libGDX Box2D: How to destroy body after collision?
Question
Thanks for your time.
I am creating a Pong clone using Box2D via libGDX. When trying to delete the Ball body as a result of the Ball body contacting with one of the two Goal Sensor Bodies (image below), I am having a sticking point that results in a Null Pointer Exception.
I want to add the contacting Ball body to a list so that I can later iterate through the list to delete the Ball body (and multiple Ball bodies in the future).
The stack trace:
Exception in thread "LWJGL Application" java.lang.NullPointerException
at com.badlogic.gdx.physics.box2d.World.destroyBody(World.java:311)
at com.ckq3r.Ponger.screens.GameScreen.update(GameScreen.java:484)
at com.ckq3r.Ponger.screens.GameScreen.render(GameScreen.java:114)
at com.badlogic.gdx.Game.render(Game.java:46)
at com.ckq3r.Ponger.PongerGame.render(PongerGame.java:236)
at com.badlogic.gdx.backends.lwjgl.LwjglApplication.mainLoop(LwjglApplication.java:204)
at com.badlogic.gdx.backends.lwjgl.LwjglApplication$1.run(LwjglApplication.java:112)
Line 484 in my GameScreen class is the line world.destroyBody(circleBody); that resides within the if(scoredGoal1 == true){stuff} conditional statement of the the public void render(float delta){stuff} method shown below.
public class GameScreen implements Screen, InputProcessor{
/*----methods and variables omitted for readability------*/
private boolean scoredGoal1 = false, scoredGoal2 = false;
ArrayList<Body> ballDeletionList = new ArrayList<Body>();
/*==============Screen implementation methods============*/
@Override
public void show(){
/*ball*/
BodyDef circleDef = new BodyDef();
Body circleBody = world.createBody(circleDef);
circleBody.setUserData(1);
CircleShape circleShape = new CircleShape();
FixtureDef circleFixture = new FixtureDef();
circleFixture.shape = circleShape;
circleBody.createFixture(circleFixture);
circleShape.dispose();
}
@Override
public void render(float delta) {
world.step(Gdx.app.getGraphics().getDeltaTime(), 8, 3);
Gdx.gl.glClearColor(0, 0, 0, 1);
Gdx.gl.glClear(GL10.GL_COLOR_BUFFER_BIT);
camera.update();
debugRenderer.render(world, camera.combined);
/*-------ball deletion experiment-------*/
if(scoredGoal1 == true){
//iterate through ballDeletionList somehow?
world.destroyBody(circleBody);
circleBody.setUserData(null);
circleBody = null;
scoredGoal1 = false;
//clear ballDeletionList
}else if(scoredGoal2 == true){
//iterate through ballDeletionList somehow?
world.destroyBody(circleBody);
circleBody.setUserData(null);
circleBody = null;
scoredGoal2 = false;
//clear ballDeletionList
}
/*-----end ball deletion experiment------*/
}
/*===========Box2D contact listener=============*/
private void createContactListener() {
world.setContactListener(new ContactListener() {
@Override
public void beginContact(Contact contact) {
Fixture fixtureA = contact.getFixtureA();
Fixture fixtureB = contact.getFixtureB();
Gdx.app.log("beginContact", "between " + fixtureA.toString() + " and " + fixtureB.toString());
if(fixtureA.getBody().getUserData().equals(1) && fixtureB.getBody().getUserData().equals(2) || fixtureA.getBody().getUserData().equals(2) && fixtureB.getBody().getUserData().equals(1)){
Gdx.app.log("HIT", "goal1 contact");
/*ball deletion experiment*/
ballDeletionList.add(circleBody);
Gdx.app.log("Ball", "circleBody added to deletion list");
scoredGoal1 = true;
/*ball deletion experiment*/
}
if(fixtureA.getBody().getUserData().equals(1) && fixtureB.getBody().getUserData().equals(3) || fixtureA.getBody().getUserData().equals(3) && fixtureB.getBody().getUserData().equals(1)){
Gdx.app.log("HIT", "goal2 contact");
/*ball deletion experiment*/
ballDeletionList.add(circleBody);
Gdx.app.log("Ball", "circleBody added to deletion list");
scoredGoal2 = true;
/*ball deletion experiment*/
}
}
});
My logic is as follows on contact between a Ball body and a Goal Sensor body:
- From within the ContactListener's beginContact() method, the Ball body contacting with the EdgeShape Sensor body will be added to the
ArrayList<Body> ballDeletionList = new ArrayList<Body>();and thescoredGoal1 = true;boolean flag will be set to true. - In the
render();check forscoredGoal1 = trueorscoredGoal2 = truethen delete the applicable Body/Bodies after theworld.step()method.
I have searched other code examples and tutorials throughout the web only to find answers that are ambiguous because either the code was case specialized or I only understand Java at the moment.
It would be great if a Java/libGDX code example solution could be posted.
1 answers
solution1
2 2013-08-05 07:20:50
You can't delete bodies in your contactlistener because that is inside your world step, and world is locked. I did exactly the same you are trying to do, in your render method just:
if(ballDeletionList.size>0) ballDeletionList.clear();
Also, using libgdx it is recommended to use Array http://libgdx.badlogicgames.com/nightlies/docs/api/com/badlogic/gdx/utils/Array.html instead of ArrayList, it generates less garbage and makes some more optimizations.
solution2
-2 2022-08-18 22:14:24
Please, do you have any simple example on how to destroy a body?
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