title: How do I tell why my UIViewController is disappearing?
body: <p>My <code>UIViewController</code> is in a navigation stack. How can I detect when the user is trying to pop up to a previous level vs. pushing a new view controller over it?</p>
  
  <p>I want to post a notification when the view is removed from the stack, as if the user had tapped a <strong>Save</strong> button that I don't have.</p>