libgdx box2D contactlistener - libgdx

Question

i am testing collision between two bodies in box2D using contactListener
this is my whole contactListener class i used world.setContactListener(new Box2DWorld(this)) to add it
public class Box2DWorld implements ContactListener{
World world;
GameScreen gameScreen;
Car car;
public enum BodyName{
frontWheel,rearWheel,carShell,ground,finishSensor
}
/**
* this class handles collisions and body userdata
* #param gameScreen
*/
public Box2DWorld(GameScreen gameScreen) {
this.gameScreen = gameScreen;
world = gameScreen.world;
this.car = gameScreen.car;
}
/*
* the debug should be like
*
* BEGONCONTACT,preSolve,postSolve,......postsolve,ENDCONTACT,BEGINCONTACT,preSolve,postSolve
*
* but i am getting debug like
*
* BEGINCONTACT,preSolve,postSolve,......postsolve,BEGINCONTACT,presolve,ENDCONTACT,preSolve,postSolve...
*
* i can't understand why begincontact appears twice without endcontact in between and also bodies are in contact
* even after endcontact
*
*/
#Override
public void beginContact(Contact contact) {
BodyName bodyA = (BodyName) contact.getFixtureA().getBody().getUserData();
BodyName bodyB = (BodyName) contact.getFixtureB().getBody().getUserData();
if((bodyA == BodyName.frontWheel && bodyB == BodyName.ground )||(bodyA == BodyName.ground && bodyB == BodyName.frontWheel)){
car.frontWheel.onGround = true;
System.out.println("begin contact");
}
}
#Override
public void endContact(Contact contact) {
BodyName bodyA = (BodyName) contact.getFixtureA().getBody().getUserData();
BodyName bodyB = (BodyName) contact.getFixtureB().getBody().getUserData();
if((bodyA == BodyName.frontWheel && bodyB == BodyName.ground )||(bodyA == BodyName.ground && bodyB == BodyName.frontWheel)){
car.frontWheel.onGround = false;
System.out.println("contact end");
}
}
#Override
public void preSolve(Contact contact, Manifold oldManifold) {
BodyName bodyA = (BodyName) contact.getFixtureA().getBody().getUserData();
BodyName bodyB = (BodyName) contact.getFixtureB().getBody().getUserData();
if((bodyA == BodyName.frontWheel && bodyB == BodyName.ground )||(bodyA == BodyName.ground && bodyB == BodyName.frontWheel)){
System.out.println("PreSolve");
}
}
#Override
public void postSolve(Contact contact, ContactImpulse impulse) {
BodyName bodyA = (BodyName) contact.getFixtureA().getBody().getUserData();
BodyName bodyB = (BodyName) contact.getFixtureB().getBody().getUserData();
if((bodyA == BodyName.frontWheel && bodyB == BodyName.ground )||(bodyA == BodyName.ground && bodyB == BodyName.frontWheel)){
System.out.println("postSolve");
}
}
my dedub goes like this
beginContact
preSolve
postSolve
.
.
.
.
.
.
preSolve
postSolve
beginContact
preSolve
endcontact
postSolve
presolve
.
.
.
.
i can't understand why begincontact is appearing before endcontact
both ground and frontWheel bodies have only 1 fixture each
i can't figure out this from a week pls help me

Answer 1

Remember that a contact is between two fixtures, not two bodies.
I would guess that your ground is made from many fixtures, and they are all on the same ground body right? As the wheel rolls over the fixtures it can touch more than one fixture at a time, so you can get begin(fixture1), begin(fixture2), end(fixture1) in that order. It's quite normal. I think if you print out the ground fixture involved in each contact event, you would see what I mean.
If you want to know when the wheel is touching the ground, you need to track the total number of fixtures currently touching. You can do that by incrementing a counter when you get a 'begin' event, and decrement it when you get an 'end' event. Then you can check the counter any time, if it is more than zero the wheel is touching the ground.