title: swapping images using pan gesture
body: <p>I have multiple <code>UIImageView</code> added into the main view, each <code>UIImageView</code> is displayed in its own parent <code>UIView</code> for cropping purpose, each <code>UIImageView</code> is also attached to a <code>PanGestureRecognizer</code>. </p>
  
  <p>I am curious on how to implement both pan gesturerecgonizer and also allow image swap by dragging one image (A) to hover over the other image (B) and hold, then the image A and B will be swapped (so A is now displayed in old B's UIView). </p>
  
  <p>This is something like the iPhoto app just implemented. </p>
  
  <p>Any ideas?</p>