SpriteKit - Contains a 2D physics engine similar to Box 2D but not as full featured (joints, rigid bodies, shapes, polygons). The whole engine is similar to Cocos2D but ships with iOS 7. A particle editor and texture atlas generator is now built-in to Xcode because of it.
UI Dynamics - Contains a set of constraints and joints which can be used to flip and throw views around the screen. Lots of API hooks for responding to / taking user interaction into account. This is used for effects like when you scroll the messages list (the message bubbles bounce and collide) or when you drop notification center down (it bounces).
Edit: from a brief look at the source code, this example uses the UIDynamicAnimator API, so it is simulated entirely with the UI dynamics stuff.
Yes, that's what I meant when I said SpriteKit contains an engine similar to Box 2D. I thought my comment was reasonably clear about SpriteKit not just being the physics engine.
According to my tests it is not Box 2D, it produces significantly different results and only implements a subset of functionality. I've been told by others with more experience that it isn't Box 2D also, but if you have more information I'd like to know for sure.
Thanks for that, that's really interesting. It behaved differently in my experiments so I assumed it wasn't the same — but it must just be a different configuration.
SpriteKit - Contains a 2D physics engine similar to Box 2D but not as full featured (joints, rigid bodies, shapes, polygons). The whole engine is similar to Cocos2D but ships with iOS 7. A particle editor and texture atlas generator is now built-in to Xcode because of it.
UI Dynamics - Contains a set of constraints and joints which can be used to flip and throw views around the screen. Lots of API hooks for responding to / taking user interaction into account. This is used for effects like when you scroll the messages list (the message bubbles bounce and collide) or when you drop notification center down (it bounces).
Edit: from a brief look at the source code, this example uses the UIDynamicAnimator API, so it is simulated entirely with the UI dynamics stuff.