How do I identify a collision with a specific body?

Question

How can I identify when the ground body collides with another b2Body? I can use b2ContactListener to be notified of collision events, but how do I tell which bodies collided?

Answer 1

This is the point of b2Body's UserData member. It's a pointer you can set to anything you want. During the collision you can check this pointer and change how you react to the collision depending on its value.

For a trivial case, you can make the user data a NSString. For example,

groundBody->SetUserData(@"Ground Body");

Then inside your b2ContactListener subclass, or when you loop over bodies checking their contact points, you can use:

if ([bodyA->GetUserData() isEqual:@"Ground Body"]) {
    // Do something special if the body is the ground body.
}

(n.b. This is not compatible with ARC. I wouldn't recommend using ARC with games yet if you're an Objective-C beginner. The libraries aren't ready and the best idioms have not gelled).