title: Observing changes to a UIView's window and superview properties
body: <p>I'm looking for a way to be notified when a generic UIView is added or removed from the visible view hierarchy. KVO looked like the perfect thing to use in this case, but observing changes to a view's window or superview properties doesn't do anything. Changes to properties like frame, or backgroundColor work as expected but changed to properties relating to the view hierarchy doesn't seem to ever call observeValueForKeyPath. </p>
  
  <p>I checked to see if UIView supports KVO on those properties by calling automaticallyNotifiesObserversForKey, and UIView reported YES for both, leaving me at a loss. So my questions are:</p>
  
  <p>1) Is there a way to use KVO to be notified of events relating to a view being added/removed to the view hierarchy?</p>
  
  <p>2) If not is there another way to be notified of such events that doesn't involve sub-classing UIView?</p>