title: CATiledLayer blanking tiles before drawing contents
body: <p>All,</p>
  
  <p>I'm having trouble getting behavior that I want from CATiledLayer.  Is there a way that I can trigger the tiles to redraw without having the side-effect that their areas are cleared to white first?  I've already subclassed CATiledLayer to set fadeDuration to return 0.</p>
  
  <p>To be more specific, here are the details of what I'm seeing and what I'm trying to achieve:</p>
  
  <ul>
  <li>I have a UIScrollView with a big content size...~12000x800.  Its content view is a UIView backed by a CATiledLayer.</li>
  <li>The UIView is rendered with a lot of custom-drawn lines</li>
  <li>Everything works fine, but the contents of the UIView sometimes change.  When that happens, I'd like to redraw the tiles as seamlessly as possible.  When I use setNeedsDisplay on the view, the tiles redraw but they are first cleared to white and there's a fraction-of-a-second delay before the new content is drawn.  I've already subclassed CATiledLayer so that fadeDuration is set to 0.</li>
  <li>The behavior that I want seems like it should be possible...when you zoom in on the scrollview and the content gets redrawn at a higher resolution, there's no blanking before the redraw; the new content is drawn right on top of the old one.  That's what I'm looking for.</li>
  </ul>
  
  <p>Thanks; I appreciate your ideas.</p>
  
  <p><strong>Update</strong>:</p>
  
  <p>Just to follow up - I realized that the tiles weren't being cleared to white before the redraw, they're being taken out entirely; the white that I was seeing is the color of the view that's beneath my CATiledLayer-backed view.</p>
  
  <p>As a quick hack/fix, I put a UIImageView beneath the UIScrollView, and before triggering a redraw of the CATiledLayer-backed view I render its visible section into the UIImageView and let it show.  This smooths out the redraw significantly.</p>
  
  <p>If anyone has a better solution, like keeping the redraw-targeted tiles from going away before being redrawn in the first place, I'd still love to hear it.</p>