Previously I mentioned the excellent Pop animation engine for creating more dynamic interfaces.
Here’s an in-depth video tutorial submitted by Whitney Rhodes showing how to create a touch based dynamically sized progress animation using the Facebook Pop library.
The tutorial uses the previously mentioned UAProgressView circular progress indicator component, and iOS 8’s majorRadius to gather the size of the touch event.
This image from the tutorial shows off the animation running in the musx app where it is shown when the user is previewing a song:
You can find the tutorial over on the SavvyApps blog.
A nice guide to building a great component.