b2ContactListener in general concept

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b2ContactListener in general concept

(10 posts) (6 voices)

Started 2 years ago by vennt
Latest reply from GloryFish

Tags:

vennt
Member
I'm not quite understanding how to use the b2ContactListener. I'm basically trying to detect a contact and have the contacted object call a method, could be a sound, but in this case its just a log.
```
Box.mm

-(id) initWithWorld:(b2World*)pWorld
{
	if((self = [super init]))
	{
		world = pWorld;

		b2PolygonShape boxShape;
		boxShape.SetAsBox(1.0f, 0.1f);

		b2FixtureDef boxFixture;
		boxFixture.shape = &boxShape;
		boxFixture.density = 1.0f;

		b2BodyDef boxBody;
		boxBody.position.Set(150.0/PTM_RATIO, 250.0/PTM_RATIO);

		box = world->CreateBody(&boxBody);
		box->CreateFixture(&boxFixture);
	}
	return self;
}

-(void) saySomething
{
		NSLog(@"Boing!");
}
```
I've been searching threads here and google for contact listener stuff, but I can't find something that I can wrap my head around. There are a couple of complex (to me) examples from badawe and others. And I just switched over from chipmunk (which I also had trouble doing the collision stuff with).

XrissKenn's blog post here is what I'm currently implementing: http://blog.xyris.ca/?p=40
It was very helpful. And the logging is occurring.
```
ContactListener.mm

void ContactListener::BeginContact(b2Contact* contact)
{
	b2Fixture* fixtureA = contact->GetFixtureA();
	b2Fixture* fixtureB = contact->GetFixtureB();
	if (contact->IsSolid()) {
		NSLog(@"Contact is solid");
	}
}

void ContactListener::EndContact(b2Contact* contact)
{
	NSLog(@"end contact");
}
```
Instead of having the log created in the if statement there, I'd like it to be fired from my contacted objects method (saySomething).

But I don't think I'm close to that yet, I've read about including the SetUserData, into my fixture, but I still don't know how I can call [box saySomething]; into the contact method.
Posted 2 years ago #
Lam
Member
Well that's kind of right to use SetUserData... but why into fixture? Why not have the b2Body store the userdata?

You're on the right track but it would be much easier to set the userData on the body.

A b2Body can have a number of b2Fixtures. Setting the userData on the b2Body makes more sense to me since you aren't repeatedly setting it into every fixture you create for that one body.

SetUserData is just a void* to any data you want stored.
So lets say that 'userData' is the address of an instantiated CocosNode*.

Does that help? I hope that isn't as complex as the other tutorials.
```
// returns a body from a given fixture
b2Body *b = fixtureA->GetBody();

// hmmm... we can get a userData from the body
id userData = (id)b->GetUserData();

// if that user data is our class type... bingo!!
if([userData isKindOfClass:[Box class]){
   [(Box*)userData saySomething];
}
```
Posted 2 years ago #
vennt
Member

Lam this makes sense to me, thank you very much for the explanation. I did see it was possible to set user data on the body, it seems to have a different syntax using the .setUserData, rather than the ->SetUserData used in the fixture (from most other examples I've seen).

It also makes sense that since a body can have many fixtures than the "main" data should be attached to the body.

It's a great starting point for me, thanks! I hope to test this tonight and see if I can make progress. I guess I have to check both fixtures to see since I don't know which is assigned.

Posted 2 years ago #
Lam
Member

Yeah you just have to check the two fixtures. If you have any more questions just ask.

Posted 2 years ago #
umer sufyan
Member

i m a new one to box2 i was looking for contact listener but i find the older one add , remove etc etc. that are declared in iphone xcode .h file and are implemented in .mm file but the new one Begincontact EndContact how to use this if some one has some clue or some sample please do share with me how can i use this...

Posted 1 year ago #
robodude666
Member
It' rather simple, actually:
```
class MyGameContactListener : public b2ContactListener
{

public:

	void EndContact(b2Contact* contact)
	{
		Actor *actorA = (Actor*)contact->GetFixtureA()->GetBody()->GetUserData();
		Actor *actorB = (Actor*)contact->GetFixtureB()->GetBody()->GetUserData();

		if([actorA isKindOfClass:[Ball class]] && [actorB isKindOfClass:[Square class]])
		{
			// Ball A hit square B
		}
		else if ([actorB isKindOfClass:[Ball class]] && [actorA isKindOfClass:[Square class]])
		{
			// Ball B hit square A
		}
	}
};
```
For example :). Make sure that you're testing them against the class type you have defined to be their userdata. I'm personally using a custom class for each of my items so I can set the userdata to self and test against the class's name.

Take note that if you want a ball to hit a square, for example, you have to test to see if:

1) shape1 is a ball and it hit shape2 which is a square
and
2) shape1 is a square and it hit shape2 which is a ball.

Other things to keep in mind: You can't modify body's parameters within the contact listener. If you change speed, rotation, etc. it won't be seen. You also can't delete bodies within a contact listener.

If you need to do any of these, have the contact listener only call a method that sets up a flag which handles this after your step function is called.

-robodude666
Posted 1 year ago #
steg90
Member

Hi,

Check Ray Wenderlich website, he has some nice articles that may help, although I think robodude666 has explained it very well :-)

http://www.raywenderlich.com/457/intro-to-box2d-with-cocos2d-tutorial-bouncing-balls

Posted 1 year ago #
GloryFish
Member

Related to this, once you get your contact listener up and running, logging collisions, how do you then get the point, in node space, of the collision?

Say I have two objects colliding and I want to trigger a particle effect at the point of collision, how do I get that info?

Posted 1 year ago #
vennt
Member

^ Good question, I am just hopping back on and I see my old post :)

I'm at this point as well trying to: another old post that might help click

Posted 1 year ago #

GloryFish
Member

I just got this working using the following code in my contact listener:

void BoardContactListener::BeginContact(b2Contact* contact) {
  b2Fixture* fixtureA = contact->GetFixtureA();
  b2Fixture* fixtureB = contact->GetFixtureB();

  b2Body* bodyA = fixtureA->GetBody();
  b2Body* bodyB = fixtureB->GetBody();

  b2Shape* shapeA = fixtureA->GetShape();
  b2Shape* shapeB = fixtureB->GetShape();

  const b2Transform xfA = bodyA->GetTransform();
  const b2Transform xfB = bodyB->GetTransform();

  b2Manifold manifold;

  contact->Evaluate(&manifold, xfA, xfB);

  b2WorldManifold worldManifold;
  worldManifold.Initialize(&manifold, xfA, shapeA->m_radius, xfB, shapeB->m_radius);
  contact->GetWorldManifold(&worldManifold);

  if(sizeof(worldManifold.points) > 0) {
    b2Vec2 point = worldManifold.points[0]; // point of collision

    CGPoint worldPoint = MTPCGPoint(point);
    CCLOG(@"Collision at %@", NSStringFromCGPoint(worldPoint));
  }
}

I am using a helper function MTPCGPoint which I cribbed from:

https://bitbucket.org/favoyang/b2bridge/src/tip/b2SpaceManager.h

to convert the Box2D point to node space.

I haven't done any major testing. I literally got it to log the point five minutes ago and wanted to share. Post your experiences here.

It is certainly not straightforward to get the collision point and I have concerns about the performance of the above code. It would be nice if there was an example of the right way to do this in the Box2D tests.

b2ContactListener in general concept

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