Proper way to use ContactListener in LibGDX

Question

I'm developing a game with LibGDX using Box2D. I'm a newbie, so I've been following Brent Aureli's tutorial in which he creates a class that implements ContactListener in order to detect all the collisions and switch depending on the contact.

I think it's a bit excessive as I don't want to detect all the collisions but only a few in certains bodies.

Is there a way to implement the ContactListener only in a certain body? If not, is there a better way to resolve the collisions more efficiently?

Answer 1

In a Box2D ContactListener the BeginContact method will be called every time a contact initiates whether you have code in there to process it or not.

As an example, the below code is an Excerpt from a ContactListener from one of my projects.

@Override
public void beginContact(Contact contact) {
    //System.out.println("Contact");
    Fixture fa = contact.getFixtureA();
    Fixture fb = contact.getFixtureB();

    if(fa.getBody().getUserData() instanceof Brick){
        this.brickHit((Brick) fa.getBody().getUserData(), fb);
    }else if(fb.getBody().getUserData() instanceof Brick){
        this.brickHit((Brick) fb.getBody().getUserData(), fa);
    }

    if(fa.getBody().getUserData() instanceof Pad){
        this.padHit((Pad) fa.getBody().getUserData(), fb);
    }else if(fb.getBody().getUserData() instanceof Pad){
        this.padHit((Pad) fb.getBody().getUserData(), fa);
    }

    if(fa.getBody().getUserData() instanceof Bin){
        objectInVoid(fb);
    }else if(fb.getBody().getUserData() instanceof Bin){
        objectInVoid(fa);
    }

    if(fa.getBody().getUserData() instanceof Ball){
        ballHitSomething((Ball) fa.getBody().getUserData(),fb);
    }else if(fb.getBody().getUserData() instanceof Ball){
        ballHitSomething((Ball) fb.getBody().getUserData(),fa);
    }

    if(fa.getBody().getUserData() instanceof Bomb){
        bombHitSomething((Bomb) fa.getBody().getUserData(),fb);
    }else if(fb.getBody().getUserData() instanceof Bomb){
        bombHitSomething((Bomb) fb.getBody().getUserData(),fa);
    }

    if(fa.getBody().getUserData() instanceof LocalEffectEntity){
        if(fa.isSensor()){
            inLocalEffectRange((LocalEffectEntity) fa.getBody().getUserData(),fb);
        }else{
            objectInVoid(fb);
        }
    }else if(fb.getBody().getUserData() instanceof LocalEffectEntity){
        if(fb.isSensor()){
            inLocalEffectRange((LocalEffectEntity) fb.getBody().getUserData(),fa);
        }else{
            objectInVoid(fa);
        }
    }
}