linux guys):
devredis01:6383 (master)
devredis02:6383 (slave)
devredis03:6383 (slave)
devredis01:26379 (sentinel)
devredis02:26379 (sentinel)
devredis03:26379 (sentinel)
I am able to connect the StackExchange client to the redis servers and write/read and verify that the data is being replicated across all redis instances using Redis Desktop Manager.
I can also connect to the sentinel services using a different ConnectionMultiplexer, query the config, ask for master redis node, ask for slaves etc.
We can also kill the master redis node and verify that one of the slaves is promoted to master and replication to the other slave continues to work. We can observe the redis connection trying to reconnect to the master, and also if I recreate the ConnectionMultiplexer I can write/read again to the newly promoted master and read from the slave.
So far so good!
The bit I'm missing is how do you bring it all together in a production system?
Should I be getting the redis endpoints from sentinel and using 2 ConnectionMultiplexers?
What exactly do I have to do to detect that a node has gone down?
Can StackExchange do this for me automatically or does it pass an event so I can reconnect my redis ConnectionMultiplexer?
Should I handle the ConnectionFailed event and then reconnect in order for the ConnectionMuliplexer to find out what the new master is?
Presumably while I am reconnecting any attempts to write will be lost?
I hope I'm not missing something very obvious here I'm just struggling to put it all together.
Thanks in advance!
| I was able to spend some time last week with the Linux guys testing scenarios and working on the C# side of this implementation and am using the following approach:
Read the sentinel addresses from config and create a ConnectionMultiplexer to connect to them
Subscribe to the +switch-master channel
Ask each sentinel server in turn what they think the master redis and slaves are, compare them all to make sure they all agree
Create a new ConnectionMultiplexer with the redis server addresses read from sentinel and connect, add event handler to ConnectionFailed and ConnectionRestored.
When I receive the +switch-master message I call Configure() on the redis ConnectionMultiplexer
As a belt and braces approach I always call Configure() on the redis ConnectionMultiplexer 12 seconds after receiving a connectionFailed or connectionRestored event when the connection type is ConnectionType.Interactive.
I find that generally I am working and reconfigured after about 5 seconds of losing the redis master. During this time I can't write but I can read (since you can read off a slave). 5 seconds is ok for us since our data updates very quickly and becomes stale after a few seconds (and is subsequently overwritten).
One thing I wasn't sure about was whether or not I should remove the redis server from the redis ConnectionMultiplexer when an instance goes down, or let it continue to retry the connection. I decided to leave it retrying as it comes back into the mix as a slave as soon as it comes back up. I did some performance testing with and without a connection being retried and it seemed to make little difference. Maybe someone can clarify whether this is the correct approach.
Every now and then bringing back an instance that was previously a master did seem to cause some confusion - a few seconds after it came back up I would receive an exception from writing - "READONLY" suggesting I can't write to a slave. This was rare but I found that my "catch-all" approach of calling Configure() 12 seconds after a connection state change caught this problem. Calling Configure() seems very cheap and therefore calling it twice regardless of whether or not it's necessary seemed OK.
Now that I have slaves I have offloaded some of my data cleanup code which does key scans to the slaves, which makes me happy.
All in all I'm pretty satisfied, it's not perfect but for something that should very rarely happen it's more than good enough.
| Sentinel | 25,385,075 | 12 |
I have attempted everything recommended by the following error message:
(error) DENIED Redis is running in protected mode because protected mode is enabled, no bind address was specified, no authentication password is requested to clients. In this mode connections are only accepted from the loopback interface. If you want to connect from external computers to Redis you may adopt one of the following solutions: 1) Just disable protected mode sending the command 'CONFIG SET protected-mode no' from the loopback interface by connecting to Redis from the same host the server is running, however MAKE SURE Redis is not publicly accessible from internet if you do so. Use CONFIG REWRITE to make this change permanent. 2) Alternatively you can just disable the protected mode by editing the Redis configuration file, and setting the protected mode option to 'no', and then restarting the server. 3) If you started the server manually just for testing, restart it with the '--protected-mode no' option. 4) Setup a bind address or an authentication password. NOTE: You only need to do one of the above things in order for the server to start accepting connections from the outside.
My /etc/redis/sentinel.conf:
daemonize yes
sentinel myid XXX
sentinel monitor master XXX 6379 2
sentinel down-after-milliseconds master 60000
sentinel config-epoch master 0
protected-mode no
bind 0.0.0.0
port 26379
EDIT: My /etc/redis/redis.conf:
port 6379
bind 0.0.0.0
protected-mode no
I've also tried adding sentinel auth-pass master XXX.
My entire backend is on private subnets. I'm VPN'd into my datacenter behind the firewall, coming from the same private network, and I can still only connect locally without getting that frustrating error message.
Server Environment: Debian 8, Redis 3.2.6
Client Environment: Ubuntu 16.10, redis-cli 3.2.1
Redis instances: 3
Sentinel instances: 3
I've done not just one, but 3/4 of the things suggested (didn't set the command-line flags). Does anyone have any guidance or ideas? I'm clearly missing something that I've been unable to figure out from the error message, documentation, Stackoverflow, Google, and trial & error. I figured I'd post a question here first, before diving into the source code.
Any help is appreciated. Thanks!
... and, yes, I've restarted the daemons after configuration changes. :)
| https://www.reddit.com/r/redis/comments/3zv85m/new_security_feature_redis_protected_mode/
As you know we got several problems from unprotected Redis instances exposed to the internet. I covered the reason why a restrictive binding to 127.0.0.1 by default may be an usability concern and, even worse, may not fix the problem (hey just comment the "bind" statement and restart!) in my blog post.
The same blog post introduced an attack that was heavily used by script kiddies to break into Redis instances (serious security researchers where already able to do this, I guess).
So I finally decided to do something before Redis 3.2 official release: Protected mode is the result and will be merged into 3.2 RC2.
The feature is already available in the unstable branch, introduced by this commit. This is how it works.
If and only if:
Protected mode is enabled (this is the default both in the configuration file and in the configless default).
AND IF No AUTH password is configured.
AND IF No "bind" directive is used in order to restrict Redis to certain interfaces.
Then Redis only accepts connections from the loopback IPv4 and IPv6 addresses. External connections are accepted just for the time to send the client an error that makes the user aware of what is happening:
> PING
(error) DENIED Redis is running in protected mode because protected mode is enabled, no bind address was specified, no authentication password is requested to clients.
In this mode connections are only accepted from the lookback interface. If you want to connect from external computers to Redis you may adopt one of the following solutions:
Just disable protected mode sending the command 'CONFIG SET protected-mode no' from the loopback interface by connecting to Redis from the same host the server is running, however MAKE SURE Redis is not publicly accessible from internet if you do so. Use CONFIG REWRITE to make this change permanent.
Alternatively you can just disable the protected mode by editing the Redis configuration file, and setting the protected mode option to 'no', and then restarting the server.
If you started the server manually just for testing, restart it with the --protected-mode no option.
Setup a bind address or an authentication password. NOTE: You only need to do one of the above things in order for the server to start accepting connections from the outside.
This should protect errors in a reasonable way while providing users with a clue instead of a connection refused. Please share your feedbacks so that we can make changes to this feature if needed, before it will get merged into Redis 3.2 RC2. Thanks.
| Sentinel | 43,107,552 | 12 |
Ok, I feel like I'm missing some crucial piece of information.
Locally I have 1 master and 1 slave redis server running on different ports
http://redis.io/topics/sentinel
I also have 3 sentinels and they all appear to be aware of each other and working as expected.
Now I have a big of java code pointing to 127.0.0.1:6379 where my master redis server is.
If i take down the master, sentinel does everthing as expected promoting the slave to master so now the new master is on
127.0.0.1:6380
My question is how does my code know this and auto switch?
| You have to subscribe to sentinel messages on one of their pubsub channels. You can see at the link that you posted that the sentinel will publish out messages like
+odown <instance details> -- The specified instance is now in Objectively Down state.
-odown <instance details> -- The specified instance is no longer in Objectively Down state.
+failover-takedown <instance details> -- 25% of the configured failover timeout has elapsed, but this sentinel can't see any progress, and is the new leader. It starts to act as the new leader reconfiguring the remaining slaves to replicate with the new master.
+failover-triggered <instance details> -- We are starting a new failover as a the leader sentinel.
So when you see a sentinel publish on one of those channels, you need to parse the message and have your client respond accordingly. Redis is not smart - you have to handle these things using a client library.
Specifically, the most useful channels are
+odown
+failover-detected
+switch-master
| Sentinel | 15,437,334 | 10 |
I've googled but not been able to find out what the swift equivalent to respondsToSelector: is.
This is the only thing I could find (Swift alternative to respondsToSelector:) but isn't too relevant in my case as its checking the existence of the delegate, I don't have a delegate I just want to check if a new API exists or not when running on the device and if not fall back to a previous version of the api.
| As mentioned, in Swift most of the time you can achieve what you need with the ? optional unwrapper operator. This allows you to call a method on an object if and only if the object exists (not nil) and the method is implemented.
In the case where you still need respondsToSelector:, it is still there as part of the NSObject protocol.
If you are calling respondsToSelector: on an Obj-C type in Swift, then it works the same as you would expect. If you are using it on your own Swift class, you will need to ensure your class derives from NSObject.
Here's an example of a Swift class that you can check if it responds to a selector:
class Worker : NSObject
{
func work() { }
func eat(food: AnyObject) { }
func sleep(hours: Int, minutes: Int) { }
}
let worker = Worker()
let canWork = worker.respondsToSelector(Selector("work")) // true
let canEat = worker.respondsToSelector(Selector("eat:")) // true
let canSleep = worker.respondsToSelector(Selector("sleep:minutes:")) // true
let canQuit = worker.respondsToSelector(Selector("quit")) // false
It is important that you do not leave out the parameter names. In this example, Selector("sleep::") is not the same as Selector("sleep:minutes:").
| Swift | 24,167,791 | 217 |
I am starting to learn Swift, and have been following the very good Stanford University video lectures on YouTube. Here is a link if you are interested or it helps (although it isn't required to understand my problem):
Developing iOS 8 Apps with Swift - 2. More Xcode and Swift, MVC
While following the lectures I got to a point where (as far as I could tell) my code was identical to the code in the video but on my system I got a compiler error. After a lot of trial and error I have managed to reduce my code to two examples, one of which generates an error, the other or which doesn't, but I have no idea what is actually causing the error or how to resolve it.
The code which creates the error is:
import UIKit
class BugViewController: UIViewController
{
func perform(operation: (Double) -> Double) {
}
func perform(operation: (Double, Double) -> Double) {
}
}
This creates the following compiler error:
Method 'perform' with Objective-C selector 'perform: ' conflicts with previous declaration with the same Objective-C selector
By simply removing the sub-classing of UIViewController the code compiles:
import UIKit
class BugViewController
{
func perform(operation: (Double) -> Double) {
}
func perform(operation: (Double, Double) -> Double) {
}
}
Some other information which may or may not be relevant:
I have recently upgraded to Yosemite.
When I installed Xcode, I ended up with a Beta version (Version 6.3 (6D543q)) because (if I remember correctly) this was the version I needed to run on my version of OS X.
I am half hoping this is a bug in the compiler because otherwise this doesn't make any sense to me. Any help very gratefully received!
| I myself am also taking the Standford course and I got stuck here for a long time too, but after some searching, I found something from here: Xcode release notes and it mentioned something below:
Swift 1.2 is strict about checking type-based overloading of @objc
methods and initializers, something not supported by Objective-C.
// Has the Objective-C selector "performOperation:".
func performOperation(op: NSOperation) { /* do something */ }
// Also has the selector "performOperation:".
func performOperation(fn: () -> Void) {
self.performOperation(NSBlockOperation(block: fn))
}
This code would work when invoked from Swift, but could easily crash
if invoked from Objective-C. To solve this problem, use a type that is
not supported by Objective-C to prevent the Swift compiler from
exposing the member to the Objective-C runtime:
If it makes sense, mark the member as private to disable inference of @objc.
Otherwise, use a dummy parameter with a default value, for
example: _ nonobjc: () = (). (19826275)
Overrides of methods exposed
to Objective-C in private subclasses are not inferred to be @objc,
causing the Swift compiler to crash. Explicitly add the @objc
attribute to any such overriding methods. (19935352)
Symbols from SDKs are not available when using Open Quickly in a
project or workspace that uses Swift. (20349540)
what i did was just adding "private" in front of the override method like this:
private func performOperation(operation: Double -> Double) {
if operandStack.count >= 1 {
displayValue = operation(operandStack.removeLast())
enter()
}
}
| Swift | 29,457,720 | 216 |
I have a (somewhat?) basic question regarding time conversions in Swift.
I have an integer that I would like converted into Hours / Minutes / Seconds.
Example: Int = 27005 would give me:
7 Hours 30 Minutes 5 Seconds
I know how to do this in PHP, but alas, swift isn't PHP.
| Define
func secondsToHoursMinutesSeconds(_ seconds: Int) -> (Int, Int, Int) {
return (seconds / 3600, (seconds % 3600) / 60, (seconds % 3600) % 60)
}
Use
> secondsToHoursMinutesSeconds(27005)
(7,30,5)
or
let (h,m,s) = secondsToHoursMinutesSeconds(27005)
The above function makes use of Swift tuples to return three values at once. You destructure the tuple using the let (var, ...) syntax or can access individual tuple members, if need be.
If you actually need to print it out with the words Hours etc then use something like this:
func printSecondsToHoursMinutesSeconds(_ seconds: Int) {
let (h, m, s) = secondsToHoursMinutesSeconds(seconds)
print ("\(h) Hours, \(m) Minutes, \(s) Seconds")
}
Note that the above implementation of secondsToHoursMinutesSeconds() works for Int arguments. If you want a Double version you'll need to decide what the return values are - could be (Int, Int, Double) or could be (Double, Double, Double). You could try something like:
func secondsToHoursMinutesSeconds(seconds: Double) -> (Double, Double, Double) {
let (hr, minf) = modf(seconds / 3600)
let (min, secf) = modf(60 * minf)
return (hr, min, 60 * secf)
}
| Swift | 26,794,703 | 216 |
I get this error after adding a Swift class to an old Xcode project.
dyld: Library not loaded: @rpath/libswift_stdlib_core.dylib
How can I make the project run again?
| For me none of the previous solutions worked. We discovered that there is a flag ALWAYS_EMBED_SWIFT_STANDARD_LIBRARIES (in earlier versions: "Embedded Content Contains Swift Code") in the Build Settings that needs to be set to YES. It was NO by default!
| Swift | 24,002,836 | 216 |
Is it possible to use the range operator ... and ..< with if statement. Maye something like this:
let statusCode = 204
if statusCode in 200 ..< 299 {
NSLog("Success")
}
| You can use the "pattern-match" operator ~=:
if 200 ... 299 ~= statusCode {
print("success")
}
Or a switch-statement with an expression pattern (which uses the pattern-match
operator internally):
switch statusCode {
case 200 ... 299:
print("success")
default:
print("failure")
}
Note that ..< denotes a range that omits the upper value, so you probably want
200 ... 299 or 200 ..< 300.
Additional information: When the above code is compiled in Xcode 6.3 with
optimizations switch on, then for the test
if 200 ... 299 ~= statusCode
actually no function call is generated at all, only three assembly instruction:
addq $-200, %rdi
cmpq $99, %rdi
ja LBB0_1
this is exactly the same assembly code that is generated for
if statusCode >= 200 && statusCode <= 299
You can verify that with
xcrun -sdk macosx swiftc -O -emit-assembly main.swift
As of Swift 2, this can be written as
if case 200 ... 299 = statusCode {
print("success")
}
using the newly introduced pattern-matching for if-statements.
See also Swift 2 - Pattern matching in "if".
| Swift | 24,893,110 | 215 |
How to concatenate string in Swift?
In Objective-C we do like
NSString *string = @"Swift";
NSString *resultStr = [string stringByAppendingString:@" is a new Programming Language"];
or
NSString *resultStr=[NSString stringWithFormat:@"%@ is a new Programming Language",string];
But I want to do this in Swift-language.
| You can concatenate strings a number of ways:
let a = "Hello"
let b = "World"
let first = a + ", " + b
let second = "\(a), \(b)"
You could also do:
var c = "Hello"
c += ", World"
I'm sure there are more ways too.
Bit of description
let creates a constant. (sort of like an NSString). You can't change its value once you have set it. You can still add it to other things and create new variables though.
var creates a variable. (sort of like NSMutableString) so you can change the value of it. But this has been answered several times on Stack Overflow, (see difference between let and var).
Note
In reality let and var are very different from NSString and NSMutableString but it helps the analogy.
| Swift | 24,034,174 | 214 |
is there any way to get absolute value from an integer?
for example
-8
to
8
I already tried to use UInt() assuming it will convert the Int to unsigned value but it didn't work.
| The standard abs() function works great here:
let c = -8
print(abs(c))
// 8
| Swift | 24,159,627 | 213 |
Suppose I have an array and I want to pick one element at random.
What would be the simplest way to do this?
The obvious way would be array[random index]. But perhaps there is something like ruby's array.sample? Or if not could such a method be created by using an extension?
| Swift 4.2 and above
The new recommended approach is a built-in method on the Collection protocol: randomElement(). It returns an optional to avoid the empty case I assumed against previously.
let array = ["Frodo", "Samwise", "Merry", "Pippin"]
print(array.randomElement()!) // Using ! knowing I have array.count > 0
If you don't create the array and aren't guaranteed count > 0, you should do something like:
if let randomElement = array.randomElement() {
print(randomElement)
}
Swift 4.1 and below
Just to answer your question, you can do this to achieve random array selection:
let array = ["Frodo", "Samwise", "Merry", "Pippin"]
let randomIndex = Int(arc4random_uniform(UInt32(array.count)))
print(array[randomIndex])
The castings are ugly, but I believe they're required unless someone else has another way.
| Swift | 24,003,191 | 213 |
How can I convert a String "Hello" to an Array ["H","e","l","l","o"] in Swift?
