Listening for collisions

If you remember, right after we created the world, we called its SetContactListener function and passed in a reference to GameWorld. We could do this because our GameWorld class inherits not only from CCLayer, but also from b2ContactListener. As a result of this, GameWorld can receive information about any and all pairs of fixtures colliding by implementing the BeginContact function as follows:

void GameWorld::BeginContact(b2Contact *contact)
{
  b2Body* body_a = contact->GetFixtureA()->GetBody();
  b2Body* body_b = contact->GetFixtureB()->GetBody();

  // only need to observe collisions that involve GameObjects
  if(body_a->GetUserData() == NULL || body_b->GetUserData() == NULL)
  {
    return;
  }

  // identify type of the objects involved in the collision
  EGameObjectType game_object_a_type = 
    ((GameObject*)body_a->GetUserData())->GetType();
  EGameObjectType game_object_b_type = 
    ((GameObject*)body_b->GetUserData())->GetType...