Question

Hello everyone I’m trying to develop a game like Jetpack so i want to set a gravity that normally push down and when the user tap the screen the gravity change it push up. I searched for a week a tutorial that explain how to do that but I didn’t find what I search. Someone can explain me how to do that or post a link to a tutorial ?

Thank you !!

    main = new Sprite(sX, sY, mainTextureRegion);
    main.setScale(1);
    main.setFlippedHorizontal(true);
    scene.attachChild(main);
    mPhysicsWorld = new PhysicsWorld(new Vector2(0, SensorManager.GRAVITY_EARTH), false);
    final FixtureDef objectFixtureDef = PhysicsFactory.createFixtureDef(1, 0.5f, 0.5f);
    final Body body = PhysicsFactory.createBoxBody(mPhysicsWorld, main, BodyType.DynamicBody, objectFixtureDef);
    mPhysicsWorld.registerPhysicsConnector(new PhysicsConnector(main, body, true, true));
    final Vector2 gravity = new Vector2(0, 5f);
    mPhysicsWorld.setGravity(gravity);

    scene.registerUpdateHandler(new IUpdateHandler() {
        @Override
        public void onUpdate(float pSecondsElapsed) {

      }

        @Override
        public void reset() {
            // TODO Auto-generated method stub

        }});

Answer 1

When the user touches the screen, you shouldn’t change the gravity because it is not suppose to be done like that.
When he touches the screen, just apply a LinearImpulse (check if it not already at maximum speed) or something like that to make it go upper. The gravity shouldn’t change as it should always attract all of your physics objects down. If your change the gravity you change the behavior of all your objects, not only the player one.

EDIT

To have the physics engine to work you have to call the step() method of the PhysicsWorld class at each render. The problem with Andengine is that you don’t directly access the render() method as Andengine handle it at all.
So what you have to do is to register a IUpdateHandler on the Engine object that you return in your onLoadEngine() method.
This way the onUpdate() method of the interface will be called at each render and here you can call the step() method to get the physics part to work. The float value tells you about how many seconds elapsed since last render but you don’t have to take care of that. The method is called multiple times per second.

Hope it helps.

EDIT 2

I think this should work :

//on the Scene object or on the Engine one
scene.registerUpdateHandler(new IUpdateHandler() {
    @Override
    public void onUpdate(float pSecondsElapsed) {
        mPhysicsWorld.onUpdate(pSecondsElapsed);
    }

    @Override
    public void reset() {}
});