Moving Shapes with Touches

As per request by "nickthedude" at the cocos2d-iphone forums, this is a very quick post about using touches to move/rotate shapes. We'll be using Cocos2d, Chipmunk, and SpaceManager.

So this concept we're about to discuss has been around for a bit, originally coded up by Scott Lembcke (chipmunk creator) in his example of cpMouse; I think some have even kept it alive somewhere... So anyway we're going to expand upon this original idea a bit and make it multi-touch enabled as well, this way you can use two fingers to rotate a shape for example.

So the single-touch concept is quite simple. On touch-began we detect if that touch is within a shape, if it is we immediately create a pivot constraint between the shape's body and static (non-moving) body, we set the pivot's anchor to the touch's position and... voila! The shape is suddenly attached to where I put my finger down.

The rest is easy. On touch-moved we just update the pivot's anchor, and on touch-ended we just get rid of the pivot altogether. Sounds fairly simple right? Well what would it take to make this multi-touch enabled?

Well basically instead of keeping around just one pivot, we need to have an array of them now AND be able to tell which touch they're associated with... uh oh. Hmm well what we really need is a new class that has UITouch and cpConstraint;