Question

感谢您的时间。

我正在通过libGDX使用Box2D创建一个Pong克隆。当球体与两个目标传感器主体之一（下图）接触时试图删除球体时，我有一个粘滞点导致空指针异常。

我想将接触的Ball体添加到列表中，以便稍后我可以遍历列表以删除Ball体（以及将来的多个Ball体）。

enter image description here

堆栈跟踪：

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)

我的GameScreen类中的第484行是world.destroyBody(circleBody);行，它位于下面显示的if(scoredGoal1 == true){stuff}方法的public void render(float delta){stuff}条件语句中。

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*/
            }
        }                

        });

我的逻辑如下：球体和目标传感器身体之间的接触：

在ContactListener的beginContact（）方法中，与EdgeShape传感器主体接触的球体将添加到ArrayList<Body> ballDeletionList = new ArrayList<Body>();，scoredGoal1 = true;布尔标志将设置为true。
在render();检查scoredGoal1 = true或scoredGoal2 = true，然后在world.step()方法后删除适用的正文/机构。

我在整个网络中搜索过其他代码示例和教程，但却找不到含糊不清的答案，因为代码是专用的，或者我目前只了解Java。

如果可以发布Java / libGDX代码示例解决方案，那就太好了。

Answer 1

您无法删除contactlistener中的实体，因为它位于您的世界步骤中，并且世界已被锁定。我在你的渲染方法中完成了与你想做的完全相同的事情：

if(ballDeletionList.size>0) ballDeletionList.clear();

此外，使用libgdx建议使用Array http://libgdx.badlogicgames.com/nightlies/docs/api/com/badlogic/gdx/utils/Array.html而不是ArrayList，它会产生更少的垃圾并进行更多优化。

libGDX Box2D：碰撞后如何破坏身体？

1 个答案: