title: viewDidAppear and viewWillDisappear not being called in UIViewController
body: <p>I am creating a UIViewController using the nib file ? </p>
  
  <p>I have implemented the following 2 methods </p>
  
  <ul>
  <li>(void)viewDidAppear:(BOOL)animated</li>
  <li>(void)viewWillAppear:(BOOL)animated</li>
  </ul>
  
  <p>in my view controller, but these methods are not getting called every time ?</p>
  
  <p>What is the error in the code ?</p>