title: How could a UIVIew know its child has committed suicide
body: <p>I have a quick question: </p>
  
  <p>View Controller X has added a sub-view: View Controller Y. </p>
  
  <p>Y has a "quit" button. When the button is tapped, it removes its view from X's view hierarchy. </p>
  
  <p>But I found Y's 'viewDidUnload' method is not called and its memory can't be released. How could Y release itself properly, or how could X know that Y is gone?</p>
  
  <p>I believe I can make a protocol and let Y call a method from X, but I hope there is a easier way. </p>