It may be necessary for you to change something about a Box2D body in a collision - for example:
@Override
public void beginContact(Contact contact) {
Body body = contact.getFixturaA().getBody();
body.setActive(false);
}
Disabling the body as shown in the example above will crash the game - this happens because you're changing the state of the body while the simulation is running, i.e. during the execution of World.step().
The way (most) physics engines work is by executing the physics when a method is called - in our case World.step() - which resolves the collisions, applies forces, etc. And for the simulation not to break, certain properties cannot be changed while that's happening. The properties that will crash the game when changed while the world is locked are:
- body.setActive();
- body.setBodyType();
- And a few more!
On the other hand, these ones do not crash the game, so you can change them while the world is locked with no problems:
- body.setLinearVelocity();
- body.setAngularVelocity();
- And a few more!
The crux of the issue is the fact that the execution is happening at a certain point in time - so just execute the troublesome code later! And libGDX has got us covered with Application.postRunnable(), more information is available on the libGDX wiki. With this in mind, the above code should look like this:
@Override
public void beginContact(Contact contact) {
Body body = contact.getFixturaA().getBody();
Gdx.app.postRunnable(new Runnable() {
@Override
public void run() {
body.setActive(false);
}
});
}
Like the wiki says, this will run the code next frame, but before ApplicationListener.render() is called. Note that there's no difference whether you change the state of a body the very last line before World.step() or when the frame begins, because you are doing nothing but setting some values - which will be applied only when World.step() is called.
You can also come up with a custom solution, but this is the easiest & most straightforward way.