title: Why is viewDidUnload called less often than viewDidLoad?
body: <p>I put <code>NSLog(@"%@::%@", [[self class] description], NSStringFromSelector(_cmd));</code> in both <code>viewDidLoad</code> and <code>viewDidUnload</code> of a view controller.</p>
  
  <p>In the log, I found <code>viewDidLoad</code> is called a lot more than <code>viewDidUnload</code> when the app moves to and from different .nibs.</p>
  
  <p>Why?</p>