title: Is there a notification for when a UIView's position in the window changes?
body: <p>I have a custom <code>UIView</code> which is an accessibility container. Each of the <code>UIAccessibilityElement</code>s it houses has a frame, relative to the window. Because the frame is relative to the window, it needs to be updated whenever the position of the view in the window changes.</p>
  
  <p>Is there a way to be notified when the position of the view in the window changes? This would allow me to keep all the code in this class, nice and reusable, instead of sprawling all over the view hierarchy.</p>