title: UINavigationItem.prompt appearing, moves the view below. How to listen for a new prompt?
body: <p>I have a normal UINavigationController with a UIViewController contained in it, when my view controller has a prompt applied to it (via a UINavigationItem.prompt = @"some prompt"), the prompt is animating the UIViewController's view downward. </p>
  
  <p>My question is, is there an apple what to find out that a prompt will appear (and maybe the animation time it will take) so I can relayout the view? For example: so the content appears not to move. Or do I have to write this functionality myself. What i am really looking for is a notification similar to the KeyboardWillShow/Hide notifications. </p>
  
  <p>Note that the controller is not the object applying the prompt, and the prompt will appear and disappear asynchronously. Not looking for code, just advice.</p>