NotifyWidget on preparations. When is GetMessage called?

NotifyWidget on preparations. When is GetMessage called?

  
Hi there.
When we add a NotifyWidget action to a web block, the caller web screen will have to set the corresponding callback to capture the message. The NotifyWidget reference states that "the notification is sent using Ajax but only when Screen Action flow ends". This implies that the callback which will have the "GetMessage" action will be run asynchronously to the action that called "NotifyWidget".

Is this true if we call NotifyWidget in the preparation of the web block? Since the page hasn't even finished rendering, is it possible that the callback is called during the preparation, and not after the preparation ended?