title: UIActivityViewControllerCompletionHandler How to?
body: <p>I am trying to use the <code>UIActivityViewControllerCompletionHandler</code>  , but i dont quite get how.. </p>
  
  <p>I want to detect when the user finishes or dismisses this <strong>view controller</strong> (<code>UIActivityViewController</code>).</p>
  
  <p>Anyone knows how?</p>