AndEngine GLES1: Identifying Objects during Collision

Question

I am creating a 'Chain Reaction' type game.

In onLoadResources I create 4 static bodies (ground, roof, left border and right border) to act as boundaries. I then create 30 dynamic bodies (which are balls that bounce around).

My question is, how can I detect which ball has just collided with a boundary?

The way I have it right now, I believe the last 'particle_body' that is created, is the only one that is ever modified during a collision.

Any suggestions??

Here is my code:

this.mPhysicsWorld = new FixedStepPhysicsWorld(60, new Vector2(0, 0), false); // Gravity (x, y) scene.registerUpdateHandler(mPhysicsWorld);

  // Create the ground groundFixture = PhysicsFactory.createFixtureDef(1, 1.0f, 0.5f); ground = new Rectangle(0, CAMERA_HEIGHT-1, CAMERA_WIDTH, CAMERA_HEIGHT); ground.setColor(0.0f, 0.0f, 0.0f); ground_body = PhysicsFactory.createBoxBody(this.mPhysicsWorld, this.ground, BodyType.StaticBody, groundFixture); ground_body.setUserData("ground"); mPhysicsWorld.registerPhysicsConnector(new PhysicsConnector(ground, ground_body, true, false)); scene.attachChild(ground); // Create the roof roofFixture = PhysicsFactory.createFixtureDef(1, 1.0f, 0.5f); roof = new Rectangle(0, 0, CAMERA_WIDTH, 1); roof.setColor(0.0f, 0.0f, 0.0f); roof_body = PhysicsFactory.createBoxBody(this.mPhysicsWorld, this.roof, BodyType.StaticBody, roofFixture); roof_body.setUserData("roof"); mPhysicsWorld.registerPhysicsConnector(new PhysicsConnector(roof, roof_body, true, false)); scene.attachChild(roof); // Create the left border leftBorderFixture = PhysicsFactory.createFixtureDef(1, 1.0f, 0.5f); leftBorder = new Rectangle(0, 0, 1, CAMERA_HEIGHT); leftBorder.setColor(0.0f, 0.0f, 0.0f); leftBorder_body = PhysicsFactory.createBoxBody(this.mPhysicsWorld, this.leftBorder, BodyType.StaticBody, leftBorderFixture); leftBorder_body.setUserData("leftBorder"); mPhysicsWorld.registerPhysicsConnector(new PhysicsConnector(leftBorder, leftBorder_body, true, false)); scene.attachChild(leftBorder); // Create the right border rightBorderFixture = PhysicsFactory.createFixtureDef(1, 1.0f, 0.5f); rightBorder = new Rectangle(CAMERA_WIDTH-1, 0, CAMERA_WIDTH, CAMERA_HEIGHT); rightBorder.setColor(0.0f, 0.0f, 0.0f); rightBorder_body = PhysicsFactory.createBoxBody(this.mPhysicsWorld, this.rightBorder, BodyType.StaticBody, rightBorderFixture); rightBorder_body.setUserData("rightBorder"); mPhysicsWorld.registerPhysicsConnector(new PhysicsConnector(rightBorder, rightBorder_body, true, false)); scene.attachChild(rightBorder); // Prepare a random number generator Random rand = new Random(); // Create Particles for(int i=0; i < 30; i++) { int x = rand.nextInt(CAMERA_WIDTH) - 1; int y = rand.nextInt(CAMERA_HEIGHT) - 1; this.particle = new Particle(x, y, this.mPurpleParticleTextureRegion, 100); particle.setScale(0.3f); scene.attachChild(particle); particleFixture = PhysicsFactory.createFixtureDef(10, 0.9f, 0.1f); particle_body = PhysicsFactory.createBoxBody(this.mPhysicsWorld, this.particle, BodyType.DynamicBody, particleFixture); particle_body.setUserData("particle_" + i); mPhysicsWorld.registerPhysicsConnector(new PhysicsConnector(this.particle, particle_body, true, false)); // Assign a random velocity to the particle x = rand.nextInt(20) + 3; y = rand.nextInt(20) + 3; particle_body.setLinearVelocity(new Vector2(x, y)); // x, y impulse mParticleList.add(particle); mBodyList.add(particle_body); mFixtureList.add(particleFixture); particleNumAR[i] = i; } // Create the Physics Collision Detection listener mPhysicsWorld.setContactListener(new ContactListener() { public void endContact(Contact contact) { } public void beginContact(Contact contact) { if (contact.getFixtureA().getBody().getUserData() != null && contact.getFixtureB().getBody().getUserData() != null) { final String objA = (String)contact.getFixtureA().getBody().getUserData(); final String objB = (String)contact.getFixtureB().getBody().getUserData(); // Particle hits ground if (objA.equals("ground") || objB.equals("ground")) { Random rand2 = new Random(); int xx = rand2.nextInt(20) + 3; int yy = rand2.nextInt(10) + 3; particle_body.setLinearVelocity(new Vector2(xx, yy)); // x, y impulse } // Particle hits roof if (objA.equals("roof") || objB.equals("roof")) { Random rand2 = new Random(); int xx = rand2.nextInt(20) + 3; int yy = rand2.nextInt(10) + 3; particle_body.setLinearVelocity(new Vector2(xx, yy)); // x, y impulse } // Particle hits left border if (objA.equals("leftBorder") || objB.equals("leftBorder")) { Random rand2 = new Random(); int xx = rand2.nextInt(10) + 3; int yy = rand2.nextInt(20) + 3; particle_body.setLinearVelocity(new Vector2(xx, yy)); // x, y impulse } // Particle hits right border if (objA.equals("rightBorder") || objB.equals("rightBorder")) { Random rand2 = new Random(); int xx = rand2.nextInt(10) + 3; int yy = rand2.nextInt(20) + 3; particle_body.setLinearVelocity(new Vector2(xx, yy)); // x, y impulse } } } @Override public void preSolve(Contact contact, Manifold oldManifold) { } @Override public void postSolve(Contact contact, ContactImpulse impulse) { } }); 

Answer 1

You can use the other fixture of the contact. One of the fixtures will be one of your walls. The other fixture will be your particle that collided.

So you could do something like this (untested pseudo code):

// I don't know what class the Box2D body is in this case (b2Body maybe? or just Body?)
// so please insert appropriate type

final String objA = (String)contact.getFixtureA().getBody().getUserData();
Body bodyA = contact.getFixtureA().getBody();
final String objB = (String)contact.getFixtureB().getBody().getUserData();
Body bodyB = contact.getFixtureB().getBody();


Body particleBody = objA.startsWith("particle_") ? bodyA : bodyB; 

...

// then use particleBody for setting the velocity in your different if cases

particleBody.setLinearVelocity(new Vector2(x,y));