In Objective-C I have used this:
NSMutableArray *characters = [[NSMutableArray alloc] initWithCapacity:[myString length]];
for (int i=0; i < [myString length]; i++) {
NSString *ichar = [NSString stringWithFormat:@"%c", [myString characterAtIndex:i]];
[characters addObject:ichar];
}
| It is even easier in Swift:
let string : String = "Hello 🐶🐮 🇩🇪"
let characters = Array(string)
println(characters)
// [H, e, l, l, o, , 🐶, 🐮, , 🇩🇪]
This uses the facts that
an Array can be created from a SequenceType, and
String conforms to the SequenceType protocol, and its sequence generator
enumerates the characters.
And since Swift strings have full support for Unicode, this works even with characters
outside of the "Basic Multilingual Plane" (such as 🐶) and with extended grapheme
clusters (such as 🇩🇪, which is actually composed of two Unicode scalars).
Update: As of Swift 2, String does no longer conform to
SequenceType, but the characters property provides a sequence of the
Unicode characters:
let string = "Hello 🐶🐮 🇩🇪"
let characters = Array(string.characters)
print(characters)
This works in Swift 3 as well.
Update: As of Swift 4, String is (again) a collection of its
Characters:
let string = "Hello 🐶🐮 🇩🇪"
let characters = Array(string)
print(characters)
// ["H", "e", "l", "l", "o", " ", "🐶", "🐮", " ", "🇩🇪"]
| Swift | 25,921,204 | 212 |
In my app I have a function that makes an NSRURLSession and sends out an NSURLRequest using
sesh.dataTaskWithRequest(req, completionHandler: {(data, response, error)
In the completion block for this task, I need to do some computation that adds a UIImage to the calling viewcontroller. I have a func called
func displayQRCode(receiveAddr, withAmountInBTC:amountBTC)
that does the UIImage-adding computation. If I try to run the view-adding code inside of the completion block, Xcode throws an error saying that I can't use the layout engine while in a background process. So I found some code on SO that tries to queue a method on the main thread:
let time = dispatch_time(DISPATCH_TIME_NOW, Int64(0.0 * Double(NSEC_PER_MSEC)))
dispatch_after(time, dispatch_get_main_queue(), {
let returned = UIApplication.sharedApplication().sendAction("displayQRCode:", to: self.delegate, from: self, forEvent: nil)
})
However, I don't know how to add the parameters "receiveAddr" and "amountBTC" to this function call. How would I do this, or can someone suggest an optimal way for adding a method call to the application's main queue?
| Modern versions of Swift use DispatchQueue.main.async to dispatch to the main thread:
DispatchQueue.main.async {
// your code here
}
To dispatch after on the main queue, use:
DispatchQueue.main.asyncAfter(deadline: .now() + 0.1) {
// your code here
}
Older versions of Swift used:
dispatch_async(dispatch_get_main_queue(), {
let delegateObj = UIApplication.sharedApplication().delegate as YourAppDelegateClass
delegateObj.addUIImage("yourstring")
})
| Swift | 24,985,716 | 212 |
Now I would like to migrate my ObjC framework to Swift and I got the following error:
include of non-modular header inside framework module 'SOGraphDB'
The references is to a header file which just define a protocol and I use this header file in some classes to use this protocol.
Is seems related to the module feature but it is at the moment not quite clear how to fix, do you know a solution?
UPDATE:
This is a Swift compiler error.
UPDATE 2:
A quick fix (but not solving the root cause) is to set the following setting to yes:
CLANG_ALLOW_NON_MODULAR_INCLUDES_IN_FRAMEWORK_MODULES = YES
| Is your header public?
Select the header file in the project explorer. Then in the section on the right in xcode, you'll notice there is a dropdown next to the target. Change that from "project" to "public". This worked for me.
| Swift | 24,103,169 | 212 |
I am beginning to learn swift by following the iBook-The Swift Programming Language on Swift provided by Apple. The book says to create an empty dictionary one should use [:] same as while declaring array as []:
I declared an empty array as follows :
let emptyArr = [] // or String[]()
But on declaring empty dictionary, I get syntax error:
let emptyDict = [:]
How do I declare an empty dictionary?
| var emptyDictionary = [String: String]()
var populatedDictionary = ["key1": "value1", "key2": "value2"]
Note: if you're planning to change the contents of the dictionary over time then declare it as a variable (var). You can declare an empty dictionary as a constant (let) but it would be pointless if you have the intention of changing it because constant values can't be changed after initialization.
| Swift | 24,033,393 | 212 |
Given the following enum:
enum Audience {
case Public
case Friends
case Private
}
How do I get the string "Public" from the audience constant below?
let audience = Audience.Public
| The idiomatic interface for 'getting a String' is to use the CustomStringConvertible interface and access the description getter. You could specify the 'raw type' as String but the use of description hides the 'raw type' implementation; avoids string comparisons in switch/case and allows for internationalization, if you so desire. Define your enum as:
enum Foo : CustomStringConvertible {
case Bing
case Bang
case Boom
var description : String {
switch self {
// Use Internationalization, as appropriate.
case .Bing: return "Bing"
case .Bang: return "Bang"
case .Boom: return "Boom"
}
}
}
In action:
> let foo = Foo.Bing
foo: Foo = Bing
> println ("String for 'foo' is \(foo)"
String for 'foo' is Bing
Updated: For Swift >= 2.0, replaced Printable with CustomStringConvertible
Note: Using CustomStringConvertible allows Foo to adopt a different raw type. For example enum Foo : Int, CustomStringConvertible { ... } is possible. This freedom can be useful.
| Swift | 24,701,075 | 210 |
in iOS6 I noticed the new Container View but am not quite sure how to access it's controller from the containing view.
Scenario:
I want to access the labels in Alert view controller from the view controller that houses the container view.
There's a segue between them, can I use that?
| Yes, you can use the segue to get access the child view controller (and its view and subviews). Give the segue an identifier (such as alertview_embed), using the Attributes inspector in Storyboard. Then have the parent view controller (the one housing the container view) implement a method like this:
- (void) prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
{
NSString * segueName = segue.identifier;
if ([segueName isEqualToString: @"alertview_embed"]) {
AlertViewController * childViewController = (AlertViewController *) [segue destinationViewController];
AlertView * alertView = childViewController.view;
// do something with the AlertView's subviews here...
}
}
| Swift | 13,279,105 | 210 |
From Apple book
"One of the most important differences between structures and classes is that structures are always copied when they are passed around in your code, but classes are passed by reference."
Can anyone help me understand what that means? To me, classes and structs seem to be the same.
| Here's an example with a class. Note how when the name is changed, the instance referenced by both variables is updated. Bob is now Sue, everywhere that Bob was ever referenced.
class SomeClass {
var name: String
init(name: String) {
self.name = name
}
}
var aClass = SomeClass(name: "Bob")
var bClass = aClass // aClass and bClass now reference the same instance!
bClass.name = "Sue"
println(aClass.name) // "Sue"
println(bClass.name) // "Sue"
And now with a struct we see that the values are copied and each variable keeps its own set of values. When we set the name to Sue, the Bob struct in aStruct does not get changed.
struct SomeStruct {
var name: String
init(name: String) {
self.name = name
}
}
var aStruct = SomeStruct(name: "Bob")
var bStruct = aStruct // aStruct and bStruct are two structs with the same value!
bStruct.name = "Sue"
println(aStruct.name) // "Bob"
println(bStruct.name) // "Sue"
So for representing a stateful complex entity, a class is awesome. But for values that are simply a measurement or bits of related data, a struct makes more sense so that you can easily copy them around and calculate with them or modify the values without fear of side effects.
| Swift | 24,217,586 | 209 |
After updating to the latest version of Xcode at the moment (version 10.0) the project is unable to build because it found some errors regarding some "Command CompileSwift failed with a nonzero exit code" error.
How do I solve this errors?
They appear in most of the Pods (I use CocoaPods) I use inside my project.
I have tried updating the pod version and the pods to the latest version available, but the problem is still there.
I have searched a lot through the web and there is very little information regarding this issue.
| For me, just cleaning project works using
ShiftCommandK & OptionShiftCommandK.
| Swift | 52,387,452 | 208 |
This is my setup: I have an UIScrollView with leading,top, trialing edge set to 0. Inside this I add an UIStackView with this constraints:
stackView.centerYAnchor.constraintEqualToAnchor(selectedContactsScrollView.centerYAnchor).active = true
stackView.leadingAnchor.constraintEqualToAnchor(selectedContactsScrollView.leadingAnchor).active = true
Inside the stack view I add some views.
My issue is that because of the constraints the first view added to stack view will also have leading edge = 0.
What are the ways that I could add some padding to the first view ? Without adjusting the scroll view constraints.
| When isLayoutMarginsRelativeArrangement property is true, the stack view will layout its arranged views relative to its layout margins.
stackView.layoutMargins = UIEdgeInsets(top: 0, left: 20, bottom: 0, right: 20)
stackView.isLayoutMarginsRelativeArrangement = true
But it affects all arranged views inside to the stack view. If you want this padding for only one arranged view, you need to use nested UIStackView
| Swift | 32,551,890 | 207 |
I have a dictionary containing UIColor objects hashed by an enum value, ColorScheme:
var colorsForColorScheme: [ColorScheme : UIColor] = ...
I would like to be able to extract an array of all the colors (the values) contained by this dictionary. I thought I could use the values property, as is used when iterating over dictionary values (for value in dictionary.values {...}), but this returns an error:
let colors: [UIColor] = colorsForColorSchemes.values
~~~~~~~~~~~~~~~~~~~~~^~~~~~~
'LazyBidrectionalCollection<MapCollectionView<Dictionary<ColorScheme, UIColor>, UIColor>>' is not convertible to 'UIColor'
It seems that rather than returning an Array of values, the values method returns a more abstract collection type. Is there a way to get an Array containing the dictionary's values without extracting them in a for-in loop?
| As of Swift 2.0, Dictionary’s values property now returns a LazyMapCollection instead of a LazyBidirectionalCollection. The Array type knows how to initialise itself using this abstract collection type:
let colors = Array(colorsForColorSchemes.values)
Swift's type inference already knows that these values are UIColor objects, so no type casting is required, which is nice!
| Swift | 26,988,167 | 207 |
I'm trying to make a calculator of growth rate (Double) that will round the result to the nearest Integer and recalculate from there, as such:
let firstUsers = 10.0
let growth = 0.1
var users = firstUsers
var week = 0
while users < 14 {
println("week \(week) has \(users) users")
users += users * growth
week += 1
}
but I've been unable so far.
EDIT
I kinda did it like so:
var firstUsers = 10.0
let growth = 0.1
var users:Int = Int(firstUsers)
var week = 0
while users <= 14 {
println("week \(week) has \(users) users")
firstUsers += firstUsers * growth
users = Int(firstUsers)
week += 1
}
Although I don't mind that it is always rounding down, I don't like it because firstUsers had to become a variable and change throughout the program (in order to make the next calculation), which I don't want it to happen.
| There is a round available in the Foundation library (it's actually in Darwin, but Foundation imports Darwin and most of the time you'll want to use Foundation instead of using Darwin directly).
import Foundation
users = round(users)
Running your code in a playground and then calling:
print(round(users))
Outputs:
15.0
round() always rounds up when the decimal place is >= .5 and down when it's < .5 (standard rounding). You can use floor() to force rounding down, and ceil() to force rounding up.
If you need to round to a specific place, then you multiply by pow(10.0, number of places), round, and then divide by pow(10, number of places):
Round to 2 decimal places:
let numberOfPlaces = 2.0
let multiplier = pow(10.0, numberOfPlaces)
let num = 10.12345
let rounded = round(num * multiplier) / multiplier
print(rounded)
Outputs:
10.12
Note: Due to the way floating point math works, rounded may not always be perfectly accurate. It's best to think of it more of an approximation of rounding. If you're doing this for display purposes, it's better to use string formatting to format the number rather than using math to round it.
| Swift | 26,350,977 | 207 |
In Swift 2.0, Apple introduced a new way to handle errors (do-try-catch).
And few days ago in Beta 6 an even newer keyword was introduced (try?).
Also, knew that I can use try!.
What's the difference between the 3 keywords, and when to use each?
| Updated for Swift 5.1
Assume the following throwing function:
enum ThrowableError: Error {
case badError(howBad: Int)
}
func doSomething(everythingIsFine: Bool = false) throws -> String {
if everythingIsFine {
return "Everything is ok"
} else {
throw ThrowableError.badError(howBad: 4)
}
}
try
You have 2 options when you try calling a function that may throw.
You can take responsibility of handling errors by surrounding your call within a do-catch block:
do {
let result = try doSomething()
}
catch ThrowableError.badError(let howBad) {
// Here you know about the error
// Feel free to handle or to re-throw
// 1. Handle
print("Bad Error (How Bad Level: \(howBad)")
// 2. Re-throw
throw ThrowableError.badError(howBad: howBad)
}
Or just try calling the function, and pass the error along to the next caller in the call chain:
func doSomeOtherThing() throws -> Void {
// Not within a do-catch block.
// Any errors will be re-thrown to callers.
let result = try doSomething()
}
try!
What happens when you try to access an implicitly unwrapped optional with a nil inside it? Yes, true, the app will CRASH!
Same goes with try! it basically ignores the error chain, and declares a “do or die” situation. If the called function didn’t throw any errors, everything goes fine. But if it failed and threw an error, your application will simply crash.
let result = try! doSomething() // if an error was thrown, CRASH!
try?
A new keyword that was introduced in Xcode 7 beta 6. It returns an optional that unwraps successful values, and catches error by returning nil.
if let result = try? doSomething() {
// doSomething succeeded, and result is unwrapped.
} else {
// Ouch, doSomething() threw an error.
}
Or we can use guard:
guard let result = try? doSomething() else {
// Ouch, doSomething() threw an error.
}
// doSomething succeeded, and result is unwrapped.
One final note here, by using try? note that you’re discarding the error that took place, as it’s translated to a nil.
Use try? when you’re focusing more on successes and failure, not on why things failed.
Using Coalescing Operator ??
You can use the coalescing operator ?? with try? to provide a default value incase of failure:
let result = (try? doSomething()) ?? "Default Value"
print(result) // Default Value
| Swift | 32,390,611 | 206 |
Is there a way to get the index of the array in map or reduce in Swift? I'm looking for something like each_with_index in Ruby.
func lunhCheck(number : String) -> Bool
{
var odd = true;
return reverse(number).map { String($0).toInt()! }.reduce(0) {
odd = !odd
return $0 + (odd ? ($1 == 9 ? 9 : ($1 * 2) % 9) : $1)
} % 10 == 0
}
lunhCheck("49927398716")
lunhCheck("49927398717")
I would like to get rid of the odd variable above.
| You can use enumerate to convert a sequence (Array, String, etc.) to a sequence of tuples with an integer counter and and element paired together. That is:
let numbers = [7, 8, 9, 10]
let indexAndNum: [String] = numbers.enumerate().map { (index, element) in
return "\(index): \(element)"
}
print(indexAndNum)
// ["0: 7", "1: 8", "2: 9", "3: 10"]
Link to enumerate definition
Note that this isn't the same as getting the index of the collection—enumerate gives you back an integer counter. This is the same as the index for an array, but on a string or dictionary won't be very useful. To get the actual index along with each element, you can use zip:
let actualIndexAndNum: [String] = zip(numbers.indices, numbers).map { "\($0): \($1)" }
print(actualIndexAndNum)
// ["0: 7", "1: 8", "2: 9", "3: 10"]
When using an enumerated sequence with reduce, you won't be able to separate the index and element in a tuple, since you already have the accumulating/current tuple in the method signature. Instead, you'll need to use .0 and .1 on the second parameter to your reduce closure:
let summedProducts = numbers.enumerate().reduce(0) { (accumulate, current) in
return accumulate + current.0 * current.1
// ^ ^
// index element
}
print(summedProducts) // 56
Swift 3.0 and above
Since Swift 3.0 syntax is quite different.
Also, you can use short-syntax/inline to map array on dictionary:
let numbers = [7, 8, 9, 10]
let array: [(Int, Int)] = numbers.enumerated().map { ($0, $1) }
// ^ ^
// index element
That produces:
[(0, 7), (1, 8), (2, 9), (3, 10)]
| Swift | 28,012,205 | 206 |
So I have converted an NSURL to a String.
So if I println it looks like file:///Users/... etc.
Later I want this back as an NSURL so I try and convert it back as seen below, but I lose two of the forward slashes that appear in the string version above, that in turn breaks the code as the url is invalid.
Why is my conversion back to NSURL removing two forward slashes from the String I give it, and how can I convert back to the NSURL containing three forward slashes?
var urlstring: String = recordingsDictionaryArray[selectedRow]["path"] as String
println("the url string = \(urlstring)")
// looks like file:///Users/........etc
var url = NSURL.fileURLWithPath(urlstring)
println("the url = \(url!)")
// looks like file:/Users/......etc
| In Swift 5, Swift 4 and Swift 3
To convert String to URL:
URL(string: String)
or,
URL.init(string: "yourURLString")
And to convert URL to String:
URL.absoluteString
The one below converts the 'contents' of the url to string
String(contentsOf: URL)
| Swift | 27,062,454 | 206 |
I'd like to map a function on all keys in the dictionary. I was hoping something like the following would work, but filter cannot be applied to dictionary directly. What's the cleanest way of achieving this?
