title: Call a method every time a UIViewController is shown
body: <p>I want to show a full page image Ad every time a <code>UIViewController</code> is shown.
  I think I have to call the method inside a <code>viewDidAppear</code> or <code>ViewWillAppear</code>, but they are being called once.</p>
  
  <pre><code>- (void) viewDidAppear:(BOOL)animated{
      [self showAds];
  }
  
  - (void) showAds{
      //Do Something
  }
  </code></pre>
  
  <p>What should I do to call a method every time a uiviewcontroller is shown( even if its already created)? </p>