title: How to mask UIViews in iOS
body: <p>I've seen similar questions, but haven't found workable answers.</p>
  
  <p>I want to mask a UIView using a grey image (need to convert to alpha scale for masking).  The UIView has background. It should be easy to mask an image, but I want to mask any UIView.</p>
  
  <p>Any clues will be appreciated.</p>