In this example, I'm trying to increment each value by 1. However this is incidental for the example - the main purpose is to figure out how to apply map() to a dictionary.
var d = ["foo" : 1, "bar" : 2]
d.map() {
$0.1 += 1
}
| Swift 4+
Good news! Swift 4 includes a mapValues(_:) method which constructs a copy of a dictionary with the same keys, but different values. It also includes a filter(_:) overload which returns a Dictionary, and init(uniqueKeysWithValues:) and init(_:uniquingKeysWith:) initializers to create a Dictionary from an arbitrary sequence of tuples. That means that, if you want to change both the keys and values, you can say something like:
let newDict = Dictionary(uniqueKeysWithValues:
oldDict.map { key, value in (key.uppercased(), value.lowercased()) })
There are also new APIs for merging dictionaries together, substituting a default value for missing elements, grouping values (converting a collection into a dictionary of arrays, keyed by the result of mapping the collection over some function), and more.
During discussion of the proposal, SE-0165, that introduced these features, I brought up this Stack Overflow answer several times, and I think the sheer number of upvotes helped demonstrate the demand. So thanks for your help making Swift better!
| Swift | 24,116,271 | 206 |
I would like a for in loop to send off a bunch of network requests to firebase, then pass the data to a new view controller once the the method finishes executing. Here is my code:
var datesArray = [String: AnyObject]()
for key in locationsArray {
let ref = Firebase(url: "http://myfirebase.com/" + "\(key.0)")
ref.observeSingleEventOfType(.Value, withBlock: { snapshot in
datesArray["\(key.0)"] = snapshot.value
})
}
// Segue to new view controller here and pass datesArray once it is complete
I have a couple concerns. First, how do I wait until the for loop is finished and all the network requests are complete? I can't modify the observeSingleEventOfType function, it is part of the firebase SDK. Also, will I create some sort of race condition by trying to access the datesArray from different iterations of the for loop (hope that makes sense)? I've been reading about GCD and NSOperation but I'm a bit lost as this is the first app I've built.
Note: Locations array is an array containing the keys I need to access in firebase. Also, it's important that the network requests are fired off asynchronously. I just want to wait until ALL the asynchronous requests complete before I pass the datesArray to the next view controller.
| You can use dispatch groups to fire an asynchronous callback when all your requests finish.
Here's an example using dispatch groups to execute a callback asynchronously when multiple networking requests have all finished.
override func viewDidLoad() {
super.viewDidLoad()
let myGroup = DispatchGroup()
for i in 0 ..< 5 {
myGroup.enter()
Alamofire.request("https://httpbin.org/get", parameters: ["foo": "bar"]).responseJSON { response in
print("Finished request \(i)")
myGroup.leave()
}
}
myGroup.notify(queue: .main) {
print("Finished all requests.")
}
}
Output
Finished request 1
Finished request 0
Finished request 2
Finished request 3
Finished request 4
Finished all requests.
| Swift | 35,906,568 | 205 |
I have been reading about Optionals in Swift, and I have seen examples where if let is used to check if an Optional holds a value, and in case it does – do something with the unwrapped value.
However, I have seen that in Swift 2.0 the keyword guard let is used mostly. I wonder whether if let has been removed from Swift 2.0 or if it still possible to be used.
Should I change my programs that contain if let to guard let?
| if let and guard let serve similar, but distinct purposes.
The "else" case of guard must exit the current scope. Generally that means it must call return or abort the program. guard is used to provide early return without requiring nesting of the rest of the function.
if let nests its scope, and does not require anything special of it. It can return or not.
In general, if the if-let block was going to be the rest of the function, or its else clause would have a return or abort in it, then you should be using guard instead. This often means (at least in my experience), when in doubt, guard is usually the better answer. But there are plenty of situations where if let still is appropriate.
| Swift | 32,256,834 | 205 |
I'm trying to learn how to use UICollectionView. The documentation is a little hard to understand and the tutorials that I found were either in Objective C or long complicated projects.
When I was learning how to use UITableView, We ❤ Swift's How to make a simple tableview with iOS 8 and Swift had a very basic setup and explanation to get me going. Is there anything like this for UICollectionView?
The answer below is my attempt to learn to do this.
| This project has been tested with Xcode 10 and Swift 4.2.
Create a new project
It can be just a Single View App.
Add the code
Create a new Cocoa Touch Class file (File > New > File... > iOS > Cocoa Touch Class). Name it MyCollectionViewCell. This class will hold the outlets for the views that you add to your cell in the storyboard.
import UIKit
class MyCollectionViewCell: UICollectionViewCell {
@IBOutlet weak var myLabel: UILabel!
}
We will connect this outlet later.
Open ViewController.swift and make sure you have the following content:
import UIKit
class ViewController: UIViewController, UICollectionViewDataSource, UICollectionViewDelegate {
let reuseIdentifier = "cell" // also enter this string as the cell identifier in the storyboard
var items = ["1", "2", "3", "4", "5", "6", "7", "8", "9", "10", "11", "12", "13", "14", "15", "16", "17", "18", "19", "20", "21", "22", "23", "24", "25", "26", "27", "28", "29", "30", "31", "32", "33", "34", "35", "36", "37", "38", "39", "40", "41", "42", "43", "44", "45", "46", "47", "48"]
// MARK: - UICollectionViewDataSource protocol
// tell the collection view how many cells to make
func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
return self.items.count
}
// make a cell for each cell index path
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
// get a reference to our storyboard cell
let cell = collectionView.dequeueReusableCell(withReuseIdentifier: reuseIdentifier, for: indexPath as IndexPath) as! MyCollectionViewCell
// Use the outlet in our custom class to get a reference to the UILabel in the cell
cell.myLabel.text = self.items[indexPath.row] // The row value is the same as the index of the desired text within the array.
cell.backgroundColor = UIColor.cyan // make cell more visible in our example project
return cell
}
// MARK: - UICollectionViewDelegate protocol
func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
// handle tap events
print("You selected cell #\(indexPath.item)!")
}
}
Notes
UICollectionViewDataSource and UICollectionViewDelegate are the protocols that the collection view follows. You could also add the UICollectionViewFlowLayout protocol to change the size of the views programmatically, but it isn't necessary.
We are just putting simple strings in our grid, but you could certainly do images later.
Set up the storyboard
Drag a Collection View to the View Controller in your storyboard. You can add constraints to make it fill the parent view if you like.
Make sure that your defaults in the Attribute Inspector are also
Items: 1
Layout: Flow
The little box in the top left of the Collection View is a Collection View Cell. We will use it as our prototype cell. Drag a Label into the cell and center it. You can resize the cell borders and add constraints to center the Label if you like.
Write "cell" (without quotes) in the Identifier box of the Attributes Inspector for the Collection View Cell. Note that this is the same value as let reuseIdentifier = "cell" in ViewController.swift.
And in the Identity Inspector for the cell, set the class name to MyCollectionViewCell, our custom class that we made.
Hook up the outlets
Hook the Label in the collection cell to myLabel in the MyCollectionViewCell class. (You can Control-drag.)
Hook the Collection View delegate and dataSource to the View Controller. (Right click Collection View in the Document Outline. Then click and drag the plus arrow up to the View Controller.)
Finished
Here is what it looks like after adding constraints to center the Label in the cell and pinning the Collection View to the walls of the parent.
Making Improvements
The example above works but it is rather ugly. Here are a few things you can play with:
Background color
In the Interface Builder, go to your Collection View > Attributes Inspector > View > Background.
Cell spacing
Changing the minimum spacing between cells to a smaller value makes it look better. In the Interface Builder, go to your Collection View > Size Inspector > Min Spacing and make the values smaller. "For cells" is the horizontal distance and "For lines" is the vertical distance.
Cell shape
If you want rounded corners, a border, and the like, you can play around with the cell layer. Here is some sample code. You would put it directly after cell.backgroundColor = UIColor.cyan in code above.
cell.layer.borderColor = UIColor.black.cgColor
cell.layer.borderWidth = 1
cell.layer.cornerRadius = 8
See this answer for other things you can do with the layer (shadow, for example).
Changing the color when tapped
It makes for a better user experience when the cells respond visually to taps. One way to achieve this is to change the background color while the cell is being touched. To do that, add the following two methods to your ViewController class:
// change background color when user touches cell
func collectionView(_ collectionView: UICollectionView, didHighlightItemAt indexPath: IndexPath) {
let cell = collectionView.cellForItem(at: indexPath)
cell?.backgroundColor = UIColor.red
}
// change background color back when user releases touch
func collectionView(_ collectionView: UICollectionView, didUnhighlightItemAt indexPath: IndexPath) {
let cell = collectionView.cellForItem(at: indexPath)
cell?.backgroundColor = UIColor.cyan
}
Here is the updated look:
Further study
A Simple UICollectionView Tutorial
UICollectionView Tutorial Part 1: Getting Started
UICollectionView Tutorial Part 2: Reusable Views and Cell Selection
UITableView version of this Q&A
UITableView example for Swift
| Swift | 31,735,228 | 205 |
I'm really confused with regards to how we create an empty array in Swift. Could you please show me the different ways we have to create an empty array with some detail?
| Here you go:
var yourArray = [String]()
The above also works for other types and not just strings. It's just an example.
Adding Values to It
I presume you'll eventually want to add a value to it!
yourArray.append("String Value")
Or
let someString = "You can also pass a string variable, like this!"
yourArray.append(someString)
Add by Inserting
Once you have a few values, you can insert new values instead of appending. For example, if you wanted to insert new objects at the beginning of the array (instead of appending them to the end):
yourArray.insert("Hey, I'm first!", atIndex: 0)
Or you can use variables to make your insert more flexible:
let lineCutter = "I'm going to be first soon."
let positionToInsertAt = 0
yourArray.insert(lineCutter, atIndex: positionToInsertAt)
You May Eventually Want to Remove Some Stuff
var yourOtherArray = ["MonkeysRule", "RemoveMe", "SwiftRules"]
yourOtherArray.remove(at: 1)
The above works great when you know where in the array the value is (that is, when you know its index value). As the index values begin at 0, the second entry will be at index 1.
Removing Values Without Knowing the Index
But what if you don't? What if yourOtherArray has hundreds of values and all you know is you want to remove the one equal to "RemoveMe"?
if let indexValue = yourOtherArray.index(of: "RemoveMe") {
yourOtherArray.remove(at: indexValue)
}
This should get you started!
| Swift | 30,430,550 | 204 |
I'm using Xcode 8.0 beta 4.
In previous version, UIViewController have method to set the status bar style
public func preferredStatusBarStyle() -> UIStatusBarStyle
However, I found it changed to a "Get ONLY varaiable" in Swift 3.
public var preferredStatusBarStyle: UIStatusBarStyle { get }
How can provide the style to use in my UIViewController?
| [UPDATED] For Xcode 10+ & Swift 4.2+
This is the preferred method for iOS 7 and higher
In your application's Info.plist, set View controller-based status bar appearance to YES.
Override preferredStatusBarStyle (Apple docs) in each of your view controllers. For example:
override var preferredStatusBarStyle: UIStatusBarStyle {
return .lightContent
}
If you have preferredStatusBarStyle returning a different preferred status bar style based on something that changes inside of your view controller (for example, whether the scroll position or whether a displayed image is dark), then you will want to call setNeedsStatusBarAppearanceUpdate() when that state changes.
iOS before version 7, deprecated method
Apple has deprecated this, so it will be removed in the future. Use the above method so that you don't have to rewrite it when the next iOS version is released.
If your application will support In your application's Info.plist, set View controller-based status bar appearance to NO.
In appDelegate.swift, the didFinishLaunchingWithOptions function, add:
UIApplication.shared.statusBarStyle = .lightContent
For Navigation Controller
If you use a navigation controller and you want the preferred status bar style of each view controller to be used and set View controller-based status bar appearance to YES in your application's info.plist
extension UINavigationController {
open override var preferredStatusBarStyle: UIStatusBarStyle {
return topViewController?.preferredStatusBarStyle ?? .default
}
}
| Swift | 38,740,648 | 203 |
I have the following class:
class ReportView: NSView {
var categoriesPerPage = [[Int]]()
var numPages: Int = { return categoriesPerPage.count }
}
Compilation fails with the message:
Instance member 'categoriesPerPage' cannot be used on type
'ReportView'
What does this mean?
| Sometimes Xcode when overrides methods adds class func instead of just func. Then in static method you can't see instance properties. It is very easy to overlook it. That was my case.
| Swift | 32,351,343 | 203 |
It's possible to add extensions to existing Swift object types using extensions, as described in the language specification.
As a result, it's possible to create extensions such as:
extension String {
var utf8data:NSData {
return self.dataUsingEncoding(NSUTF8StringEncoding, allowLossyConversion: false)!
}
}
However, what's the best naming practice for Swift source files containing such extensions?
In the past, the convention was to use extendedtype+categoryname.m for the Objective-C
type as discussed in the Objective-C guide. But the Swift example doesn't have a category name, and calling it String.swift doesn't seem appropriate.
So the question is: given the above String extension, what should the swift source file be called?
| Most examples I have seen mimic the Objective-C approach. The example extension above would be:
String+UTF8Data.swift
The advantages are that the naming convention makes it easy to understand that it is an extension, and which Class is being extended.
The problem with using Extensions.swift or even StringExtensions.swift is that it's not possible to infer the purpose of the file by its name without looking at its contents.
Using xxxable.swift approach as used by Java works okay for protocols or extensions that only define methods. But again, the example above defines an attribute so that UTF8Dataable.swift doesn't make much grammatical sense.
| Swift | 26,319,660 | 203 |
I want to leave a bit of space at the beginning of a UITextField, just like here:
Add lefthand margin to UITextField
But I don't know how to do that with Swift.
| This is what I am using right now:
Swift 4.2, 5
class TextField: UITextField {
let padding = UIEdgeInsets(top: 0, left: 5, bottom: 0, right: 5)
override open func textRect(forBounds bounds: CGRect) -> CGRect {
return bounds.inset(by: padding)
}
override open func placeholderRect(forBounds bounds: CGRect) -> CGRect {
return bounds.inset(by: padding)
}
override open func editingRect(forBounds bounds: CGRect) -> CGRect {
return bounds.inset(by: padding)
}
}
Swift 4
class TextField: UITextField {
let padding = UIEdgeInsets(top: 0, left: 5, bottom: 0, right: 5)
override open func textRect(forBounds bounds: CGRect) -> CGRect {
return UIEdgeInsetsInsetRect(bounds, padding)
}
override open func placeholderRect(forBounds bounds: CGRect) -> CGRect {
return UIEdgeInsetsInsetRect(bounds, padding)
}
override open func editingRect(forBounds bounds: CGRect) -> CGRect {
return UIEdgeInsetsInsetRect(bounds, padding)
}
}
Swift 3:
class TextField: UITextField {
let padding = UIEdgeInsets(top: 0, left: 5, bottom: 0, right: 5)
override func textRect(forBounds bounds: CGRect) -> CGRect {
return UIEdgeInsetsInsetRect(bounds, padding)
}
override func placeholderRect(forBounds bounds: CGRect) -> CGRect {
return UIEdgeInsetsInsetRect(bounds, padding)
}
override func editingRect(forBounds bounds: CGRect) -> CGRect {
return UIEdgeInsetsInsetRect(bounds, padding)
}
}
I never set a other padding but you can tweak. This class doesn't take care of the rightView and leftView on the textfield. If you want that to be handle correctly you can use something like (example in objc and I only needed the rightView:
- (CGRect)textRectForBounds:(CGRect)bounds {
CGRect paddedRect = UIEdgeInsetsInsetRect(bounds, self.insets);
if (self.rightViewMode == UITextFieldViewModeAlways || self.rightViewMode == UITextFieldViewModeUnlessEditing) {
return [self adjustRectWithWidthRightView:paddedRect];
}
return paddedRect;
}
- (CGRect)placeholderRectForBounds:(CGRect)bounds {
CGRect paddedRect = UIEdgeInsetsInsetRect(bounds, self.insets);
if (self.rightViewMode == UITextFieldViewModeAlways || self.rightViewMode == UITextFieldViewModeUnlessEditing) {
return [self adjustRectWithWidthRightView:paddedRect];
}
return paddedRect;
}
- (CGRect)editingRectForBounds:(CGRect)bounds {
CGRect paddedRect = UIEdgeInsetsInsetRect(bounds, self.insets);
if (self.rightViewMode == UITextFieldViewModeAlways || self.rightViewMode == UITextFieldViewModeWhileEditing) {
return [self adjustRectWithWidthRightView:paddedRect];
}
return paddedRect;
}
- (CGRect)adjustRectWithWidthRightView:(CGRect)bounds {
CGRect paddedRect = bounds;
paddedRect.size.width -= CGRectGetWidth(self.rightView.frame);
return paddedRect;
}
| Swift | 25,367,502 | 203 |
I have an Objective-C project in Xcode 8 Beta 3. Since updating, whenever I try to build I receive the following error:
“Use Legacy Swift Language Version” (SWIFT_VERSION) is required to be configured correctly for targets which use Swift. Use the [Edit > Convert > To Current Swift Syntax…] menu to choose a Swift version or use the Build Settings editor to configure the build setting directly.
Has anyone encountered this? Since it's an Objective-C project there's no build setting to configure Swift. I have also made sure none of the project dependencies or CocoaPods are using Swift. The only solution I have is to use Beta 2. Any ideas how I might fix this issue?
I should also mention I'm running OSX 10.12 Beta 2.
| If you are using CocoaPods and want it to be fixed automatically every time you are doing a pod install, then you can add these lines to the end of your Podfile:
post_install do |installer|
installer.pods_project.targets.each do |target|
target.build_configurations.each do |config|
config.build_settings['SWIFT_VERSION'] = '3.0'
end
end
end
EDIT: This problem is now fixed if you use CocoaPods v1.1.1 or later.
