title: What method is called when UINavigationController is presented?
body: <p>I am trying to subclass the <code>UINavigationController</code> to add a custom subview underneath its <code>UINavigationBar</code>. The problem is when I set my custom <code>UINavigationController</code> as the <code>rootViewController</code> of my <code>window</code>, the <code>UINavigationBar</code> position changes, so I want to update the position of my custom view accordingly. My question is where should I put my code to update my custom view. </p>
  
  <pre><code>    var navController = TabbedNavigationController(rootViewController: firstView)
  
      // navController.navigationBar.frame.origin.y = 0
      self.window?.rootViewController = navController
      self.window?.makeKeyAndVisible()
      // navController.navigationBar.frame.origin.y = 20.0
  </code></pre>