(BTW- This is great work, I just hope UI devs don't overdo it...)
https://developer.apple.com/library/ios/documentation/UIKit/...:
As well as a full-on physics with collisions, etc:
http://www.raywenderlich.com/50197/uikit-dynamics-tutorial
https://developer.apple.com/library/ios/documentation/UIKit/...:
As well as a full-on physics with collisions, etc:
http://www.raywenderlich.com/50197/uikit-dynamics-tutorial