title: CATiledLayer requests the same tile multiple times
body: <p>Please see this simple CATiledLayer example <a href="https://github.com/seanhess/CATiledLayer-Example" rel="nofollow">https://github.com/seanhess/CATiledLayer-Example</a></p>
  
  <p>It consists of one viewController with a hierarchy like this: </p>
  
  <pre><code>view: (frame = window size)
      scrollView: (frame = window size, content size = 200 x 4000)
          contentView: (frame = content size = 200 x 4000, tile size = 100 x 100)
  </code></pre>
  
  <p>The content view's layer has been overriden to be a CATiledLayer. </p>
  
  <p><strong>If you run the linked code, you'll see that tiles with the same rect are requested multiple times</strong>. It happens both when you first run the code, and when you scroll. </p>
  
  <p>Switch to branch "one-column" - it only happens on init, never when you scroll down. </p>
  
  <p>Switch to branch "default-tile-size" - it only happens on init, but very rarely (you have to run it multiple times before it happens)</p>
  
  <p>I'm trying to write some code in <code>drawLayer:inContext:</code> that locates the correct data and draws it. It could be expensive, and I don't want to do it more than once. </p>
  
  <p>Any idea what is going on? What could I do differently? </p>