Don't forget to remove the ALWAYS_EMBED_SWIFT_STANDARD_LIBRARIES setting from your main project targets.
| Swift | 38,446,097 | 202 |
I'm programming an app in swift and when I run the test app on the iPhone simulator everything works, but then I try to swipe right, which is a gesture that I added for it to go to the next Page(View Controller Two) it crashes and shows this error report in the console log.
2014-10-18 12:07:34.400 soundtest[17081:818922] *** Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[<soundtest.ViewControllerTwo 0x7f92f1f20090> setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key sfdfa.'
*** First throw call stack:
(
0 CoreFoundation 0x00000001067813f5 __exceptionPreprocess + 165
1 libobjc.A.dylib 0x00000001082afbb7 objc_exception_throw + 45
2 CoreFoundation 0x0000000106781039 -[NSException raise] + 9
3 Foundation 0x0000000106b984d3 -[NSObject(NSKeyValueCoding) setValue:forKey:] + 259
4 CoreFoundation 0x00000001066cb400 -[NSArray makeObjectsPerformSelector:] + 224
5 UIKit 0x00000001072ce97d -[UINib instantiateWithOwner:options:] + 1506
6 UIKit 0x000000010712f698 -[UIViewController _loadViewFromNibNamed:bundle:] + 242
7 UIKit 0x000000010712fc88 -[UIViewController loadView] + 109
8 UIKit 0x000000010712fef9 -[UIViewController loadViewIfRequired] + 75
9 UIKit 0x000000010713038e -[UIViewController view] + 27
10 UIKit 0x00000001076cd83f -[_UIFullscreenPresentationController _setPresentedViewController:] + 65
11 UIKit 0x000000010710bc49 -[UIPresentationController initWithPresentedViewController:presentingViewController:] + 105
12 UIKit 0x000000010713c121 -[UIViewController _presentViewController:withAnimationController:completion:] + 1746
13 UIKit 0x000000010713e461 __62-[UIViewController presentViewController:animated:completion:]_block_invoke + 132
14 UIKit 0x000000010713e385 -[UIViewController presentViewController:animated:completion:] + 229
15 UIKit 0x00000001073bb9d6 _UIGestureRecognizerSendActions + 262
16 UIKit 0x00000001073ba679 -[UIGestureRecognizer _updateGestureWithEvent:buttonEvent:] + 532
17 UIKit 0x00000001073bf296 ___UIGestureRecognizerUpdate_block_invoke662 + 51
18 UIKit 0x00000001073bf192 _UIGestureRecognizerRemoveObjectsFromArrayAndApplyBlocks + 254
19 UIKit 0x00000001073b520d _UIGestureRecognizerUpdate + 2796
20 UIKit 0x00000001070520a6 -[UIWindow _sendGesturesForEvent:] + 1041
21 UIKit 0x0000000107052cd3 -[UIWindow sendEvent:] + 667
22 UIKit 0x000000010701fae1 -[UIApplication sendEvent:] + 246
23 UIKit 0x000000010702cbad _UIApplicationHandleEventFromQueueEvent + 17370
24 UIKit 0x0000000107008233 _UIApplicationHandleEventQueue + 1961
25 CoreFoundation 0x00000001066b6ad1 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 17
26 CoreFoundation 0x00000001066ac99d __CFRunLoopDoSources0 + 269
27 CoreFoundation 0x00000001066abfd4 __CFRunLoopRun + 868
28 CoreFoundation 0x00000001066aba06 CFRunLoopRunSpecific + 470
29 GraphicsServices 0x000000010a1699f0 GSEventRunModal + 161
30 UIKit 0x000000010700b550 UIApplicationMain + 1282
31 soundtest 0x000000010624503e top_level_code + 78
32 soundtest 0x000000010624507a main + 42
33 libdyld.dylib 0x000000010ae4a145 start + 1
34 ??? 0x0000000000000001 0x0 + 1
)
libc++abi.dylib: terminating with uncaught exception of type NSException
(lldb)
| CMYR - "his could also happen if you've wired up a button to an IBAction that doesn't exist anymore (or has been renamed)"
If you're running into this problem make sure that you go to Main.storyboard, RIGHT click on the yellow box icon (view controller) at the top of the phone outline and DELETE the outlet(s) with yellow flags.
What happens in instances like this is you probably named an action, then renamed it. You need to delete the old name and if that was the only issue will start right up in sim!
| Swift | 26,442,414 | 200 |
I am familiar with switch statements in Swift, but wondering how to replace this piece of code with a switch:
if someVar < 0 {
// do something
} else if someVar == 0 {
// do something else
} else if someVar > 0 {
// etc
}
| Here's one approach. Assuming someVar is an Int or other Comparable, you can optionally assign the operand to a new variable. This lets you scope it however you want using the where keyword:
var someVar = 3
switch someVar {
case let x where x < 0:
print("x is \(x)")
case let x where x == 0:
print("x is \(x)")
case let x where x > 0:
print("x is \(x)")
default:
print("this is impossible")
}
This can be simplified a bit:
switch someVar {
case _ where someVar < 0:
print("someVar is \(someVar)")
case 0:
print("someVar is 0")
case _ where someVar > 0:
print("someVar is \(someVar)")
default:
print("this is impossible")
}
You can also avoid the where keyword entirely with range matching:
switch someVar {
case Int.min..<0:
print("someVar is \(someVar)")
case 0:
print("someVar is 0")
default:
print("someVar is \(someVar)")
}
| Swift | 31,656,642 | 199 |
I'm going through the iOS tutorial from Apple developer page.
It seems to me that protocol and interface almost have the same functionality.
Are there any differences between the two?
the different usage in the project?
Updated
Yes, I did read the link above and I'm still not sure what the differences and usage between protocol and interface. When I ask a question like this, I would like to see a simple explanation about the topic. Sometime it could be tough to get everything from the documentation.
| Essentially protocols are very similar to Java interfaces except for:
Swift protocols can also specify properties that must be implemented (i.e. fields)
Swift protocols need to deal with value/reference through the use of the mutating keyword (because protocols can be implemented by structures, enumerations or classes).
you can combine protocols at any point using "Protocol Composition". This replaces the older swift protocol<A, B> way of protocol composition. For example, declaring a function parameter that must adhere to protocol Named and Aged as:
func wishHappyBirthday(to celebrator: Named & Aged) {}
These are the immediately apparent differences for a Java developer (or at least what I've spotted so far). There's more info here.
| Swift | 30,859,334 | 199 |
Arrays in Swift support the += operator to add the contents of one Array to another. Is there an easy way to do that for a dictionary?
eg:
var dict1 = ["a" : "foo"]
var dict2 = ["b" : "bar"]
var combinedDict = ... (some way of combining dict1 & dict2 without looping)
| You can define += operator for Dictionary, e.g.,
func += <K, V> (left: inout [K:V], right: [K:V]) {
for (k, v) in right {
left[k] = v
}
}
| Swift | 24,051,904 | 199 |
Does Swift have something like _.findWhere in Underscore.js?
I have an array of structs of type T and would like to check if array contains a struct object whose name property is equal to Foo.
Tried to use find() and filter() but they only work with primitive types, e.g. String or Int. Throws an error about not conforming to Equitable protocol or something like that.
| SWIFT 5
Check if the element exists
if array.contains(where: {$0.name == "foo"}) {
// it exists, do something
} else {
//item could not be found
}
Get the element
if let foo = array.first(where: {$0.name == "foo"}) {
// do something with foo
} else {
// item could not be found
}
Get the element and its offset
if let foo = array.enumerated().first(where: {$0.element.name == "foo"}) {
// do something with foo.offset and foo.element
} else {
// item could not be found
}
Get the offset
if let fooOffset = array.firstIndex(where: {$0.name == "foo"}) {
// do something with fooOffset
} else {
// item could not be found
}
| Swift | 28,727,845 | 197 |
Finally now with Beta 5 we can programmatically pop to a parent View. However, there are several places in my app where a view has a "Save" button that concludes a several step process and returns to the beginning. In UIKit, I use popToRootViewController(), but I have been unable to figure out a way to do the same in SwiftUI.
Below is a simple example of the pattern I'm trying to achieve.
How can I do it?
import SwiftUI
struct DetailViewB: View {
@Environment(\.presentationMode) var presentationMode: Binding<PresentationMode>
var body: some View {
VStack {
Text("This is Detail View B.")
Button(action: { self.presentationMode.value.dismiss() } )
{ Text("Pop to Detail View A.") }
Button(action: { /* How to do equivalent to popToRootViewController() here?? */ } )
{ Text("Pop two levels to Master View.") }
}
}
}
struct DetailViewA: View {
@Environment(\.presentationMode) var presentationMode: Binding<PresentationMode>
var body: some View {
VStack {
Text("This is Detail View A.")
NavigationLink(destination: DetailViewB() )
{ Text("Push to Detail View B.") }
Button(action: { self.presentationMode.value.dismiss() } )
{ Text("Pop one level to Master.") }
}
}
}
struct MasterView: View {
var body: some View {
VStack {
Text("This is Master View.")
NavigationLink(destination: DetailViewA() )
{ Text("Push to Detail View A.") }
}
}
}
struct ContentView: View {
var body: some View {
NavigationView {
MasterView()
}
}
}
| iOS 16 Update: NavigationPath was added to make this easier. Use with the new NavigationStack that also fixes a lot of bugs.
Setting the view modifier isDetailLink to false on a NavigationLink is the key to getting pop-to-root to work. isDetailLink is true by default and is adaptive to the containing View. On iPad landscape for example, a Split view is separated and isDetailLink ensures the destination view will be shown on the right-hand side. Setting isDetailLink to false consequently means that the destination view will always be pushed onto the navigation stack; thus can always be popped off.
Along with setting isDetailLink to false on NavigationLink, pass the isActive binding to each subsequent destination view. At last when you want to pop to the root view, set the value to false and it will automatically pop everything off:
import SwiftUI
struct ContentView: View {
@State var isActive : Bool = false
var body: some View {
NavigationView {
NavigationLink(
destination: ContentView2(rootIsActive: self.$isActive),
isActive: self.$isActive
) {
Text("Hello, World!")
}
.isDetailLink(false)
.navigationBarTitle("Root")
}
}
}
struct ContentView2: View {
@Binding var rootIsActive : Bool
var body: some View {
NavigationLink(destination: ContentView3(shouldPopToRootView: self.$rootIsActive)) {
Text("Hello, World #2!")
}
.isDetailLink(false)
.navigationBarTitle("Two")
}
}
struct ContentView3: View {
@Binding var shouldPopToRootView : Bool
var body: some View {
VStack {
Text("Hello, World #3!")
Button (action: { self.shouldPopToRootView = false } ){
Text("Pop to root")
}
}.navigationBarTitle("Three")
}
}
struct ContentView_Previews: PreviewProvider {
static var previews: some View {
ContentView()
}
}
| Swift | 57,334,455 | 196 |
I want to convert the string "2014-07-15 06:55:14.198000+00:00" to an NSDate in Swift.
| try this:
let dateFormatter = NSDateFormatter()
dateFormatter.dateFormat = /* find out and place date format from
* http://userguide.icu-project.org/formatparse/datetime
*/
let date = dateFormatter.dateFromString(/* your_date_string */)
For further query, check NSDateFormatter and DateFormatter classes of Foundation framework for Objective-C and Swift, respectively.
Swift 3 and later
let dateFormatter = DateFormatter()
dateFormatter.dateFormat = /* date_format_you_want_in_string from
* http://userguide.icu-project.org/formatparse/datetime
*/
guard let date = dateFormatter.date(from: /* your_date_string */) else {
fatalError("ERROR: Date conversion failed due to mismatched format.")
}
// use date constant here
Edit:
Alternative date time format reference
https://unicode-org.github.io/icu/userguide/format_parse/datetime/
| Swift | 24,777,496 | 196 |
Getting this error in Swift 2.0.
Binary operator '|' cannot be applied to two UIViewAutoresizing operands
Here is the code:
let view = UIView(frame: CGRect(x: 0, y: 0, width: 320, height: 568))
addSubview(view)
view.autoresizingMask = UIViewAutoresizing.FlexibleWidth | UIViewAutoresizing.FlexibleHeight
Any idea what can be the problem?
| The OptionSetType got an updated syntax for Swift 2.x and another update for Swift 3.x
Swift 3.x
view.autoresizingMask = [.flexibleWidth, .flexibleHeight]
Swift 2.x
view.autoresizingMask = [.FlexibleWidth, .FlexibleHeight]
| Swift | 30,867,325 | 195 |
I haven't read too much into Swift but one thing I noticed is that there are no exceptions.
So how do they do error handling in Swift? Has anyone found anything related to error-handling?
| Swift 2 & 3
Things have changed a bit in Swift 2, as there is a new error-handling mechanism, that is somewhat more similar to exceptions but different in detail.
1. Indicating error possibility
If function/method wants to indicate that it may throw an error, it should contain throws keyword like this
func summonDefaultDragon() throws -> Dragon
Note: there is no specification for type of error the function actually can throw. This declaration simply states that the function can throw an instance of any type implementing ErrorType or is not throwing at all.
2. Invoking function that may throw errors
In order to invoke function you need to use try keyword, like this
try summonDefaultDragon()
this line should normally be present do-catch block like this
do {
let dragon = try summonDefaultDragon()
} catch DragonError.dragonIsMissing {
// Some specific-case error-handling
} catch DragonError.notEnoughMana(let manaRequired) {
// Other specific-case error-handlng
} catch {
// Catch all error-handling
}
Note: catch clause use all the powerful features of Swift pattern matching so you are very flexible here.
You may decided to propagate the error, if your are calling a throwing function from a function that is itself marked with throws keyword:
func fulfill(quest: Quest) throws {
let dragon = try summonDefaultDragon()
quest.ride(dragon)
}
Alternatively, you can call throwing function using try?:
let dragonOrNil = try? summonDefaultDragon()
This way you either get the return value or nil, if any error occurred. Using this way you do not get the error object.
Which means that you can also combine try? with useful statements like:
if let dragon = try? summonDefaultDragon()
or
guard let dragon = try? summonDefaultDragon() else { ... }
Finally, you can decide that you know that error will not actually occur (e.g. because you have already checked are prerequisites) and use try! keyword:
let dragon = try! summonDefaultDragon()
If the function actually throws an error, then you'll get a runtime error in your application and the application will terminate.
3. Throwing an error
In order to throw an error you use throw keyword like this
throw DragonError.dragonIsMissing
You can throw anything that conforms to ErrorType protocol. For starters NSError conforms to this protocol but you probably would like to go with enum-based ErrorType which enables you to group multiple related errors, potentially with additional pieces of data, like this
enum DragonError: ErrorType {
case dragonIsMissing
case notEnoughMana(requiredMana: Int)
...
}
Main differences between new Swift 2 & 3 error mechanism and Java/C#/C++ style exceptions are follows:
Syntax is a bit different: do-catch + try + defer vs traditional try-catch-finally syntax.
Exception handling usually incurs much higher execution time in exception path than in success path. This is not the case with Swift 2.0 errors, where success path and error path cost roughly the same.
All error throwing code must be declared, while exceptions might have been thrown from anywhere. All errors are "checked exceptions" in Java nomenclature. However, in contrast to Java, you do not specify potentially thrown errors.
Swift exceptions are not compatible with ObjC exceptions. Your do-catch block will not catch any NSException, and vice versa, for that you must use ObjC.
Swift exceptions are compatible with Cocoa NSError method conventions of returning either false (for Bool returning functions) or nil (for AnyObject returning functions) and passing NSErrorPointer with error details.
As an extra syntatic-sugar to ease error handling, there are two more concepts
deferred actions (using defer keyword) which let you achieve the same effect as finally blocks in Java/C#/etc
guard statement (using guard keyword) which let you write little less if/else code than in normal error checking/signaling code.
Swift 1
Runtime errors:
As Leandros suggests for handling runtime errors (like network connectivity problems, parsing data, opening file, etc) you should use NSError like you did in ObjC, because the Foundation, AppKit, UIKit, etc report their errors in this way. So it's more framework thing than language thing.
Another frequent pattern that is being used are separator success/failure blocks like in AFNetworking:
var sessionManager = AFHTTPSessionManager(baseURL: NSURL(string: "yavin4.yavin.planets"))
sessionManager.HEAD("/api/destoryDeathStar", parameters: xwingSquad,
success: { (NSURLSessionDataTask) -> Void in
println("Success")
},
failure:{ (NSURLSessionDataTask, NSError) -> Void in
println("Failure")
})
Still the failure block frequently received NSError instance, describing the error.
Programmer errors:
For programmer errors (like out of bounds access of array element, invalid arguments passed to a function call, etc) you used exceptions in ObjC. Swift language does not seem to have any language support for exceptions (like throw, catch, etc keyword). However, as documentation suggests it is running on the same runtime as ObjC, and therefore you are still able to throw NSExceptions like this:
NSException(name: "SomeName", reason: "SomeReason", userInfo: nil).raise()
You just cannot catch them in pure Swift, although you may opt for catching exceptions in ObjC code.
