title: UIView animation based on UIPanGestureRecognizer velocity
body: <p>I would like to be able to move a subview on and off the screen much like you browse between images in the iPhone's build in Photos app, so if the subview is more than 1/2 off screen when I let go with my finger it must animate off screen, but it must also support swipe so if the swipe/pan velocity is high enough it must animate off screen even though is might be less than 1/2 off screen.</p>
  
  <p>My idea was to use UIPanGestureRecognizer and then test on the velocity. This works, but how do I set a correct animation duration for the moving of the UIView based on the view's current location and velocity of the pan so that it seems smooth? If I set a fixed value, the animation either starts to slow or to fast compared to my fingers swipe speed.</p>