title: Transparent (alpha) gradient with Core Graphic iPhone
body: <p>anyone knows how to programmatically create a transparent gradient (alpha gradient), over an image with Core Graphic?
  I need it both for display an image and save it. </p>
  
  <p>An example of what I want to obtain: 
  <img src="https://i.stack.imgur.com/bGZTu.png" alt="enter image description here"></p>