The questions is whether you should throw exceptions for programmer errors, or rather use assertions as Apple suggests in the language guide.
| Swift | 24,010,569 | 195 |
I am trying to register my application for local notifications this way:
UIApplication.sharedApplication().registerUserNotificationSettings(UIUserNotificationSettings(forTypes: UIUserNotificationType.Alert | UIUserNotificationType.Badge, categories: nil))
In Xcode 7 and Swift 2.0 - I get error Binary Operator "|" cannot be applied to two UIUserNotificationType operands. Please help me.
| In Swift 2, many types that you would typically do this for have been updated to conform to the OptionSetType protocol. This allows for array like syntax for usage, and In your case, you can use the following.
let settings = UIUserNotificationSettings(forTypes: [.Alert, .Badge], categories: nil)
UIApplication.sharedApplication().registerUserNotificationSettings(settings)
And on a related note, if you want to check if an option set contains a specific option, you no longer need to use bitwise AND and a nil check. You can simply ask the option set if it contains a specific value in the same way that you would check if an array contained a value.
let settings = UIUserNotificationSettings(forTypes: [.Alert, .Badge], categories: nil)
if settings.types.contains(.Alert) {
// stuff
}
In Swift 3, the samples must be written as follows:
let settings = UIUserNotificationSettings(types: [.alert, .badge], categories: nil)
UIApplication.shared.registerUserNotificationSettings(settings)
and
let settings = UIUserNotificationSettings(types: [.alert, .badge], categories: nil)
if settings.types.contains(.alert) {
// stuff
}
| Swift | 30,761,996 | 194 |
I give it a try to understand new error handling thing in swift 2. Here is what I did: I first declared an error enum:
enum SandwichError: Error {
case NotMe
case DoItYourself
}
And then I declared a method that throws an error (not an exception folks. It is an error.). Here is that method:
func makeMeSandwich(names: [String: String]) throws -> String {
guard let sandwich = names["sandwich"] else {
throw SandwichError.NotMe
}
return sandwich
}
The problem is from the calling side. Here is the code that calls this method:
let kitchen = ["sandwich": "ready", "breakfast": "not ready"]
do {
let sandwich = try makeMeSandwich(kitchen)
print("i eat it \(sandwich)")
} catch SandwichError.NotMe {
print("Not me error")
} catch SandwichError.DoItYourself {
print("do it error")
}
After the do line compiler says Errors thrown from here are not handled because the enclosing catch is not exhaustive. But in my opinion, it is exhaustive because there are only two cases in SandwichError enum.
For regular switch statements, swift can understand it is exhaustive when every case is handled.
| There are two important points to the Swift 2 error handling model: exhaustiveness and resiliency. Together, they boil down to your do/catch statement needing to catch every possible error, not just the ones you know you can throw.
Notice that you don't declare what types of errors a function can throw, only whether it throws at all. It's a zero-one-infinity sort of problem: as someone defining a function for others (including your future self) to use, you don't want to have to make every client of your function adapt to every change in the implementation of your function, including what errors it can throw. You want code that calls your function to be resilient to such change.
Because your function can't say what kind of errors it throws (or might throw in the future), the catch blocks that catch it errors don't know what types of errors it might throw. So, in addition to handling the error types you know about, you need to handle the ones you don't with a universal catch statement -- that way if your function changes the set of errors it throws in the future, callers will still catch its errors.
do {
let sandwich = try makeMeSandwich(kitchen)
print("i eat it \(sandwich)")
} catch SandwichError.NotMe {
print("Not me error")
} catch SandwichError.DoItYourself {
print("do it error")
} catch let error {
print(error.localizedDescription)
}
But let's not stop there. Think about this resilience idea some more. The way you've designed your sandwich, you have to describe errors in every place where you use them. That means that whenever you change the set of error cases, you have to change every place that uses them... not very fun.
The idea behind defining your own error types is to let you centralize things like that. You could define a description method for your errors:
extension SandwichError: CustomStringConvertible {
var description: String {
switch self {
case NotMe: return "Not me error"
case DoItYourself: return "Try sudo"
}
}
}
And then your error handling code can ask your error type to describe itself -- now every place where you handle errors can use the same code, and handle possible future error cases, too.
do {
let sandwich = try makeMeSandwich(kitchen)
print("i eat it \(sandwich)")
} catch let error as SandwichError {
print(error.description)
} catch {
print("i dunno")
}
This also paves the way for error types (or extensions on them) to support other ways of reporting errors -- for example, you could have an extension on your error type that knows how to present a UIAlertController for reporting the error to an iOS user.
| Swift | 30,720,497 | 194 |
I need to capture multiple groups of the same pattern. Suppose, I have the following string:
HELLO,THERE,WORLD
And I've written the following pattern
^(?:([A-Z]+),?)+$
What I want it to do is to capture every single word, so that Group 1 is : "HELLO", Group 2 is "THERE" and Group 3 is "WORLD". What my regex is actually capturing is only the last one, which is "WORLD".
I'm testing my regular expression here and I want to use it with Swift (maybe there's a way in Swift to get intermediate results somehow, so that I can use them?)
I don't want to use split. I just need to now how to capture all the groups that match the pattern, not only the last one.
| With one group in the pattern, you can only get one exact result in that group. If your capture group gets repeated by the pattern (you used the + quantifier on the surrounding non-capturing group), only the last value that matches it gets stored.
You have to use your language's regex implementation functions to find all matches of a pattern, then you would have to remove the anchors and the quantifier of the non-capturing group (and you could omit the non-capturing group itself as well).
Alternatively, expand your regex and let the pattern contain one capturing group per group you want to get in the result:
^([A-Z]+),([A-Z]+),([A-Z]+)$
| Swift | 37,003,623 | 192 |
The background text in the status bar is still black. How do I change the color to white?
// io8, swift, Xcode 6.0.1
override func viewDidLoad() {
super.viewDidLoad()
self.navigationController?.navigationBar.barTintColor = UIColor.blackColor()
self.navigationController?.navigationBar.titleTextAttributes = [NSForegroundColorAttributeName: UIColor.orangeColor()]
}
| In AppDelegate.swift, in application(_:didFinishLaunchingWithOptions:) I put the following:
UINavigationBar.appearance().barTintColor = UIColor(red: 234.0/255.0, green: 46.0/255.0, blue: 73.0/255.0, alpha: 1.0)
UINavigationBar.appearance().tintColor = UIColor.white
UINavigationBar.appearance().titleTextAttributes = [NSAttributedString.Key.foregroundColor : UIColor.white]
(For Swift 4 or earlier use NSAttributedStringKey instead of NSAttributedString.Key)
For titleTextAttributes, the docs say:
You can specify the font, text color, text shadow color, and text
shadow offset for the title in the text attributes dictionary
| Swift | 26,008,536 | 192 |
In Objective C, one could do the following to check for strings:
if ([myString isEqualToString:@""]) {
NSLog(@"myString IS empty!");
} else {
NSLog(@"myString IS NOT empty, it is: %@", myString);
}
How does one detect empty strings in Swift?
| There is now the built in ability to detect empty string with .isEmpty:
if emptyString.isEmpty {
print("Nothing to see here")
}
Apple Pre-release documentation: "Strings and Characters".
| Swift | 24,133,157 | 192 |
I'm currently working on a iOS app developed in Swift and I need to store some user-created content on the device but I can't seem to find a simple and quick way to store/receive the users content on the device.
Could someone explain how to store and access local storage?
The idea is to store the data when the user executes an action and receive it when the app starts.
| The simplest solution for storing a few strings or common types is UserDefaults.
The UserDefaults class provides convenience methods for accessing common types such as floats, doubles, integers, Boolean values, and URLs.
UserDefaults lets us store objects against a key of our choice, It's a good idea to store these keys somewhere accessible so we can reuse them.
Keys
struct DefaultsKeys {
static let keyOne = "firstStringKey"
static let keyTwo = "secondStringKey"
}
Setting
let defaults = UserDefaults.standard
defaults.set(
"Some String Value",
forKey: DefaultsKeys.keyOne
)
defaults.set(
"Another String Value",
forKey: DefaultsKeys.keyTwo
)
Getting
let defaults = UserDefaults.standard
if let stringOne = defaults.string(
forKey: DefaultsKeys.keyOne
) {
print(stringOne) // Some String Value
}
if let stringTwo = defaults.string(
forKey: DefaultsKeys.keyTwo
) {
print(stringTwo) // Another String Value
}
Swift 2.0
In Swift 2.0 UserDefaults was called NSUserDefaults and the setters and getters were named slightly differently:
Setting
let defaults = NSUserDefaults.standardUserDefaults()
defaults.setObject(
"Some String Value",
forKey: DefaultsKeys.keyOne
)
defaults.setObject(
"Another String Value",
forKey: DefaultsKeys.keyTwo
)
Getting
let defaults = NSUserDefaults.standardUserDefaults()
if let stringOne = defaults.stringForKey(
DefaultsKeys.keyOne
) {
print(stringOne) // Some String Value
}
if let stringTwo = defaults.stringForKey(
DefaultsKeys.keyTwo
) {
print(stringTwo) // Another String Value
}
For anything more serious than minor config you should consider using a more robust persistent store:
CoreData
Realm
SQLite
| Swift | 28,628,225 | 191 |
How can we measure the time elapsed for running a function in Swift? I am trying to display the elapsed time like this: "Elapsed time is .05 seconds". Saw that in Java, we can use System.nanoTime(), are there any equivalent methods available in Swift to accomplish this?
Please have a look at the sample program:
func isPrime(_ number: Int) -> Bool {
var i = 0;
for i=2; i<number; i++ {
if number % i == 0, i != 0 {
return false
}
}
return true
}
var number = 5915587277
if isPrime(number) {
print("Prime number")
} else {
print("NOT a prime number")
}
| Update
With Swift 5.7, everything below becomes obsolete. Swift 5.7 introduces the concept of a Clock which has a function designed to do exactly what is required here.
There are two concrete examples of a Clock provided: ContinuousClock and SuspendingClock. The former keeps ticking when the system is suspending and the latter does not.
The following is an example of what to do in Swift 5.7
func doSomething()
{
for i in 0 ..< 1000000
{
if (i % 10000 == 0)
{
print(i)
}
}
}
let clock = ContinuousClock()
let result = clock.measure(doSomething)
print(result) // On my laptop, prints "0.552065882 seconds"
It also allows you to measure closures directly, of course
let clock = ContinuousClock()
let result = clock.measure {
for i in 0 ..< 1000000
{
if (i % 10000 == 0)
{
print(i)
}
}
}
print(result) // "0.534663798 seconds"
Pre Swift 5.7
Here's a Swift function I wrote to measure Project Euler problems in Swift
As of Swift 3, there is now a version of Grand Central Dispatch that is "swiftified". So the correct answer is probably to use the DispatchTime API.
My function would look something like:
// Swift 3
func evaluateProblem(problemNumber: Int, problemBlock: () -> Int) -> Answer
{
print("Evaluating problem \(problemNumber)")
let start = DispatchTime.now() // <<<<<<<<<< Start time
let myGuess = problemBlock()
let end = DispatchTime.now() // <<<<<<<<<< end time
let theAnswer = self.checkAnswer(answerNum: "\(problemNumber)", guess: myGuess)
let nanoTime = end.uptimeNanoseconds - start.uptimeNanoseconds // <<<<< Difference in nano seconds (UInt64)
let timeInterval = Double(nanoTime) / 1_000_000_000 // Technically could overflow for long running tests
print("Time to evaluate problem \(problemNumber): \(timeInterval) seconds")
return theAnswer
}
Old answer
For Swift 1 and 2, my function uses NSDate:
// Swift 1
func evaluateProblem(problemNumber: Int, problemBlock: () -> Int) -> Answer
{
println("Evaluating problem \(problemNumber)")
let start = NSDate() // <<<<<<<<<< Start time
let myGuess = problemBlock()
let end = NSDate() // <<<<<<<<<< end time
let theAnswer = self.checkAnswer(answerNum: "\(problemNumber)", guess: myGuess)
let timeInterval: Double = end.timeIntervalSinceDate(start) // <<<<< Difference in seconds (double)
println("Time to evaluate problem \(problemNumber): \(timeInterval) seconds")
return theAnswer
}
Note that using NSdate for timing functions is discouraged: "The system time may decrease due to synchronization with external time references or due to an explicit user change of the clock.".
| Swift | 24,755,558 | 191 |
I have a framework (in this instance it's RxSwift) which I've compiled using Xcode 11.0 into the traditional RxSwift.framework style package
This imported fine into Xcode 11.0 and also 11.1 never had any problems with it
Today, upon Apple's release of Xcode 11.2, I upgraded, and I am presented with the error:
Module compiled with Swift 5.1 cannot be imported by the Swift 5.1.2 compiler
I'm used to swift compiler mismatches, and I'm aware I can just recompile RxSwift using Xcode 11.2 and carry on, but the headline feature of Swift 5.1 was module stability.
I was under the impression that now that we have module stability, frameworks won't need to keep getting recompiled with every new Xcode release, yet this is clearly not the case.
If anyone can explain what is going on here I would much appreciate it. Is Xcode 11.2 exhibiting a bug? or did I somehow need to tell it I wanted module stability when I originally compiled with Xcode 11.0?
| OK, Turns out if you watch the WWDC video, they explain it:
https://developer.apple.com/videos/play/wwdc2019/416/
You need to set the Build Settings > Build Options > Build Libraries for Distribution option to Yes in your framework's build settings, otherwise the swift compiler doesn't generate the neccessary .swiftinterface files which are the key to future compilers being able to load your old library.
This ends up in your project.pbxproj file as:
BUILD_LIBRARY_FOR_DISTRIBUTION = YES;
After setting this flag, a framework I compiled using Xcode 11.0 (swift 5.1) was able to be used by Xcode 11.2 (swift 5.1.2) and everything appears to be working correctly.
Hopefully this question/answer will serve as a useful reference for everyone who hasn't watched all the WWDC videos
If the error still persists go to Product > Clean Build Folder and Build again.
| Swift | 58,654,714 | 190 |
How do you import CommonCrypto in a Swift framework for iOS?
I understand how to use CommonCrypto in a Swift app:
You add #import <CommonCrypto/CommonCrypto.h> to the bridging header.
However, Swift frameworks don't support bridging headers. The documentation says:
You can import external frameworks that have a pure Objective-C codebase, a pure Swift codebase, or a mixed-language codebase. The
process for importing an external framework is the same whether the
framework is written in a single language or contains files from both
languages. When you import an external framework, make sure the
Defines Module build setting for the framework you’re importing is set
to Yes.
You can import a framework into any Swift file within a different
target using the following syntax:
import FrameworkName
Unfortunately, import CommonCrypto doesn't work. Neither does adding #import <CommonCrypto/CommonCrypto.h> to the umbrella header.
| Something a little simpler and more robust is to create an Aggregate target called "CommonCryptoModuleMap" with a Run Script phase to generate the module map automatically and with the correct Xcode/SDK path:
The Run Script phase should contain this bash:
# This if-statement means we'll only run the main script if the CommonCryptoModuleMap directory doesn't exist
# Because otherwise the rest of the script causes a full recompile for anything where CommonCrypto is a dependency
# Do a "Clean Build Folder" to remove this directory and trigger the rest of the script to run
if [ -d "${BUILT_PRODUCTS_DIR}/CommonCryptoModuleMap" ]; then
echo "${BUILT_PRODUCTS_DIR}/CommonCryptoModuleMap directory already exists, so skipping the rest of the script."
exit 0
fi
mkdir -p "${BUILT_PRODUCTS_DIR}/CommonCryptoModuleMap"
cat <<EOF > "${BUILT_PRODUCTS_DIR}/CommonCryptoModuleMap/module.modulemap"
module CommonCrypto [system] {
header "${SDKROOT}/usr/include/CommonCrypto/CommonCrypto.h"
export *
}
EOF
Using shell code and ${SDKROOT} means you don't have to hard code the Xcode.app path which can vary system-to-system, especially if you use xcode-select to switch to a beta version, or are building on a CI server where multiple versions are installed in non-standard locations. You also don't need to hard code the SDK so this should work for iOS, macOS, etc. You also don't need to have anything sitting in your project's source directory.
After creating this target, make your library/framework depend on it with a Target Dependencies item:
This will ensure the module map is generated before your framework is built.
macOS note: If you're supporting macOS as well, you'll need to add macosx to the Supported Platforms build setting on the new aggregate target you just created, otherwise it won't put the module map in the correct Debug derived data folder with the rest of the framework products.
Next, add the module map's parent directory, ${BUILT_PRODUCTS_DIR}/CommonCryptoModuleMap, to the "Import Paths" build setting under the Swift section (SWIFT_INCLUDE_PATHS):
Remember to add a $(inherited) line if you have search paths defined at the project or xcconfig level.
That's it, you should now be able to import CommonCrypto
Update for Xcode 10
Xcode 10 now ships with a CommonCrypto module map making this workaround unnecessary. If you would like to support both Xcode 9 and 10 you can do a check in the Run Script phase to see if the module map exists or not, e.g.
COMMON_CRYPTO_DIR="${SDKROOT}/usr/include/CommonCrypto"
if [ -f "${COMMON_CRYPTO_DIR}/module.modulemap" ]
then
echo "CommonCrypto already exists, skipping"
else
# generate the module map, using the original code above
fi
| Swift | 25,248,598 | 190 |
How can I simply scan barcodes on iPhone and/or iPad?
| We produced the 'Barcodes' application for the iPhone. It can decode QR Codes. The source code is available from the zxing project; specifically, you want to take a look at the iPhone client and the partial C++ port of the core library. The port is a little old, from circa the 0.9 release of the Java code, but should still work reasonably well.
If you need to scan other formats, like 1D formats, you could continue the port of the Java code within this project to C++.
EDIT: Barcodes and the iphone code in the project were retired around the start of 2014.
| Swift | 838,724 | 190 |
I have a protocol:
enum DataFetchResult {
case success(data: Data)
case failure
}
protocol DataServiceType {
func fetchData(location: String, completion: (DataFetchResult) -> (Void))
func cachedData(location: String) -> Data?
