[Guide] Contact Listener

[Mathew]

Contact Listener

Lately many people keep asking about how to make collision/contact check's between bodies, it was explained couple of times - more or less, I will create this brief guide, to keep most important informations about this subject in one thread. * If it there is already similar guide, please remove this one (sorry in advance)

Short description: as we know (or some of you don't) we can check collisions between shapes (for example sprites), with simple check:

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1. if (x.collidesWith(y)
2. {
3.      // action
4. }

Parsed in 0.030 seconds, using GeSHi 1.0.8.4

But, its not enough for bodies, that's why we can use Contact Listener.

1. Creating new contact listener:

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1.         private ContactListener contactListener2()
2.         {
3.                 ContactListener contactListener = new ContactListener()
4.                 {
5.                         @Override
6.                         public void beginContact(Contact contact)
7.                         {
8.                                
9.                         }
10.  
11.                         @Override
12.                         public void endContact(Contact contact)
13.                         {
14.                                
15.                         }
16.  
17.                         @Override
18.                         public void preSolve(Contact contact, Manifold oldManifold)
19.                         {
20.                                
21.                         }
22.  
23.                         @Override
24.                         public void postSolve(Contact contact, ContactImpulse impulse)
25.                         {
26.                                
27.                         }
28.                 };
29.                 return contactListener;
30.         }

Parsed in 0.032 seconds, using GeSHi 1.0.8.4

There are implemented, two most important functions (beginContact and endContact)

Also we have to register this contact listener, to our physic world.

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1.                 mPhysicsWorld.setContactListener(contactListener());

Parsed in 0.033 seconds, using GeSHi 1.0.8.4

Those give us possibilities to do certain actions, when there is contact between 2 bodies (or contact is ended)

Obviously, we have to 'receive' those 2 bodies somehow, to reference to them

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1.         Fixture x1 = contact.getFixtureA();
2.         Fixture x2 = contact.getFixtureB();

Parsed in 0.035 seconds, using GeSHi 1.0.8.4

And now lets say we want to Log info while there is contact between two bodies, so we have to add this check in beginContact

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1. if (x2.getUserData().equals("player") || x1.getUserData().equals("monster"))
2. {                                              
3.         Log.i("CONTACT", "BETWEEN PLAYER AND MONSTER!");
4. }

Parsed in 0.035 seconds, using GeSHi 1.0.8.4

as you most likely noticed, we used getUserData() function, to retrieve "data/reference name of the object/body/shape"

It means, you can use setUserData() function, to 'name' somehow your object.

For example:

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1. Body b = PhysicsFactory.createBoxBody(mPhysicsWorld, testSprite, BodyType.KinematicBody, wallFixtureDef);
2. b.setUserData("monster");

Parsed in 0.035 seconds, using GeSHi 1.0.8.4

Then we can reference to those names, using contact listener (with way posted above)

Hopefully its easy to understand, thanks!

[nivangerow]

Can more than one body have the same userdata? Because I have some blocks that are loaded from a text file, into an ArrayList. Also, will the method contactListener2() be updated automatically?

[Mathew]

For both of your question is yes.

[nivangerow]

Ok, I have tried it, and I dont get any output.... Here is my code (all linked up to the scene)

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1. Problem solved.
2.  

Parsed in 0.035 seconds, using GeSHi 1.0.8.4

Am I doing something wrong?

[Mathew]

You just set user data for player, I couldn't see any object with "block" data.

Also take into consideration, that x1 OR x2 might be player and x1 or x2 might be 2nd object (Ex. block)

[nivangerow]

You just set user data for player, I couldn't see any object with "block" data.

Also take into consideration, that x1 OR x2 might be player and x1 or x2 might be 2nd object (Ex. block)

That is in a different class. DO i have to refer to that class or can i run it all over the Main class with the scene object?

[Mathew]

Oh, I'm an idiot, forget to mention that ... we have to register this contact listener to our physic world (...)

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1. mPhysicsWorld.setContactListener(contactListener());

Parsed in 0.035 seconds, using GeSHi 1.0.8.4

[nivangerow]

You need to change your if function inside the beginContact method to something like this:

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1. if(x2.getBody().getUserData().equals("player") || x1.getBody().getUserData().equals("monster"))

Parsed in 0.036 seconds, using GeSHi 1.0.8.4

I got an error, because you forgot to add the .getBody() after the x2 or x1

[nivangerow]

The problem is also fixed...

[spaine]

Where do you place this code?

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1. Fixture x1 = contact.getFixtureA();
2. Fixture x2 = contact.getFixtureB();
3.  

Parsed in 0.036 seconds, using GeSHi 1.0.8.4

I tried doing this code in my game but it closes the intent and the game restarts.

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1. ContactListener cl = new ContactListener() {
2.                                
3.                         @Override
4.                         public void preSolve(Contact contact, Manifold oldManifold) {
5.                                 // TODO Auto-generated method stub
6.                                
7.                         }
8.                        
9.                         @Override
10.                         public void postSolve(Contact contact, ContactImpulse impulse) {
11.                                 // TODO Auto-generated method stub
12.                                
13.                         }
14.                        
15.                         @Override
16.                         public void endContact(Contact contact) {
17.                                 // TODO Auto-generated method stub
18.                                
19.                         }
20.                        
21.                         @Override
22.                         public void beginContact(Contact contact) {
23.                                 // TODO Auto-generated method stub
24.                                 Fixture x1 = contact.getFixtureA();
25.                         Fixture x2 = contact.getFixtureB();
26.                         if (x2.getUserData().equals("angel") || x1.getUserData().equals("enemy"))
27.                         {                                              
28.                                 Log.i("CONTACT", "BETWEEN PLAYER AND MONSTER!");
29.                            
30.                                
31.                         }
32.                                
33.                         }
34.                 };
35.  

Parsed in 0.039 seconds, using GeSHi 1.0.8.4

What am I doing wrong?