code: function initialize() {
      if (isInitialized) {
        return;
      }
      isInitialized = true;
      if (window.parent === window) return;
  
      // We kick off communication with the parent window by sending a "hello" message. Then we wait
      // for a handshake (another "hello" message) from the parent window.
      startPostingHello();
      window.addEventListener('message', messageListener, false);
    }
docs: Initialize communication with the parent frame. This should not be called until the app's custom
  listeners are registered (via our 'addListener' public method) because, once we open the
  communication, the parent window may send any messages it may have queued. Messages for which
  we don't have handlers will be silently ignored.