src/camera_input_live/frontend/streamlit-component-lib.js

// Borrowed minimalistic Streamlit API from Thiago
// https://discuss.streamlit.io/t/code-snippet-create-components-without-any-frontend-tooling-no-react-babel-webpack-etc/13064
function sendMessageToStreamlitClient(type, data) {
  console.log(type, data)
  const outData = Object.assign({
      isStreamlitMessage: true,
      type: type,
  }, data);
  window.parent.postMessage(outData, "*");
}

const Streamlit = {
  setComponentReady: function() {
      sendMessageToStreamlitClient("streamlit:componentReady", {apiVersion: 1});
  },
  setFrameHeight: function(height) {
      sendMessageToStreamlitClient("streamlit:setFrameHeight", {height: height});
  },
  setComponentValue: function(value) {
      sendMessageToStreamlitClient("streamlit:setComponentValue", {value: value});
  },
  RENDER_EVENT: "streamlit:render",
  events: {
      addEventListener: function(type, callback) {
          window.addEventListener("message", function(event) {
              if (event.data.type === type) {
                  event.detail = event.data
                  callback(event);
              }
          });
      }
  }
}