Trace the beginning and the end of a collision

As the names suggest, the Begin Contact Event is called when two fixtures begin to overlap, while the End Contact Event is called when two fixtures cease to overlap.

1. Let's analyze the BeginContact function line by line:

   var fixtureA:b2Fixture=contact.GetFixtureA();
   var fixtureB:b2Fixture=contact.GetFixtureB();

   As you can see, BeginContact has a b2Contact object passed as an argument. It contains all collision information we are looking for at the moment. GetFixtureA and GetFixtureB methods return the fixtures involved in the collision. I am saving them in fixtureA and fixtureB variables.

2. Then, as I am looking for bodies, I need to get the body from a fixture. You should already know about the GetBody method:

   var bodyA:b2Body=fixtureA.GetBody();
   var bodyB:b2Body=fixtureB.GetBody();

3. Now bodyA and bodyB are the colliding bodies. Time to print some text in the output window:

   trace("first body: "+bodyA.GetUserData());
   trace("second body: "+bodyB.GetUserData...