title: Flutter: how to traverse a Widget children?
body: <p>Could anybody explain me how to traverse the children of a Widget Tree so that I can get the RenderObject of a specific child, based on its type?</p>
  
  <p>The practical use case I have at the moment is that I need to get the dimensions of a Scaffold AppBar (since I have a generic Widget that needs to be positioned via a PopupMenu, just underneath the AppBar which might or not have a bottom).</p>
  
  <p>As the AppBar does not expose its AppBarState, giving it a key is useless, I need to traverse the Scaffold children to find it back.</p>
  
  <p>Many thanks</p>