title: UICollectionView: how to detect when scrolling has stopped
body: <p>I'm using a <code>UICollectionView</code> to scroll through a set of thumbnails quickly. Once scrolling ends, I'd like to display a larger hi-res version of the current thumbnail. </p>
  
  <p>How can I detect when the user has completed scrolling? I do implement <code>didEndDisplayingCell</code>, but that only tells me when a particular cell has scrolled off; it doesn't tell me when the scroll motion actually completes.</p>