title: Non-linear property interpolation for interactive transitions
body: <p>In the implementation of an interactive transition, I'd like to mimic the animated transition that uses the non-linear easing functions. After searching around CA, it looks like interpolation machinery is all hidden behind private APIs.</p>
  
  <ul>
  <li>Is there a way to leverage Core Animation to do the grunt work for me? </li>
  <li>Do I need to roll my own value interpolation? </li>
  <li>Is there some existing open source lib that does this?</li>
  </ul>