title: Adding and removing observers to NSNotificationCenter in a UIViewController
body: <p>Looking at various Apple examples (for example <a href="http://developer.apple.com/library/ios/#samplecode/AddMusic/Introduction/Intro.html" rel="noreferrer">Add Music</a>) in which I see they add observers to the default <code>NSNotificationCenter</code>  in <code>viewDidLoad</code>, then remove them in <code>dealloc</code>. This seems dangerous as <code>viewDidLoad</code> can be called multiple times without <code>dealloc</code> being called. This would then add the same observer multiple times, causing the handler to be called multiple times. </p>
  
  <p>A solution to this would be to also remove observers in <code>viewDidUnload</code>, but this would then mean the same observer could be removed for a second time in <code>dealloc</code> which seems like a potential problem.</p>
  
  <p>What am I missing?</p>