title: How can I stop UIScrollView calling drawRect: on sublayers that go in/out of the visible region?
body: <p>I've got an animation that is a bit laggy, and I've narrowed it down to this:</p>
  
  <p>As part of the animation, a <code>UIScrollView</code> is auto scrolled. Even though the subviews appear to be loaded, if they are outside the scroll views bounds, then <code>drawRect:</code> will be called on these views as they are animated back into the visible region (I'm implementing my own <code>drawRect:</code> on each of these views). This is what's making my animation stutter.</p>
  
  <p>I've got shouldRasterize set to true for each of the scroll view's subviews, no difference. How can I force <code>UIScrollView</code> to keep each of the views in memory even if goes out of the scroll view's bounds?</p>