title: How to achieve dynamic UIView masking?
body: <p>I'm trying to achieve a sort of dynamic UIView masking effect. Here is a sketch:
  <img src="https://i.stack.imgur.com/nK5s3.png" alt="enter image description here"></p>
  
  <p>So as you can see, I'm trying to create a UIView that can effectively cut through an image to reveal the image behind it. I already know how to return an image with a mask statically, however I would like the "revealer" to be draggable (I'll use pan gesture) and live.</p>
  
  <p>Does anyone have any ideas or starting points on how to achieve this? Thanks</p>
  
  <p>(NOTE: My demo says White layer, but I'd actually like to show another image or photo). </p>