Bodies and sprites

This article will teach us how to draw sprites on top of our Forge2D bodies.

Let's start by creating a Forge2D scene in which a box or a ball will appear at the top of the screen each time we click the screen.

We start by creating the class Box:

class Box extends BodyComponent {

  @override
  Body createBody() {
    final bodyDef = BodyDef(
      position: Vector2(worldSize.x / 2, 0),
      type: BodyType.dynamic,
    );

    final shape = PolygonShape()..setAsBoxXY(.25, .25);
    final fixtureDef = FixtureDef(shape)
      ..density = 5
      ..friction = .5
      ..restitution = .5;
    return world.createBody(bodyDef)
      ..createFixture(fixtureDef)
      ..angularVelocity = radians(180);
  }
}

Notice that we added an angularVelocity so the box will rotate on its axis. Everything else is what we have been doing in previous tutorials.

Now let's create a class Ball:

class Ball extends BodyComponent {

  @override
  Body createBody() {
    final bodyDef = BodyDef(
      position: Vector2(worldSize.x / 2, 0),
      type: BodyType.dynamic,
    );

    final shape = CircleShape()..radius = .25;
    final fixtureDef = FixtureDef(shape)
      ..density = 5
      ..friction = .5
      ..restitution = .5;
    return world.createBody(bodyDef)
      ..createFixture(fixtureDef)
      ..angularVelocity = radians(180);
  }
}

Again, there is nothing special in this class. This ball will be rotating on its axis too.

Every time we click the screen, we want to add a ball or box randomly. Let's add this code:

class GameLesson05 extends MyGame with TapDetector {
  @override
  Future<void> onLoad() async {
    super.onLoad();
    add(Floor());
  }

  @override
  void onTapDown(TapDownInfo info) {
    super.onTapDown(info);
    if (Random().nextBool()) {
      add(Ball());
    } else {
      add(Box());
    }
  }
}

If we run the previous code, the result is this:

Adding sprites

Let's make the scene of white boxes and balls more fun. First, we will add these two images to the assets folder:

Box and ball sprites

Then, we update the pubspec.yaml file to add the assets' path.

  assets:
    - assets/images/

We will load the sprites and store them in the cache. On the class GameLesson05, we add this code to the onLoad() function:

  Future<void> onLoad() async {
    super.onLoad();
    await loadSprite('ball.png');
    await loadSprite('box.png');

    add(Floor());
  }

Now we must retrieve the loaded sprites from the cache and add them to the box and the ball.

Let's start by overriding the onLoad() function from the Ball class:

  @override
  Future<void> onLoad() async {
    await super.onLoad();
    final sprite = Sprite(gameRef.images.fromCache('ball.png'));
    add(
      SpriteComponent(
        sprite: sprite,
        size: Vector2(.5, .5),
        anchor: Anchor.center,
      ),
    );
  }

The previous code retrieves the image from the cache and creates a new Sprite, then creates a new SpriteComponent that will be in charge of drawing the image on top of the ball.

Updating the box code is very similar to the ball code. Let's take a look:

  @override
  Future<void> onLoad() async {
    await super.onLoad();
    final sprite = Sprite(gameRef.images.fromCache('box.png'));
    add(
      SpriteComponent(
        sprite: sprite,
        size: Vector2(.5, .5),
        anchor: Anchor.center,
      ),
    );
  }

Running the updated code will give us the following result:

Looks better right? Notice how Flame draws the sprites on top of the bodies, including updating the position, rotation, etc.

info

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Adding sprites​

Adding sprites