}
With an example implementation:
/// An implementation of DataServiceType protocol returning predefined results using arbitrary queue for asynchronyous mechanisms.
/// Dedicated to be used in various tests (Unit Tests).
class DataMockService: DataServiceType {
var result : DataFetchResult
var async : Bool = true
var queue : DispatchQueue = DispatchQueue.global(qos: .background)
var cachedData : Data? = nil
init(result : DataFetchResult) {
self.result = result
}
func cachedData(location: String) -> Data? {
switch self.result {
case .success(let data):
return data
default:
return nil
}
}
func fetchData(location: String, completion: (DataFetchResult) -> (Void)) {
// Returning result on arbitrary queue should be tested,
// so we can check if client can work with any (even worse) implementation:
if async == true {
queue.async { [weak self ] in
guard let weakSelf = self else { return }
// This line produces compiler error:
// "Closure use of non-escaping parameter 'completion' may allow it to escape"
completion(weakSelf.result)
}
} else {
completion(self.result)
}
}
}
The code above compiled and worked in Swift3 (Xcode8-beta5) but does not work with beta 6 anymore. Can you point me to the underlying cause?
| This is due to a change in the default behaviour for parameters of function type. Prior to Swift 3 (specifically the build that ships with Xcode 8 beta 6), they would default to being escaping – you would have to mark them @noescape in order to prevent them from being stored or captured, which guarantees they won't outlive the duration of the function call.
However, now @noescape is the default for function-typed parameters. If you want to store or capture such functions, you now need to mark them @escaping:
protocol DataServiceType {
func fetchData(location: String, completion: @escaping (DataFetchResult) -> Void)
func cachedData(location: String) -> Data?
}
func fetchData(location: String, completion: @escaping (DataFetchResult) -> Void) {
// ...
}
See the Swift Evolution proposal for more info about this change.
| Swift | 38,990,882 | 189 |
How do you play a video with AV Kit Player View Controller in Swift?
override func viewDidLoad() {
super.viewDidLoad()
let videoURLWithPath = "http://****/5.m3u8"
let videoURL = NSURL(string: videoURLWithPath)
playerViewController = AVPlayerViewController()
dispatch_async(dispatch_get_main_queue()) {
self.playerViewController?.player = AVPlayer.playerWithURL(videoURL) as AVPlayer
}
}
| SwiftUI
import SwiftUI
import AVKit
struct ContentView: View {
var body: some View {
let videoURL = URL(string: "https://test-videos.co.uk/vids/bigbuckbunny/mp4/h264/720/Big_Buck_Bunny_720_10s_5MB.mp4")
let player = AVPlayer(url: videoURL!)
VideoPlayer(player: player)
}
}
Swift 3.x - 5.x
Necessary: import AVKit, import AVFoundation
AVFoundation framework is needed even if you use AVPlayer
If you want to use AVPlayerViewController:
let videoURL = URL(string: "https://test-videos.co.uk/vids/bigbuckbunny/mp4/h264/720/Big_Buck_Bunny_720_10s_5MB.mp4")
let player = AVPlayer(url: videoURL!)
let playerViewController = AVPlayerViewController()
playerViewController.player = player
self.present(playerViewController, animated: true) {
playerViewController.player!.play()
}
or just AVPlayer:
let videoURL = URL(string: "https://test-videos.co.uk/vids/bigbuckbunny/mp4/h264/720/Big_Buck_Bunny_720_10s_5MB.mp4")
let player = AVPlayer(url: videoURL!)
let playerLayer = AVPlayerLayer(player: player)
playerLayer.frame = self.view.bounds
self.view.layer.addSublayer(playerLayer)
player.play()
It's better to put this code into the method: override func viewDidAppear(_ animated: Bool) or somewhere after.
Objective-C
AVPlayerViewController:
NSURL *videoURL = [NSURL URLWithString:@"https://test-videos.co.uk/vids/bigbuckbunny/mp4/h264/720/Big_Buck_Bunny_720_10s_5MB.mp4"];
AVPlayer *player = [AVPlayer playerWithURL:videoURL];
AVPlayerViewController *playerViewController = [AVPlayerViewController new];
playerViewController.player = player;
[self presentViewController:playerViewController animated:YES completion:^{
[playerViewController.player play];
}];
or just AVPlayer:
NSURL *videoURL = [NSURL URLWithString:@"https://test-videos.co.uk/vids/bigbuckbunny/mp4/h264/720/Big_Buck_Bunny_720_10s_5MB.mp4"];
AVPlayer *player = [AVPlayer playerWithURL:videoURL];
AVPlayerLayer *playerLayer = [AVPlayerLayer playerLayerWithPlayer:player];
playerLayer.frame = self.view.bounds;
[self.view.layer addSublayer:playerLayer];
[player play];
| Swift | 25,932,570 | 189 |
I'm working in swift on Xcode and by default it creates a test file that references XCTest.
When I set the target membership to my main project it causes this error
Cannot load underlying module for XCTest
If this target membership is not set the tests runs properly and everything works fine.
| Double check that the file in question is not in the main target but instead only the test target. Only the test target will have that framework to import.
| Swift | 29,965,397 | 188 |
I want to programmatically create a UIImage filled by a solid color. Does anyone have an idea of how to do this in Swift?
| Another nice solution,
Swift 3.0
public extension UIImage {
convenience init?(color: UIColor, size: CGSize = CGSize(width: 1, height: 1)) {
let rect = CGRect(origin: .zero, size: size)
UIGraphicsBeginImageContextWithOptions(rect.size, false, 0.0)
color.setFill()
UIRectFill(rect)
let image = UIGraphicsGetImageFromCurrentImageContext()
UIGraphicsEndImageContext()
guard let cgImage = image?.cgImage else { return nil }
self.init(cgImage: cgImage)
}
}
Swift 2.2 compatible, is to create another constructor in UIImage, in this way:
public extension UIImage {
public convenience init?(color: UIColor, size: CGSize = CGSize(width: 1, height: 1)) {
let rect = CGRect(origin: .zero, size: size)
UIGraphicsBeginImageContextWithOptions(rect.size, false, 0.0)
color.setFill()
UIRectFill(rect)
let image = UIGraphicsGetImageFromCurrentImageContext()
UIGraphicsEndImageContext()
guard let cgImage = image?.CGImage else { return nil }
self.init(CGImage: cgImage)
}
}
In this way you can create the custom colored-image in this way:
let redImage = UIImage(color: .redColor())
Or, optionally, create the image with a custom size:
let redImage200x200 = UIImage(color: .redColor(), size: CGSize(width: 200, height: 200))
| Swift | 26,542,035 | 187 |
In Swift, is there any way to check if an index exists in an array without a fatal error being thrown?
I was hoping I could do something like this:
let arr: [String] = ["foo", "bar"]
let str: String? = arr[1]
if let str2 = arr[2] as String? {
// this wouldn't run
println(str2)
} else {
// this would be run
}
But I get
fatal error: Array index out of range
| An elegant way in Swift:
let isIndexValid = array.indices.contains(index)
| Swift | 25,976,909 | 187 |
I have Encoded text(NSString) using NSData Class new API which is Added in iOS7.
using this
- (NSData *)dataUsingEncoding:(NSStringEncoding)encoding;
here is my code
NSString *base64EncodedString = [[myText dataUsingEncoding:NSUTF8StringEncoding] base64EncodedStringWithOptions:0];
NSLog(@"%@", base64EncodedString);
I am looking to decode it
| Swift 3+
let plainString = "foo"
Encoding
let plainData = plainString.data(using: .utf8)
let base64String = plainData?.base64EncodedString()
print(base64String!) // Zm9v
Decoding
if let decodedData = Data(base64Encoded: base64String!),
let decodedString = String(data: decodedData, encoding: .utf8) {
print(decodedString) // foo
}
Swift < 3
let plainString = "foo"
Encoding
let plainData = plainString.dataUsingEncoding(NSUTF8StringEncoding)
let base64String = plainData?.base64EncodedStringWithOptions(NSDataBase64EncodingOptions(rawValue: 0))
print(base64String!) // Zm9v
Decoding
let decodedData = NSData(base64EncodedString: base64String!, options: NSDataBase64DecodingOptions(rawValue: 0))
let decodedString = NSString(data: decodedData, encoding: NSUTF8StringEncoding)
print(decodedString) // foo
Objective-C
NSString *plainString = @"foo";
Encoding
NSData *plainData = [plainString dataUsingEncoding:NSUTF8StringEncoding];
NSString *base64String = [plainData base64EncodedStringWithOptions:0];
NSLog(@"%@", base64String); // Zm9v
Decoding
NSData *decodedData = [[NSData alloc] initWithBase64EncodedString:base64String options:0];
NSString *decodedString = [[NSString alloc] initWithData:decodedData encoding:NSUTF8StringEncoding];
NSLog(@"%@", decodedString); // foo
| Swift | 19,088,231 | 187 |
I have this code to display a list of custom rows.
struct ContentView : View {
var body: some View {
VStack(alignment: .leading) {
List(1...10) {_ in
CustomRow()
}
}
}
}
However, I want to remove the line on each row. I tried not using List and instead using ForEach inside ScrollView but it completely removes all the styling including its padding and margins. I just want to remove the lines and nothing else.
Please help, thank you.
| iOS 15:
This year Apple introduced a new modifier .listRowSeparator that can be used to style the separators. you can pass .hidden to hide it:
List {
ForEach(items, id:\.self) {
Text("Row \($0)")
.listRowSeparator(.hidden)
}
}
iOS 14:
you may consider using a LazyVStack inside a ScrollView instead (because iOS is NOT supporting UIAppearance for SwiftUI lists anymore).
iOS 13:
⚠️ This method is deprecated and it's not working from iOS 14
There is a UITableView behind SwiftUI's List for iOS 13. So to remove
Extra separators (below the list):
you need a tableFooterView and to remove
All separators (including the actual ones):
you need separatorStyle to be .none
Example of usage
init() {
if #available(iOS 14.0, *) {
// iOS 14 doesn't have extra separators below the list by default.
} else {
// To remove only extra separators below the list:
UITableView.appearance().tableFooterView = UIView()
}
// To remove all separators including the actual ones:
UITableView.appearance().separatorStyle = .none
}
var body: some View {
List {
Text("Item 1")
Text("Item 2")
Text("Item 3")
}
}
Note that a static list doesn't show extra separators below the list by default
| Swift | 56,553,672 | 186 |
I tried to change the UIStackView background from clear to white in Storyboard inspector, but when simulating, the background color of the stack view still has a clear color.
How can I change the background color of a UIStackView?
|
You can't do this – UIStackView is a non-drawing view, meaning that
drawRect() is never called and its background color is ignored. If you
desperately want a background color, consider placing the stack view
inside another UIView and giving that view a background color.
Reference from HERE.
EDIT:
You can add a subView to UIStackView as mentioned HERE or in this answer (below) and assign a color to it. Check out below extension for that:
extension UIStackView {
func addBackground(color: UIColor) {
let subView = UIView(frame: bounds)
subView.backgroundColor = color
subView.autoresizingMask = [.flexibleWidth, .flexibleHeight]
insertSubview(subView, at: 0)
}
}
And you can use it like:
stackView.addBackground(color: .red)
| Swift | 34,868,344 | 186 |
I have a problem with Swift class. I have a swift file for UITableViewController class and UITableViewCell class. My problem is the UITableViewCell class, and outlets. This class has an error Class "HomeCell" has no initializers, and I don't understand this problem.
Thanks for your responses.
import Foundation
import UIKit
class HomeTable: UITableViewController, UITableViewDataSource, UITableViewDelegate {
@IBOutlet var tableViex: UITableView!
var items: [(String, String, String)] = [
("Test", "123", "1.jpeg"),
("Test2", "236", "2.jpeg"),
("Test3", "678", "3.jpeg")
]
override func viewDidLoad() {
super.viewDidLoad()
var nib = UINib(nibName: "HomeCell", bundle: nil)
tableView.registerNib(nib, forCellReuseIdentifier: "bookCell")
}
// Number row
override func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
return self.items.count
}
// Style Cell
override func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
var cell:UITableViewCell = self.tableView.dequeueReusableCellWithIdentifier("bookCell") as UITableViewCell
// Style here
return cell
}
// Select row
override func tableView(tableView: UITableView, didSelectRowAtIndexPath indexPath: NSIndexPath) {
// Select
}
}
// PROBLEM HERE
class HomeCell : UITableViewCell {
@IBOutlet var imgBook: UIImageView
@IBOutlet var titleBook: UILabel
@IBOutlet var pageBook: UILabel
func loadItem(#title: String, page: String, image:String) {
titleBook.text = title
pageBook.text = page
imgBook.image = UIImage(named: image)
}
}
| You have to use implicitly unwrapped optionals so that Swift can cope with circular dependencies (parent <-> child of the UI components in this case) during the initialization phase.
@IBOutlet var imgBook: UIImageView!
@IBOutlet var titleBook: UILabel!
@IBOutlet var pageBook: UILabel!
Read this doc, they explain it all nicely.
| Swift | 27,797,351 | 186 |
Unlike Objective-C, Swift has no preprocessor, so is there still a way to manually deprecate members of a class?
I am looking for something similar to this:
-(id)method __deprecated;
| You can use the Available tag, for example :
@available(*, deprecated)
func myFunc() {
// ...
}
Where * is the platform (iOS, iOSApplicationExtension, macOS, watchOS, tvOS, * for all, etc.).
You can also specify the version of the platform from which it was introduced, deprecated, obsoleted, renamed, and a message :
@available(iOS, deprecated:6.0)
func myFunc() {
// calling this function is deprecated on iOS6+
}
Or
@available(iOS, deprecated: 6.0, obsoleted: 7.0, message: "Because !")
func myFunc() {
// deprecated from iOS6, and obsoleted after iOS7, the message "Because !" is displayed in XCode warnings
}
If your project targets multiple platforms, you can use several tags like so :
@available(tvOS, deprecated:9.0.1)
@available(iOS, deprecated:9.1)
@available(macOS, unavailable, message: "Unavailable on macOS")
func myFunc() {
// ...
}
More details in the Swift documentation.
| Swift | 25,405,133 | 186 |
I am currently opening the link in my app in a WebView, but I'm looking for an option to open the link in Safari instead.
| It's not "baked in to Swift", but you can use standard UIKit methods to do it. Take a look at UIApplication's openUrl(_:) (deprecated) and open(_:options:completionHandler:).
Swift 4 + Swift 5 (iOS 10 and above)
guard let url = URL(string: "https://stackoverflow.com") else { return }
UIApplication.shared.open(url)
Swift 3 (iOS 9 and below)
guard let url = URL(string: "https://stackoverflow.com") else { return }
UIApplication.shared.openURL(url)
Swift 2.2
guard let url = URL(string: "https://stackoverflow.com") else { return }
UIApplication.sharedApplication().openURL(url)
| Swift | 25,945,324 | 185 |
Here is my Objective-C code which I'm using to load a nib for my customised UIView:
-(id)init{
NSArray *subviewArray = [[NSBundle mainBundle] loadNibNamed:@"myXib" owner:self options:nil];
return [subviewArray objectAtIndex:0];
}
What is the equivalent code in Swift?
| My contribution:
extension UIView {
class func fromNib<T: UIView>() -> T {
return Bundle(for: T.self).loadNibNamed(String(describing: T.self), owner: nil, options: nil)![0] as! T
}
}
Then call it like this:
let myCustomView: CustomView = UIView.fromNib()
..or even:
let myCustomView: CustomView = .fromNib()
| Swift | 24,857,986 | 185 |
Before swift I would define a set of schemes for alpha, beta, and distribution builds. Each of these schemes would have a set of macros that were defined to gate certain behaviors at the project level. The simplest example is the DEBUG=1 macro that is defined by default for all Xcode projects in the default scheme for the Run build. One could query #ifdef DEBUG ... and make decisions in the code accordingly, even compiling out non-necessary code.
It seems that this type of configurational gating is not as easy using swift, as macros are not supported. Can someone suggest a comparable approach, I don't care if the code is compiled out, per se. I would like to gate features based on build scheme, though.
| In Swift you can still use the "#if/#else/#endif" preprocessor macros (although more constrained), as per Apple docs. Here's an example:
#if DEBUG
let a = 2
#else
let a = 3
#endif
Now, you must set the "DEBUG" symbol elsewhere, though. Set it in the "Swift Compiler - Custom Flags" section, "Other Swift Flags" line. You add the DEBUG symbol with the -D DEBUG entry.
(Build Settings -> Swift Compiler - Custom Flags)
As usual, you can set a different value when in Debug or when in Release.
I tested it in real code; it doesn't seem to be recognized in a playground.
| Swift | 24,111,854 | 184 |
I have a View Controller in which my value is 0 (label) and when I open that View Controller from another ViewController I have set viewDidAppear to set value 20 on label. It works fine but when I close my app and than again I open my app but the value doesn't change because viewDidLoad, viewDidAppear and viewWillAppear nothing get called. How can I call when I open my app. Do I have to do anything from applicationDidBecomeActive?
| Curious about the exact sequence of events, I instrumented an app as follows: (@Zohaib, you can use the NSNotificationCenter code below to answer your question).
