Hooking up Chipmunk bodies to UIKit components?

Question

I'm trying to get to grips with using Chipmunk (not the Obj-C version) with UIKit components on iOS, and still struggling immensely.

I'm trying to establish how, in the ChipmunkColorMatch example in the documentation, the UIButton instances are actually hooked up to any of the physics calculations. I see that the UIButtons are created inside the Ball class, and some of their properties are set, (type, image, etc.), but I'm not understanding where the cpBody or cpShape or whichever it is is actually attached to that UIButton. I assume it needs to be, else none of the physics will be reflected in the UI.

I've looked in the SimpleObjectiveChipmunk tutorial on the website too, but due to the fact that it uses libraries unavailable to me (the Obj-C libraries), I can't establish how it works there, either. Again, I see a UIButton being created and positioned on-screen, but I don't see how the cpBody (or in that case, ChipmunkBody) is linked to the button in any way.

Could anyone shed some light on how this works? Effectively what I'm going to need are some UIButton instances which can be flicked around, but I've not even got as far as working out how to create forces yet, since I can't get the bodies hooked up to the buttons.

Much obliged, thanks in advance.

EDIT: Should also point out that I am not, and do not want to use cocos2d in this project at all. I've seen tutorials using that, but that's a third layer of confusion to add in. Thanks!

Answer 1

Assuming this source is the project you're asking about, it looks like the magic happens in Ball's sync method -- it creates a CGAffineTransform representing the translation and rotation determined by the physics engine, and applies that to the button.

In turn, that method is called by the view controller's draw: method, which is timed to occur on every frame using CADisplayLink, and updates the physics engine before telling each Ball to sync.