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<topic_start> |
flutter for android developers |
this document is meant for android developers looking to apply their |
existing android knowledge to build mobile apps with flutter. |
if you understand the fundamentals of the android framework then you |
can use this document as a jump start to flutter development. |
info note |
to integrate flutter code into your android app, see |
add flutter to existing app. |
your android knowledge and skill set are highly valuable when building with |
flutter, because flutter relies on the mobile operating system for numerous |
capabilities and configurations. flutter is a new way to build UIs for mobile, |
but it has a plugin system to communicate with android (and iOS) for non-UI |
tasks. if you’re an expert with android, you don’t have to relearn everything |
to use flutter. |
this document can be used as a cookbook by jumping around and |
finding questions that are most relevant to your needs. |
<topic_end> |
<topic_start> |
views |
<topic_end> |
<topic_start> |
what is the equivalent of a view in flutter? |
how is react-style, or declarative, programming different than the |
traditional imperative style? |
for a comparison, see introduction to declarative UI. |
in android, the view is the foundation of everything that shows up on the |
screen. buttons, toolbars, and inputs, everything is a view. |
in flutter, the rough equivalent to a view is a widget. |
widgets don’t map exactly to android views, but while you’re getting |
acquainted with how flutter works you can think of them as |
“the way you declare and construct UI”. |
however, these have a few differences to a view. to start, widgets have a |
different lifespan: they are immutable and only exist until they need to be |
changed. whenever widgets or their state change, flutter’s framework creates |
a new tree of widget instances. in comparison, an android view is drawn once |
and does not redraw until invalidate is called. |
flutter’s widgets are lightweight, in part due to their immutability. |
because they aren’t views themselves, and aren’t directly drawing anything, |
but rather are a description of the UI and its semantics that get “inflated” |
into actual view objects under the hood. |
flutter includes the material components library. |
these are widgets that implement the |
material design guidelines. material design is a |
flexible design system optimized for all platforms, |
including iOS. |
but flutter is flexible and expressive enough to implement any design language. |
for example, on iOS, you can use the cupertino widgets |
to produce an interface that looks like apple’s iOS design language. |
<topic_end> |
<topic_start> |
how do i update widgets? |
in android, you update your views by directly mutating them. however, |
in flutter, widgets are immutable and are not updated directly, |
instead you have to work with the widget’s state. |
this is where the concept of stateful and stateless widgets comes from. |
a StatelessWidget is just what it sounds like—a |
widget with no state information. |
StatelessWidgets are useful when the part of the user interface |
you are describing does not depend on anything other than the configuration |
information in the object. |
for example, in android, this is similar to placing an ImageView |
with your logo. the logo is not going to change during runtime, |
so use a StatelessWidget in flutter. |
if you want to dynamically change the UI based on data received |
after making an HTTP call or user interaction then you have to work |
with StatefulWidget and tell the flutter framework that the widget’s |
state has been updated so it can update that widget. |
the important thing to note here is at the core both stateless and stateful |
widgets behave the same. they rebuild every frame, the difference is the |
StatefulWidget has a state object that stores state data across frames |
and restores it. |
if you are in doubt, then always remember this rule: if a widget changes |
(because of user interactions, for example) it’s stateful. |
however, if a widget reacts to change, the containing parent widget can |
still be stateless if it doesn’t itself react to change. |
the following example shows how to use a StatelessWidget. a common |
StatelessWidget is the text widget. if you look at the implementation of |
the text widget you’ll find that it subclasses StatelessWidget. |
<code_start> |
text( |
'i like flutter!', |
style: TextStyle(fontWeight: FontWeight.bold), |
); |
<code_end> |
as you can see, the text widget has no state information associated with it, |
it renders what is passed in its constructors and nothing more. |
but, what if you want to make “i like flutter” change dynamically, for |
example when clicking a FloatingActionButton? |
to achieve this, wrap the text widget in a StatefulWidget and |
update it when the user clicks the button. |
for example: |
<code_start> |
import 'package:flutter/material.dart'; |
void main() { |
runApp(const SampleApp()); |
} |
class SampleApp extends StatelessWidget { |
const SampleApp({super.key}); |
// this widget is the root of your application. |
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