// AppDelegate.m
- (void)applicationWillEnterForeground:(UIApplication *)application
{
NSLog(@"app will enter foreground");
}
- (void)applicationDidBecomeActive:(UIApplication *)application
{
NSLog(@"app did become active");
}
// ViewController.m
- (void)viewDidLoad
{
[super viewDidLoad];
NSLog(@"view did load");
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(appDidBecomeActive:) name:UIApplicationDidBecomeActiveNotification object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(appWillEnterForeground:) name:UIApplicationWillEnterForegroundNotification object:nil];
}
- (void)appDidBecomeActive:(NSNotification *)notification {
NSLog(@"did become active notification");
}
- (void)appWillEnterForeground:(NSNotification *)notification {
NSLog(@"will enter foreground notification");
}
- (void)viewWillAppear:(BOOL)animated {
[super viewWillAppear:animated];
NSLog(@"view will appear");
}
- (void)viewDidAppear:(BOOL)animated {
[super viewDidAppear:animated];
NSLog(@"view did appear");
}
At launch, the output looks like this:
2013-04-07 09:31:06.505 myapp[15459:11303] view did load
2013-04-07 09:31:06.507 myapp[15459:11303] view will appear
2013-04-07 09:31:06.511 myapp[15459:11303] app did become active
2013-04-07 09:31:06.512 myapp[15459:11303] did become active notification
2013-04-07 09:31:06.517 myapp[15459:11303] view did appear
Enter the background then reenter the foreground:
2013-04-07 09:32:05.923 myapp[15459:11303] app will enter foreground
2013-04-07 09:32:05.924 myapp[15459:11303] will enter foreground notification
2013-04-07 09:32:05.925 myapp[15459:11303] app did become active
2013-04-07 09:32:05.926 myapp[15459:11303] did become active notification
| Swift | 15,864,364 | 183 |
Given:
typealias Action = () -> ()
var action: Action = { }
func doStuff(stuff: String, completion: @escaping Action) {
print(stuff)
action = completion
completion()
}
func doStuffAgain() {
print("again")
action()
}
doStuff(stuff: "do stuff") {
print("swift 3!")
}
doStuffAgain()
Is there any way to make the completion parameter (and action) of type Action? and also keep @escaping ?
Changing the type gives the following error:
@escaping attribute only applies to function types
Removing the @escaping attribute, the code compiles and runs, but doesn't seem to be correct since the completion closure is escaping the scope of the function.
| from: swift-users mailing list
Basically, @escaping is valid only on closures in function parameter position. The noescape-by-default rule only applies to these closures at function parameter position, otherwise they are escaping. Aggregates, such as enums with associated values (e.g. Optional), tuples, structs, etc., if they have closures, follow the default rules for closures that are not at function parameter position, i.e. they are escaping.
So optional function parameter is @escaping by default.
@noeascape only apply to function parameter by default.
| Swift | 39,618,803 | 182 |
I started my search by wanting to know how I could share to other apps in iOS. I discovered that two important ways are
UIActivityViewController
UIDocumentInteractionController
These and other methods are compared in this SO answer.
Often when I am learning a new concept I like to see a basic example to get me started. Once I get something basic set up I can modify it how I like later.
There are many SO questions related to UIActivityViewController, but I couldn't find any that were just asking for a simple example. Since I just learned how to do this, I will provide my own answer below. Feel free to add a better one (or an Objective-C version).
| UIActivityViewController Example Project
Set up your storyboard with two buttons and hook them up to your view controller (see code below).
Add an image to your Assets.xcassets. I called mine "lion".
Code
import UIKit
class ViewController: UIViewController {
// share text
@IBAction func shareTextButton(_ sender: UIButton) {
// text to share
let text = "This is some text that I want to share."
// set up activity view controller
let textToShare = [ text ]
let activityViewController = UIActivityViewController(activityItems: textToShare, applicationActivities: nil)
activityViewController.popoverPresentationController?.sourceView = self.view // so that iPads won't crash
// exclude some activity types from the list (optional)
activityViewController.excludedActivityTypes = [ UIActivity.ActivityType.airDrop, UIActivity.ActivityType.postToFacebook ]
// present the view controller
self.present(activityViewController, animated: true, completion: nil)
}
// share image
@IBAction func shareImageButton(_ sender: UIButton) {
// image to share
let image = UIImage(named: "Image")
// set up activity view controller
let imageToShare = [ image! ]
let activityViewController = UIActivityViewController(activityItems: imageToShare, applicationActivities: nil)
activityViewController.popoverPresentationController?.sourceView = self.view // so that iPads won't crash
// exclude some activity types from the list (optional)
activityViewController.excludedActivityTypes = [ UIActivity.ActivityType.airDrop, UIActivity.ActivityType.postToFacebook ]
// present the view controller
self.present(activityViewController, animated: true, completion: nil)
}
}
Result
Clicking "Share some text" gives result on the left and clicking "Share an image" gives the result on the right.
Notes
I retested this with iOS 11 and Swift 4. I had to run it a couple times in the simulator before it worked because it was timing out. This may be because my computer is slow.
If you wish to hide some of these choices, you can do that with excludedActivityTypes as shown in the code above.
Not including the popoverPresentationController?.sourceView line will cause your app to crash when run on an iPad.
This does not allow you to share text or images to other apps. You probably want UIDocumentInteractionController for that.
See also
Add sharing to your Swift app via UIActivityViewController
UIActivityViewController from NSHipster
UIActivityViewController documentation
Share extension documentation
comparison with UIDocumentInteractionController
| Swift | 35,931,946 | 182 |
After I installed Xcode 7 beta and convert my swift code to Swift 2, I got some issue with the code that I can't figure out. I know Swift 2 is new so I search and figure out since there is nothing about it, I should write a question.
Here is the error:
Call can throw, but it is not marked with 'try' and the error is not
handled
Code:
func deleteAccountDetail(){
let entityDescription = NSEntityDescription.entityForName("AccountDetail", inManagedObjectContext: Context!)
let request = NSFetchRequest()
request.entity = entityDescription
//The Line Below is where i expect the error
let fetchedEntities = self.Context!.executeFetchRequest(request) as! [AccountDetail]
for entity in fetchedEntities {
self.Context!.deleteObject(entity)
}
do {
try self.Context!.save()
} catch _ {
}
}
Snapshot:
| You have to catch the error just as you're already doing for your save() call and since you're handling multiple errors here, you can try multiple calls sequentially in a single do-catch block, like so:
func deleteAccountDetail() {
let entityDescription = NSEntityDescription.entityForName("AccountDetail", inManagedObjectContext: Context!)
let request = NSFetchRequest()
request.entity = entityDescription
do {
let fetchedEntities = try self.Context!.executeFetchRequest(request) as! [AccountDetail]
for entity in fetchedEntities {
self.Context!.deleteObject(entity)
}
try self.Context!.save()
} catch {
print(error)
}
}
Or as @bames53 pointed out in the comments below, it is often better practice not to catch the error where it was thrown. You can mark the method as throws then try to call the method. For example:
func deleteAccountDetail() throws {
let entityDescription = NSEntityDescription.entityForName("AccountDetail", inManagedObjectContext: Context!)
let request = NSFetchRequest()
request.entity = entityDescription
let fetchedEntities = try Context.executeFetchRequest(request) as! [AccountDetail]
for entity in fetchedEntities {
self.Context!.deleteObject(entity)
}
try self.Context!.save()
}
| Swift | 30,737,262 | 182 |
I'm trying to ultimately have an NSMutableURLRequest with a valid HTTPBody, but I can't seem to get my string data (coming from a UITextField) into a usable NSData object.
I've seen this method for going the other way:
NSString(data data: NSData!, encoding encoding: UInt)
But I can't seem to find any documentation for my use case. I'm open to putting the string into some other type if necessary, but none of the initialization options for NSData using Swift seem to be what I'm looking for.
| In Swift 3
let data = string.data(using: .utf8)
In Swift 2 (or if you already have a NSString instance)
let data = string.dataUsingEncoding(NSUTF8StringEncoding)
In Swift 1 (or if you have a swift String):
let data = (string as NSString).dataUsingEncoding(NSUTF8StringEncoding)
Also note that data is an Optional<NSData> (since the conversion might fail), so you'll need to unwrap it before using it, for instance:
if let d = data {
println(d)
}
| Swift | 24,039,868 | 182 |
I was just curious as to how I would approach this. If I had a function, and I wanted something to happen when it was fully executed, how would I add this into the function? Thanks
| Say you have a download function to download a file from network, and want to be notified when download task has finished.
typealias CompletionHandler = (success:Bool) -> Void
func downloadFileFromURL(url: NSURL,completionHandler: CompletionHandler) {
// download code.
let flag = true // true if download succeed,false otherwise
completionHandler(success: flag)
}
// How to use it.
downloadFileFromURL(NSURL(string: "url_str")!, { (success) -> Void in
// When download completes,control flow goes here.
if success {
// download success
} else {
// download fail
}
})
| Swift | 30,401,439 | 181 |
I'm trying to convert some of my Obj-C class to Swift. And some other Obj-C classes still using enum in that converted class. I searched In the Pre-Release Docs and couldn't find it or maybe I missed it. Is there a way to use Swift enum in Obj-C Class? Or a link to the doc of this issue?
This is how I declared my enum in my old Obj-C code and new Swift code.
my old Obj-C Code:
typedef NS_ENUM(NSInteger, SomeEnum)
{
SomeEnumA,
SomeEnumB,
SomeEnumC
};
@interface SomeClass : NSObject
...
@end
my new Swift Code:
enum SomeEnum: NSInteger
{
case A
case B
case C
};
class SomeClass: NSObject
{
...
}
Update: From the answers. It can't be done in Swift older version than 1.2. But according to this official Swift Blog. In Swift 1.2 that released along with XCode 6.3, You can use Swift Enum in Objective-C by adding @objc in front of enum
| As of Swift version 1.2 (Xcode 6.3) you can. Simply prefix the enum declaration with @objc
@objc enum Bear: Int {
case Black, Grizzly, Polar
}
Shamelessly taken from the Swift Blog
Note: This would not work for String enums or enums with associated values. Your enum will need to be Int-bound
In Objective-C this would look like
Bear type = BearBlack;
switch (type) {
case BearBlack:
case BearGrizzly:
case BearPolar:
[self runLikeHell];
}
| Swift | 24,139,320 | 181 |
In The Swift Programming Language, it says:
Functions can also take a variable number of arguments, collecting them into an array.
func sumOf(numbers: Int...) -> Int {
...
}
When I call such a function with a comma-separated list of numbers (`sumOf(1, 2, 3, 4), they are made available as an array inside the function.
Question: what if I already have an array of numbers that I want to pass to this function?
let numbers = [1, 2, 3, 4]
sumOf(numbers)
This fails with a compiler error, “Could not find an overload for '__conversion' that accepts the supplied arguments”. Is there a way to turn an existing array into a list of elements that I can pass to a variadic function?
| Splatting is not in the language yet, as confirmed by the devs.
[SR-128] Pass array to variadic function
Workaround for now is to use an overload or wait if you cannot add overloads.
| Swift | 24,024,376 | 181 |
Can someone explain to me what is the exact difference between modal and push segue?
I know that when we use push the segue gets added to a stack, so when we keep using push it keeps occupying memory?
Can someone please show me how these two are implemented?
Modal segues can be created by simply ctrl-click and dragging to destination but when I do that with the push my app crashes.
I am pushing from a button to a UINavigationController that has a UIViewController.
| A push Segue is adding another VC to the navigation stack. This assumes that VC that originates the push is part of the same navigation controller that the VC that is being added to the stack belongs to. Memory management is not an issue with navigation controllers and a deep stack. As long as you are taking care of objects you might be passing from one VC to another, the runtime will take care of the navigation stack. See the image for a visual indication:
A modal Segue is just one VC presenting another VC modally. The VCs don't have to be part of a navigation controller and the VC being presented modally is generally considered to be a "child" of the presenting (parent) VC. The modally presented VC is usually sans any navigation bars or tab bars. The presenting VC is also responsible for dismissing the modal VC it created and presented.
| Swift | 9,392,744 | 181 |
Swift's Encodable/Decodable protocols, released with Swift 4, make JSON (de)serialization quite pleasant. However, I have not yet found a way to have fine-grained control over which properties should be encoded and which should get decoded.
I have noticed that excluding the property from the accompanying CodingKeys enum excludes the property from the process altogether, but is there a way to have more fine-grained control?
| The list of keys to encode/decode is controlled by a type called CodingKeys (note the s at the end). The compiler can synthesize this for you but can always override that.
Let's say you want to exclude the property nickname from both encoding and decoding:
struct Person: Codable {
var firstName: String
var lastName: String
var nickname: String?
private enum CodingKeys: String, CodingKey {
case firstName, lastName
}
}
If you want it to be asymmetric (i.e. encode but not decode or vice versa), you have to provide your own implementations of encode(with encoder: ) and init(from decoder: ):
struct Person: Codable {
var firstName: String
var lastName: String
// Since fullName is a computed property, it's excluded by default
var fullName: String {
return firstName + " " + lastName
}
private enum CodingKeys: String, CodingKey {
case firstName, lastName, fullName
}
// We don't want to decode `fullName` from the JSON
init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
firstName = try container.decode(String.self, forKey: .firstName)
lastName = try container.decode(String.self, forKey: .lastName)
}
// But we want to store `fullName` in the JSON anyhow
func encode(to encoder: Encoder) throws {
var container = encoder.container(keyedBy: CodingKeys.self)
try container.encode(firstName, forKey: .firstName)
try container.encode(lastName, forKey: .lastName)
try container.encode(fullName, forKey: .fullName)
}
}
| Swift | 44,655,562 | 180 |
Swift 4 added the new Codable protocol. When I use JSONDecoder it seems to require all the non-optional properties of my Codable class to have keys in the JSON or it throws an error.
Making every property of my class optional seems like an unnecessary hassle since what I really want is to use the value in the json or a default value. (I don't want the property to be nil.)
Is there a way to do this?
class MyCodable: Codable {
var name: String = "Default Appleseed"
}
func load(input: String) {
do {
if let data = input.data(using: .utf8) {
let result = try JSONDecoder().decode(MyCodable.self, from: data)
print("name: \(result.name)")
}
} catch {
print("error: \(error)")
// `Error message: "Key not found when expecting non-optional type
// String for coding key \"name\""`
}
}
let goodInput = "{\"name\": \"Jonny Appleseed\" }"
let badInput = "{}"
load(input: goodInput) // works, `name` is Jonny Applessed
load(input: badInput) // breaks, `name` required since property is non-optional
| You can implement the init(from decoder: Decoder) method in your type instead of using the default implementation:
class MyCodable: Codable {
var name: String = "Default Appleseed"
required init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
if let name = try container.decodeIfPresent(String.self, forKey: .name) {
self.name = name
}
}
}
You can also make name a constant property (if you want to):
class MyCodable: Codable {
let name: String
required init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
if let name = try container.decodeIfPresent(String.self, forKey: .name) {
self.name = name
} else {
self.name = "Default Appleseed"
}
}
}
or
required init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
self.name = try container.decodeIfPresent(String.self, forKey: .name) ?? "Default Appleseed"
}
Re your comment: With a custom extension
extension KeyedDecodingContainer {
func decodeWrapper<T>(key: K, defaultValue: T) throws -> T
where T : Decodable {
return try decodeIfPresent(T.self, forKey: key) ?? defaultValue
}
}
you could implement the init method as
required init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
self.name = try container.decodeWrapper(key: .name, defaultValue: "Default Appleseed")
}
but that is not much shorter than
self.name = try container.decodeIfPresent(String.self, forKey: .name) ?? "Default Appleseed"
| Swift | 44,575,293 | 180 |
There are two overloads for dequeueReusableCellWithIdentifier and I'm trying to determine when should I use one vs the other?
The apple docs regarding the forIndexPath function states, "This method uses the index path to perform additional configuration based on the cell’s position in the table view."
I'm not sure how to interpret that though?
| The most important difference is that the forIndexPath: version asserts (crashes) if you didn't register a class or nib for the identifier. The older (non-forIndexPath:) version returns nil in that case.
You register a class for an identifier by sending registerClass:forCellReuseIdentifier: to the table view. You register a nib for an identifier by sending registerNib:forCellReuseIdentifier: to the table view.
If you create your table view and your cell prototypes in a storyboard, the storyboard loader takes care of registering the cell prototypes that you defined in the storyboard.
Session 200 - What's New in Cocoa Touch from WWDC 2012 discusses the (then-new) forIndexPath: version starting around 8m30s. It says that “you will always get an initialized cell” (without mentioning that it will crash if you didn't register a class or nib).
The video also says that “it will be the right size for that index path”. Presumably this means that it will set the cell's size before returning it, by looking at the table view's own width and calling your delegate's tableView:heightForRowAtIndexPath: method (if defined). This is why it needs the index path.
| Swift | 25,826,383 | 179 |
I have a type in my module:
import Cocoa
class ColoredDotView : NSView {
...
}
It is used in a number of different classes with no issue:
class EditSubjectPopoverController : NSObject {
@IBOutlet internal var subjectColorDotView : ColoredDotView!
...
}
But for some reason, when I use it in one specific class, I have compilation errors on the type:
class EditTaskPopoverController : NSObject {
@IBOutlet internal var lowPriorityDotView : ColoredDotView! // Error here
@IBOutlet internal var medPriorityDotView : ColoredDotView! // And here...
@IBOutlet internal var highPriorityDotView : ColoredDotView! // And here...
...
