5.Libgdx扩展学习之Box2D_刚体的运动和贴图

主要根据这2篇文章来写的：
1. http://bbs.9ria.com/thread-135588-1-1.html
2. http://bbs.9ria.com/thread-137127-1-1.html

到现在我们已经初步的认识了Box2D， 但是我们创建的刚体，还总是在原地不断的蹦来蹦去，那么现在让刚体动起来吧！

1. 给物体一个力– ApplyForce

ApplyForce方法会在刚体上施加一个力，更进牛顿定律： F=ma，有了力就有了加速度，物体就会有速度，就会逐渐运动起来。如果我们一直给物体一个力，那么物体的速度就会不断的增加。
示例中点击按钮只是给物体施加了一个顺时力，由于设置摩擦力=0，那么物体就会保持匀速运动。

2. 速度的叠加– ApplyImpulse

与ApplyForce不同， ApplyImpulse不会产生力，而是直接影响刚体的速度。通过ApplyImpulse方法添加的速度会与刚体原来的速度叠加，产生新的速度。

3. 立马运动– SetLinearVelocity

SetLinearVelocity直接影响刚体的速度。与ApplyImpulse不同，setLinearVelocity设置的速度会直接替换刚体原来的速度。

刚体贴图

到现在我们所有的示例程序都是光秃秃的线条，以后也会是。不过Body给我们提供了方法 public void setUserData (Object userData)来设置贴图。从上面可以知道贴图可以是任何的Object(Image, Sprite )。

下面让我们来看示例代码

public class MoveBody extends ApplicationAdapter {

    World world;
    Box2DDebugRenderer box2DDebugRenderer;
    Body body;
    OrthographicCamera camera;

    float scene_width = 12.8f;
    float scene_height = 7.2f;

    Stage stage;
    SpriteBatch batch;
    Label forceLabel, impulseLabel, velocityLabel;
    Image image;
    BitmapFont font;

    @Override
    public void create() {
        world = new World(new Vector2(0.0f, -9.8f), true);
        box2DDebugRenderer = new Box2DDebugRenderer();

        image = new Image(new Texture("badlogic.jpg"));
        image.setSize(1.0f, 1.0f);
//        image.setOrigin(image.getWidth() / 2.0f, image.getHeight() / 2.0f);

        BodyDef bodyDef = new BodyDef();
        bodyDef.type = BodyDef.BodyType.DynamicBody;
        bodyDef.position.set(3, 3);
        body = world.createBody(bodyDef);
        body.setUserData(image);

        PolygonShape polygonShape = new PolygonShape();
        polygonShape.setAsBox(0.5f, 0.5f);

        FixtureDef fixtureDef = new FixtureDef();
        fixtureDef.shape = polygonShape;
        fixtureDef.density = 1.0f;
        fixtureDef.restitution = 1.0f;
        fixtureDef.friction = 0.0f;
        body.createFixture(fixtureDef);
        polygonShape.dispose();

        createGround();

        camera = new OrthographicCamera(scene_width, scene_height);
        camera.position.set(scene_width / 2, scene_height / 2, 0);
        camera.update();

        initStageUI();
    }

    @Override
    public void resize(int width, int height) {

    }

    private void initStageUI() {
        stage = new Stage();
        batch = new SpriteBatch();
        font = new BitmapFont();

        Label.LabelStyle style = new Label.LabelStyle();
        style.font = font;
        forceLabel = new Label("Force",style);
        forceLabel.setPosition(520, 440);
        impulseLabel = new Label("Impulse", style);
        impulseLabel.setPosition(520, 400);
        velocityLabel = new Label("LinearVelocity", style);
        velocityLabel.setPosition(520, 360);

        forceLabel.addListener(new ClickListener() {
            @Override
            public void clicked(InputEvent event, float x, float y) {
                forceLabel.setColor(Color.RED);
                impulseLabel.setColor(Color.WHITE);
                velocityLabel.setColor(Color.WHITE);
                // 给物理施加100N的力
                body.applyForceToCenter(new Vector2(100f, 0), true);
            }
        });
        impulseLabel.addListener(new ClickListener() {
            @Override
            public void clicked(InputEvent event, float x, float y) {
                impulseLabel.setColor(Color.RED);
                forceLabel.setColor(Color.WHITE);
                velocityLabel.setColor(Color.WHITE);
                body.applyLinearImpulse(new Vector2(0.8f, 0), body.getWorldCenter(), true);
            }
        });
        velocityLabel.addListener(new ClickListener() {
            @Override
            public void clicked(InputEvent event, float x, float y) {
                velocityLabel.setColor(Color.RED);
                forceLabel.setColor(Color.WHITE);
                impulseLabel.setColor(Color.WHITE);
                // 设置线性速度
                body.setLinearVelocity(new Vector2(1.2f, 0));
            }
        });

        stage.addActor(forceLabel);
        stage.addActor(impulseLabel);
        stage.addActor(velocityLabel);

        Gdx.input.setInputProcessor(stage);
    }

    @Override
    public void render() {
        world.step( 1/ 60f, 6, 2);

        Gdx.gl.glClearColor(0.39f, 0.58f, 0.92f, 1.0f);
        Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);

        box2DDebugRenderer.render(world, camera.combined);

        batch.setProjectionMatrix(camera.combined);
        // Image的开始坐标和Body的坐标原点不一样，需要手动调整一下
        image.setPosition(body.getPosition().x - image.getWidth() * 0.5f,
                    body.getPosition().y - image.getHeight() * 0.5f);

        batch.begin();
        image.draw(batch, 1.0f);
        batch.end();

        stage.act();
        stage.draw();

        Gdx.app.log("MoveBody", "Velocity.x=" + body.getLinearVelocity().x);
    }

    @Override
    public void dispose() {
        batch.dispose();
        stage.dispose();
        font.dispose();
        world.dispose();
        box2DDebugRenderer.dispose();
    }

    private void createGround() {
        BodyDef bodyDef = new BodyDef();
        bodyDef.position.set(scene_width * 0.5f, 0.2f);
        Body body1 = world.createBody(bodyDef);

        PolygonShape polygonShape = new PolygonShape();
        polygonShape.setAsBox(scene_width * 0.5f, 0.2f);
        body1.createFixture(polygonShape, 0.0f);

        bodyDef.position.set(0.4f, scene_height * 0.5f);
        Body body2 = world.createBody(bodyDef);

        polygonShape.setAsBox(0.2f, scene_height * 0.5f);
        body2.createFixture(polygonShape, 0);

        bodyDef.position.set(12.4f, scene_height * 0.5f);
        Body body3 = world.createBody(bodyDef);

        polygonShape.setAsBox(0.2f, scene_height * 0.5f);
        body3.createFixture(polygonShape, 0);
    }

}