title: UIViewController view being unloaded even when visible
body: <p>I have a view controller A being shown modally over another view controller B within a navigation controller container.  When simulating memory warnings when this view controller A is shown modally, I receive the didReceiveMemoryWarnings message as expected, and then I receive a message viewDidUnload, thereby making my view controller A's view disappear.</p>
  <p>This is surprising to me, and I am not sure how to resolve this. Why is viewDidUnload being called, releasing the visible view in the process?</p>
  <p>Here is a stack trace in case it helps pinpoint the problem:</p>
  <blockquote>
  <p>[UIViewController unloadViewForced:] ()</p>
  <p>0x01458535 in -[UIViewController unloadViewIfReloadable] ()</p>
  <p>0x01463eb8 in -[UINavigationController purgeMemoryForReason:] ()</p>
  <p>0x01457b6d in -[UIViewController didReceiveMemoryWarning] ()</p>
  </blockquote>
  <p>Thanks for any help!</p>