How can I track all of my Box2D collisions in a clean, manageable manner?

Question

I am using Box2D for the first time seriously in a medium sized Flash Game that I am working on. My current experience with Box2D is limited to creating a world, bodies and adding those bodies to the world in a functional manner.

I'm finding it easy enough to integrate Box2D into my game environment, maintaining well-written code and have completed a few tutorials that walk through dealing with collisions. The issue that I'm facing now is that my game will have many bodies, each interacting with other bodies in different ways, and I'm finding it hard to write my own b2ContactListener subclass without it getting extremely messy.

Based off a tutorial I used, I have created my own subclass of b2ContactListener and added an override of the BeginContact() method. The argument that BeginContact() receives when it is called will reference an instance of b2Contact, through which I can access two b2Fixture instances (the two instances that have collided). I am then able to access the b2Body instance associated with each of those b2Fixtures.

• Problem: Currently I have a roundabout way of finding out what two things collided (i.e. whether they're a wall and a missile, or the player and a tree, etc) which uses GetUserData() and looks like this as an example:

  var f1Player:Boolean = contact.GetFixtureA().GetBody().GetUserData() is Player
  var f2Player:Boolean = contact.GetFixtureB().GetBody().GetUserData() is Player
  var f1Tree:Boolean = contact.GetFixtureA().GetBody().GetUserData() is Tree
  var f2Tree:Boolean = contact.GetFixtureB().GetBody().GetUserData() is Tree
  // ... continutes with all possible combinations.
  
  
  // Example of managing a collision:
  if(f1Player && f2Tree)
  {
      // Player (FixtureA) and Tree (FixtureB)
  }
  
  if(f2Player && f1Tree)
  {
      // Player (FixtureB) and Tree (FixtureA)
  }
  

  As you can see, this is going to end up extremely long and unmanageable. I also have to write each set of actions to perform twice to cater for a certain element being FixtureA or FixtureB, or vice versa (obviously in the form of a function call with the parameters swapped around rather than literally re-written).

This is clearly not the correct approach, but I haven't been able to locate resources that more thoroughly explain collision detection management.

Does anyone have experience with collision detection management using Box2D that they can share? Also, is using SetUserData( entityThatOwnsTheBody ); the correct way to be using that method?

Answer 1

Yeah, it's a bit of a nuisance indeed. Actually I think the way you have it is quite typical.

fwiw Box2D itself has to deal with a similar problem when testing whether fixtures overlap. There are a bunch of functions such as b2CollideCircles, b2CollidePolygonAndCircle, b2CollidePolygons etc, and when two fixtures come near each other the engine chooses which of these functions should be used.

It does this by putting the function pointers in a 2-dimensional array, then looks up the appropriate function in this array by using the two shape types as index. See the first three functions in b2Contact.cpp for details.

Of course, if you can't pass around function references like this in AS3 then I guess this answer doesn't help much, but I thought I would post anyway as C/C++/JS users might come by.