How to make two bodies stick after a collision?

Question

I am really stuck on this I can successfully detect a collision but I can't make the two bodies involved in the collision to stick.

Here is my ContactListener

world.setContactListener(listener);

    listener = new ContactListener() {

        @Override
        public void preSolve(Contact contact, Manifold oldManifold) {

        }


        @Override
        public void postSolve(Contact contact, ContactImpulse impulse) {

        }

        //called when two fixtures cease to touch
        @Override
        public void endContact(Contact contact) {
            Fixture fixtureA = contact.getFixtureA();
            Fixture fixtureB = contact.getFixtureB();
            Gdx.app.log("beginContact", "between" + fixtureA.toString() + "and" + fixtureB.toString());
        }

        //called when two fixtures begin to touch
        @Override
        public void beginContact(Contact contact) {
            Fixture fixtureA = contact.getFixtureA();
            Fixture fixtureB = contact.getFixtureB();
            Gdx.app.log("beginContact", "between" + fixtureA.toString() + "and" + fixtureB.toString());
        }
    };

Also this is what I put in my render() straight after the world.step() line

int numContacts = world.getContactCount();

    if(numContacts > 0)
    {
        Gdx.app.log("contact", "start of contact list");
        for(Contact contact: world.getContactList())
        {
            Fixture fixtureA = contact.getFixtureA();
            Fixture fixtureB = contact.getFixtureB();
            Gdx.app.log("contact", "between" + fixtureA.toString() + "and" + fixtureB.toString());
        }
        Gdx.app.log("contact", "end of contact list");
    }

I am extremely stuck on what to put on post solve or pre-solve really confused. I followed the iforce2d sticky projectiles http://www.iforce2d.net/b2dtut/sticky-projectiles but I don't understand C++ and i get a lot of syntax errors when working in eclipse. Please can someone show me an example code of a working collision where bodies stick together after colliding in java please.

Answer 1

This is how you create a WeldJoint with the libgdx wrapper:

WeldJointDef wd = new WeldJointDef();
wd.bodyA = body1;
wd.bodyB = body2;
wd.referenceAngle = wd.bodyB.getAngle() - wd.bodyA.getAngle();
world.createJoint( wd );

Do not try to create Joints inside the ContactListener. Add the bodies to be glued to a list and check them just after world.step.

EDIT:

Ok, like in the iforce2d tutorial,create an object to contain 2 bodies:

public class StickyInfo{
    Body bodyA;
    Body bodyB;
    public StickyInfo(Body bodyA, Body bodyB){
        this.bodyA = bodyA;
        this.bodyB = bodyB;
    }
};

Then create a libgdx Array of StickyInfo's

Array<StickyInfo> collisionsToMakeSticky = new Array<StickyInfo>();

When the bodies collide (well, technically their fixtures), add them to this list:

collisionsToMakeSticky.add(new StickyInfo(body1, body2))

And then just after world.step, if the Array is not empty. Create the WeldJoints:

while(collisionsToMakeSticky.size>0){
    StickyInfo si = collisionsToMakeSticky.removeIndex(0);
    //Make the WeldJoint with the bodies si.bodyA and si.bodyB
}