title: How to enable a child component to make changes to a parent's appbar?
body: <p>Imagine the following scenario - the app has a global Scaffold with a global AppBar. The body of the Scaffold is one of many app components. Each component would have the global appbar render differently and also have access to the appbar's TabController (so that listeners can be attached to it).</p>
  
  <p>The easiest way to implement this would be for each of the pages to have their own Scaffold with an AppBar and hide the AppBar from the main app Scaffold. This, however, would break transitions/animations, because instead of just the page fading in, the appbar would also fade in (since its part of the page), which would look very weird.</p>
  
  <p>I've been trying to figure this out for a few days now and would appreciate some help. I've tried using Redux and changing the appearance of the global appbar depending of a value in the Redux state, but I still have the issue with the TabController of the main Scaffold being inaccessible from any of the child pages.</p>