title: navigationController.navigationItem vs navigationItem
body: <p>Just curious, why setting <code>self.navigationItem = ...</code> works, but <code>self.navigationController.navigationItem</code> fails? The same applies for <code>self.toolbarItems</code> vs <code>self.navigationController.toobarItems</code>.</p>
  
  <p>When to use <code>self.navigationController.navigationItem</code>?</p>
  
  <p>Maybe you will say, they point to different things. but why <code>self.navigationController.navigationBarHidden = YES</code> the navigation bar is hidden. doesn't it means <code>self.navigationController.navigationItem</code> point to the bar i wanted?</p>