title: Fading out items in UICollectionView
body: <p>I have a UICollectionView and I'm implementing sticky headers as per this link: <a href="http://blog.radi.ws/post/32905838158/sticky-headers-for-uicollectionview-using#notes">http://blog.radi.ws/post/32905838158/sticky-headers-for-uicollectionview-using#notes</a></p>
  
  <p>It works fantastically however my window has a background image applied, and my header views have a transparent background. Consequentially, when my items scroll above the header view, you can still see them.</p>
  
  <p>Ideally I would fade out the cells with a gradient, to the point it is invisible by the time it appears behind the header view. </p>
  
  <p>Thanks.</p>