title: Create inner shadow in UIView
body: <p>I would like create an inner shadow on my <code>UIView</code> on iPad like that :</p>
  
  <p><img src="https://i.stack.imgur.com/XIbGq.png" alt="UIView with shadow"></p>
  
  <p>This <code>UIView</code> could change size so I can't use a simple image to create this kind of shadow.</p>
  
  <p>I have tested with <code>setShadow</code> etc., but it's only a dropshadow that is created.</p>
  
  <p>Any idea how to create this kind of shadow?</p>