}
The compilation error is:
EditTaskPopoverController.swift:15:49: Use of undeclared type
'ColoredDotView'
Which I don't understand. It's the first compilation error in the file, and the rest of the errors are all symptomatic of the first. Further, there are no other files with compilation errors. I don't understand why the type is undeclared, as the file is in the same module:
I have tried cleaning the project, cleaning the build folder, and restarting Xcode, to no avail. What potential missteps can cause an undeclared type compiler error in Swift?
| For me, I encountered this error when my test target did not have some swift files that my app build target had in compile sources. It was very confusing because the 'undeclared type' was being used in so many other places with no problem, and the error seemed vague. So solution there was of course to add the file containing the 'undeclared type' to the test target.
| Swift | 25,437,891 | 179 |
In Objective-C, one can add a description method to their class to aid in debugging:
@implementation MyClass
- (NSString *)description
{
return [NSString stringWithFormat:@"<%@: %p, foo = %@>", [self class], foo _foo];
}
@end
Then in the debugger, you can do:
po fooClass
<MyClass: 0x12938004, foo = "bar">
What is the equivalent in Swift? Swift's REPL output can be helpful:
1> class MyClass { let foo = 42 }
2>
3> let x = MyClass()
x: MyClass = {
foo = 42
}
But I'd like to override this behavior for printing to the console:
4> println("x = \(x)")
x = C11lldb_expr_07MyClass (has 1 child)
Is there a way to clean up this println output? I've seen the Printable protocol:
/// This protocol should be adopted by types that wish to customize their
/// textual representation. This textual representation is used when objects
/// are written to an `OutputStream`.
protocol Printable {
var description: String { get }
}
I figured this would automatically be "seen" by println but it does not appear to be the case:
1> class MyClass: Printable {
2. let foo = 42
3. var description: String { get { return "MyClass, foo = \(foo)" } }
4. }
5>
6> let x = MyClass()
x: MyClass = {
foo = 42
}
7> println("x = \(x)")
x = C11lldb_expr_07MyClass (has 1 child)
And instead I have to explicitly call description:
8> println("x = \(x.description)")
x = MyClass, foo = 42
Is there a better way?
| To implement this on a Swift type you must implement the CustomStringConvertible protocol and then also implement a string property called description.
For example:
class MyClass: CustomStringConvertible {
let foo = 42
var description: String {
return "<\(type(of: self)): foo = \(foo)>"
}
}
print(MyClass()) // prints: <MyClass: foo = 42>
Note: type(of: self) gets the type of the current instances instead of explicitly writing ‘MyClass’.
| Swift | 24,108,634 | 179 |
If so, are there any key differences that weren't otherwise present when using key-value observation in Objective-C?
| You can use KVO in Swift, but only for dynamic properties of NSObject subclass. Consider that you wanted to observe the bar property of a Foo class. In Swift 4, specify bar as dynamic property in your NSObject subclass:
class Foo: NSObject {
@objc dynamic var bar = 0
}
You can then register to observe changes to the bar property. In Swift 4 and Swift 3.2, this has been greatly simplified, as outlined in Using Key-Value Observing in Swift:
class MyObject {
private var token: NSKeyValueObservation
var objectToObserve = Foo()
init() {
token = objectToObserve.observe(\.bar) { [weak self] object, change in // the `[weak self]` is to avoid strong reference cycle; obviously, if you don't reference `self` in the closure, then `[weak self]` is not needed
print("bar property is now \(object.bar)")
}
}
}
Note, in Swift 4, we now have strong typing of keypaths using the backslash character (the \.bar is the keypath for the bar property of the object being observed). Also, because it's using the completion closure pattern, we don't have to manually remove observers (when the token falls out of scope, the observer is removed for us) nor do we have to worry about calling the super implementation if the key doesn't match. The closure is called only when this particular observer is invoked. For more information, see WWDC 2017 video, What's New in Foundation.
In Swift 3, to observe this, it's a bit more complicated, but very similar to what one does in Objective-C. Namely, you would implement observeValue(forKeyPath keyPath:, of object:, change:, context:) which (a) makes sure we're dealing with our context (and not something that our super instance had registered to observe); and then (b) either handle it or pass it on to the super implementation, as necessary. And make sure to remove yourself as an observer when appropriate. For example, you might remove the observer when it is deallocated:
In Swift 3:
class MyObject: NSObject {
private var observerContext = 0
var objectToObserve = Foo()
override init() {
super.init()
objectToObserve.addObserver(self, forKeyPath: #keyPath(Foo.bar), options: [.new, .old], context: &observerContext)
}
deinit {
objectToObserve.removeObserver(self, forKeyPath: #keyPath(Foo.bar), context: &observerContext)
}
override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
guard context == &observerContext else {
super.observeValue(forKeyPath: keyPath, of: object, change: change, context: context)
return
}
// do something upon notification of the observed object
print("\(keyPath): \(change?[.newKey])")
}
}
Note, you can only observe properties that can be represented in Objective-C. Thus, you cannot observe generics, Swift struct types, Swift enum types, etc.
For a discussion of the Swift 2 implementation, see my original answer, below.
Using the dynamic keyword to achieve KVO with NSObject subclasses is described in the Key-Value Observing section of the Adopting Cocoa Design Conventions chapter of the Using Swift with Cocoa and Objective-C guide:
Key-value observing is a mechanism that allows objects to be notified of changes to specified properties of other objects. You can use key-value observing with a Swift class, as long as the class inherits from the NSObject class. You can use these three steps to implement key-value observing in Swift.
Add the dynamic modifier to any property you want to observe. For more information on dynamic, see Requiring Dynamic Dispatch.
class MyObjectToObserve: NSObject {
dynamic var myDate = NSDate()
func updateDate() {
myDate = NSDate()
}
}
Create a global context variable.
private var myContext = 0
Add an observer for the key-path, and override the observeValueForKeyPath:ofObject:change:context: method, and remove the observer in deinit.
class MyObserver: NSObject {
var objectToObserve = MyObjectToObserve()
override init() {
super.init()
objectToObserve.addObserver(self, forKeyPath: "myDate", options: .New, context: &myContext)
}
override func observeValueForKeyPath(keyPath: String?, ofObject object: AnyObject?, change: [String : AnyObject]?, context: UnsafeMutablePointer<Void>) {
if context == &myContext {
if let newValue = change?[NSKeyValueChangeNewKey] {
print("Date changed: \(newValue)")
}
} else {
super.observeValueForKeyPath(keyPath, ofObject: object, change: change, context: context)
}
}
deinit {
objectToObserve.removeObserver(self, forKeyPath: "myDate", context: &myContext)
}
}
[Note, this KVO discussion has subsequently been removed from the Using Swift with Cocoa and Objective-C guide, which has been adapted for Swift 3, but it still works as outlined at the top of this answer.]
It's worth noting that Swift has its own native property observer system, but that's for a class specifying its own code that will be performed upon observation of its own properties. KVO, on the other hand, is designed to register to observe changes to some dynamic property of some other class.
| Swift | 24,092,285 | 179 |
What I want to implement:
class func getSomeObject() -> [SomeObject]? {
let objects = Realm().objects(SomeObject)
return objects.count > 0 ? objects : nil
}
How can I return object as [SomeObject] instead if Results?
| Weird, the answer is very straightforward. Here is how I do it:
let array = Array(results) // la fin
| Swift | 31,100,011 | 178 |
I'm trying to use Swift's @testable declaration to expose my classes to the test target. However I'm getting this compiler error:
Intervals is the module that contains the classes I'm trying to expose. How do I get rid of this error?
| In your main target you need to set the Enable Testability build option to Yes.
As per the comment by @earnshavian below, this should only be used on debug builds as per apple release notes: "The Enable Testability build setting should be used only in your Debug configuration, because it prohibits optimizations that depend on not exporting internal symbols from the app or framework" https://developer.apple.com/library/content/releasenotes/DeveloperTools/RN-Xcode/Chapters/Introduction.html#//apple_ref/doc/uid/TP40001051-CH1-SW326
| Swift | 30,787,674 | 178 |
How do I return 3 separate data values of the same type(Int) from a function in swift?
I'm attempting to return the time of day, I need to return the Hour, Minute and Second as separate integers, but all in one go from the same function, is this possible?
I think I just don't understand the syntax for returning multiple values. This is the code I'm using, I'm having trouble with the last(return) line.
Any help would be greatly appreciated!
func getTime() -> Int
{
let date = NSDate()
let calendar = NSCalendar.currentCalendar()
let components = calendar.components(.CalendarUnitHour | .CalendarUnitMinute | .CalendarUnitSecond, fromDate: date)
let hour = components.hour
let minute = components.minute
let second = components.second
let times:String = ("\(hour):\(minute):\(second)")
return hour, minute, second
}
| Return a tuple:
func getTime() -> (Int, Int, Int) {
...
return ( hour, minute, second)
}
Then it's invoked as:
let (hour, minute, second) = getTime()
or:
let time = getTime()
println("hour: \(time.0)")
| Swift | 27,531,195 | 178 |
Let's say I have Customer data type which contains a metadata property that can contains any JSON dictionary in the customer object
struct Customer {
let id: String
let email: String
let metadata: [String: Any]
}
{
"object": "customer",
"id": "4yq6txdpfadhbaqnwp3",
"email": "[email protected]",
"metadata": {
"link_id": "linked-id",
"buy_count": 4
}
}
The metadata property can be any arbitrary JSON map object.
Before I can cast the property from a deserialized JSON from NSJSONDeserialization but with the new Swift 4 Decodable protocol, I still can't think of a way to do that.
Do anyone know how to achieve this in Swift 4 with Decodable protocol?
| With some inspiration from this gist I found, I wrote some extensions for UnkeyedDecodingContainer and KeyedDecodingContainer. You can find a link to my gist here. By using this code you can now decode any Array<Any> or Dictionary<String, Any> with the familiar syntax:
let dictionary: [String: Any] = try container.decode([String: Any].self, forKey: key)
or
let array: [Any] = try container.decode([Any].self, forKey: key)
Edit: there is one caveat I have found which is decoding an array of dictionaries [[String: Any]] The required syntax is as follows. You'll likely want to throw an error instead of force casting:
let items: [[String: Any]] = try container.decode(Array<Any>.self, forKey: .items) as! [[String: Any]]
EDIT 2: If you simply want to convert an entire file to a dictionary, you are better off sticking with api from JSONSerialization as I have not figured out a way to extend JSONDecoder itself to directly decode a dictionary.
guard let json = try JSONSerialization.jsonObject(with: data, options: []) as? [String: Any] else {
// appropriate error handling
return
}
The extensions
// Inspired by https://gist.github.com/mbuchetics/c9bc6c22033014aa0c550d3b4324411a
struct JSONCodingKeys: CodingKey {
var stringValue: String
init?(stringValue: String) {
self.stringValue = stringValue
}
var intValue: Int?
init?(intValue: Int) {
self.init(stringValue: "\(intValue)")
self.intValue = intValue
}
}
extension KeyedDecodingContainer {
func decode(_ type: Dictionary<String, Any>.Type, forKey key: K) throws -> Dictionary<String, Any> {
let container = try self.nestedContainer(keyedBy: JSONCodingKeys.self, forKey: key)
return try container.decode(type)
}
func decodeIfPresent(_ type: Dictionary<String, Any>.Type, forKey key: K) throws -> Dictionary<String, Any>? {
guard contains(key) else {
return nil
}
guard try decodeNil(forKey: key) == false else {
return nil
}
return try decode(type, forKey: key)
}
func decode(_ type: Array<Any>.Type, forKey key: K) throws -> Array<Any> {
var container = try self.nestedUnkeyedContainer(forKey: key)
return try container.decode(type)
}
func decodeIfPresent(_ type: Array<Any>.Type, forKey key: K) throws -> Array<Any>? {
guard contains(key) else {
return nil
}
guard try decodeNil(forKey: key) == false else {
return nil
}
return try decode(type, forKey: key)
}
func decode(_ type: Dictionary<String, Any>.Type) throws -> Dictionary<String, Any> {
var dictionary = Dictionary<String, Any>()
for key in allKeys {
if let boolValue = try? decode(Bool.self, forKey: key) {
dictionary[key.stringValue] = boolValue
} else if let stringValue = try? decode(String.self, forKey: key) {
dictionary[key.stringValue] = stringValue
} else if let intValue = try? decode(Int.self, forKey: key) {
dictionary[key.stringValue] = intValue
} else if let doubleValue = try? decode(Double.self, forKey: key) {
dictionary[key.stringValue] = doubleValue
} else if let nestedDictionary = try? decode(Dictionary<String, Any>.self, forKey: key) {
dictionary[key.stringValue] = nestedDictionary
} else if let nestedArray = try? decode(Array<Any>.self, forKey: key) {
dictionary[key.stringValue] = nestedArray
}
}
return dictionary
}
}
extension UnkeyedDecodingContainer {
mutating func decode(_ type: Array<Any>.Type) throws -> Array<Any> {
var array: [Any] = []
while isAtEnd == false {
// See if the current value in the JSON array is `null` first and prevent infite recursion with nested arrays.
if try decodeNil() {
continue
} else if let value = try? decode(Bool.self) {
array.append(value)
} else if let value = try? decode(Double.self) {
array.append(value)
} else if let value = try? decode(String.self) {
array.append(value)
} else if let nestedDictionary = try? decode(Dictionary<String, Any>.self) {
array.append(nestedDictionary)
} else if let nestedArray = try? decode(Array<Any>.self) {
array.append(nestedArray)
}
}
return array
}
mutating func decode(_ type: Dictionary<String, Any>.Type) throws -> Dictionary<String, Any> {
let nestedContainer = try self.nestedContainer(keyedBy: JSONCodingKeys.self)
return try nestedContainer.decode(type)
}
}
| Swift | 44,603,248 | 177 |
I have been using DispatchQueue.main.async for a long time to perform UI related operations.
Swift provides both DispatchQueue.main.async and DispatchQueue.main.sync, and both are performed on the main queue.
Can anyone tell me the difference between them?
When should I use each?
DispatchQueue.main.async {
self.imageView.image = imageView
self.lbltitle.text = ""
}
DispatchQueue.main.sync {
self.imageView.image = imageView
self.lbltitle.text = ""
}
| Why Concurrency?
As soon as you add heavy tasks to your app like data loading it slows your UI work down or even freezes it.
Concurrency lets you perform 2 or more tasks “simultaneously”.
The disadvantage of this approach is that thread safety which is not always as easy to control. F.e. when different tasks want to access the same resources like trying to change the same variable on a different threads or accessing the resources already blocked by the different threads.
There are a few abstractions we need to be aware of.
Queues.
Synchronous/Asynchronous task performance.
Priorities.
Common troubles.
Queues
Must be serial or concurrent. As well as global or private at the same time.
With serial queues, tasks will be finished one by one while with concurrent queues, tasks will be performed simultaneously and will be finished on unexpected schedules. The same group of tasks will take the way more time on a serial queue compared to a concurrent queue.
You can create your own private queues (both serial or concurrent) or use already available global (system) queues.
The main queue is the only serial queue out of all of the global queues.
It is highly recommended to not perform heavy tasks which are not referred to UI work on the main queue (f.e. loading data from the network), but instead to do them on the other queues to keep the UI unfrozen and responsive to the user actions. If we let the UI be changed on the other queues, the changes can be made on a different and unexpected schedule and speed. Some UI elements can be drawn before or after they are needed. It can crash the UI. We also need to keep in mind that since the global queues are system queues there are some other tasks can run by the system on them.
Quality of Service / Priority
Queues also have different qos (Quality of Service) which sets the task performing priority (from highest to lowest here):
.userInteractive - as for the main queue .userInitiated - for the user initiated tasks on which user waits for some response .utility - for the tasks which takes some time and doesn't require immediate response, e.g working with data .background - for the tasks which aren't related with the visual part and which aren't strict for the completion time). There is also .default queue which does't transfer the qos information.
If it wasn't possible to detect the qos the qos will be used between .userInitiated and .utility.
Tasks can be performed synchronously or asynchronously.
Synchronous function returns control to the current queue only after the task is finished. It blocks the queue and waits until the task is finished.
Asynchronous function returns control to the current queue right after task has been sent to be performed on the different queue. It doesn't wait until the task is finished. It doesn't block the queue.
Common Troubles.
The most popular mistakes programmers make while projecting the concurrent apps are the following:
Race condition - caused when the app work depends on the order of the code parts execution.
Priority inversion - when the higher priority tasks wait for the smaller priority tasks to be finished due to some resources being blocked
Deadlock - when a few queues have infinite wait for the sources (variables, data etc.) already blocked by some of these queues.
NEVER call the sync function on the main queue.
If you call the sync function on the main queue it will block the queue as well as the queue will be waiting for the task to be completed but the task will never be finished since it will not be even able to start due to the queue is already blocked. It is called deadlock.
When to use sync?
When we need to wait until the task is finished. F.e. when we are making sure that some function/method is not double called. F.e. we have synchronization and trying to prevent it to be double called until it's completely finished. Here's some code for this concern: How to find out what caused error crash report on IOS device?
| Swift | 44,324,595 | 177 |
I have this:
class Movies {
Name:String
Date:Int
}
and an array of [Movies]. How do I sort the array alphabetically by name? I've tried:
movieArr = movieArr.sorted{ $0 < $1 }
and
movieArr = sorted(movieArr)
but that doesn't work because I'm not accessing the name attribute of Movies.
| In the closure you pass to sort, compare the properties you want to sort by. Like this:
movieArr.sorted { $0.name < $1.name }
or the following in the cases that you want to bypass cases:
movieArr.sorted { $0.name.lowercased() < $1.name.lowercased() }
Sidenote: Typically only types start with an uppercase letter; I'd recommend using name and date, not Name and Date.
Example, in a playground:
class Movie {
let name: String
var date: Int?
init(_ name: String) {
self.name = name
}
}
var movieA = Movie("A")
var movieB = Movie("B")
var movieC = Movie("C")
let movies = [movieB, movieC, movieA]
let sortedMovies = movies.sorted { $0.name < $1.name }
sortedMovies
sortedMovies will be in the order [movieA, movieB, movieC]
Swift5 Update
channelsArray = channelsArray.sorted { (channel1, channel2) -> Bool in
let channelName1 = channel1.name
let channelName2 = channel2.name
return (channelName1.localizedCaseInsensitiveCompare(channelName2) == .orderedAscending)
}
| Swift | 26,719,744 | 177 |
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