title: How to tell when a UIView gains focus
body: <p>On the iPhone, we can simply use <code>(void) viewDidAppear:(BOOL)animated;</code> to perform actions when a view becomes the focus. In some events, we have a modal view with another modal view on top of it and, on the iPhone, closing the topmost modal view will fire the viewDidAppear for the lower modal view.</p>
  
  <p>This is not the case for the iPad, as the view stays "visible" even though it's behind another modal view. Is there any way to tell from within a UIViewController when the view itself becomes the active view?</p>