title: iOS Flip UIView without shadow
body: <p>I use <code>transitionFromView</code> and <code>.TransitionFlipFromLeft</code> to replace a view to another view.
  Then I want to flip view without shadow above the views. </p>
  
  <pre><code>UIView.transitionFromView(
   self.postListView,
   toView:     self.userListView,
   duration:   0.8,
   options:    .TransitionFlipFromLeft,
   completion: nil
  )
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/3EE7a.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/3EE7a.jpg" alt="enter image description here"></a></p>