title: UIViewController's viewDidAppear/viewDidDisappear: when exactly are that methods supposed to be called?
body: <p>I definitely need some clarification on when exactly viewDidAppear/viewDidDisappear methods are supposed to be called...</p>
  
  <ol>
  <li>If the application enters background while showing some view, in this case I would expect viewDidDisappear to be called on the UIViewController linked to that view. On the other hand, if the application enters foreground after being background, I would expect viewDidAppear to be called. But it doesn't work this way.</li>
  <li>If an UINavigationController displays an UIViewController we call 'A', and that UIViewController is linked to a view that has a subview linked to another UIViewController we call 'B', the viewDidAppear method is NOT called on the controller 'B'. Do I have to propagate viewDidAppear myself? I'm confused...</li>
  </ol>
  
  <p>Thank you in advance!</p>