title: Observing change in frame of a UIView during animation
body: <p>I want to observe changes to the x coordinate of my UIView's origin while it is being animated using <code>animateWithDuration:delay:options:animations:completion:</code>. I want to track changes in the x coordinate during this animation at a granular level because I want to make a change in interaction to another view that the view being animated may make contact with. I want to make that change at the exact point of contact. I want to understand the best way to do something like this at a higher level:</p>
  
  <p>-- Should I use <code>animateWithDuration:...</code> in the completion call back at the point of contact? In other words, The first animation runs until it hits that x coordinate, and the rest of the animation takes place in the completion callback?</p>
  
  <p>-- Should I use <code>NSNotification</code> observers and observe changes to the frame property? How accurate / granular is this? Can I track every change to x? Should I do this in a separate thread?</p>
  
  <p>Any other suggestions would be welcome. I'm looking for a abest practice.</p>