title: NSNotification, addObserver and removeObserver - removeObserver both in ViewDidUnload and dealloc?
body: <p>I want to add a view controller as observer of a notification. </p>
  
  <p>When the selector method is called, I'll alert the user. I would that this alert appears only when this view controller is the top view controller.</p>
  
  <p>I thought to add it as observer in the <code>viewDidLoad</code> method. But where is the better remove the view controller as observer?</p>
  
  <p>Could it be a problem if I remove it both in <code>viewDidUnload</code> and <code>dealloc</code>?</p>