# Box2D

Although Flame does not implements Box2d itself, it bundles a forked port of the Java Box2d to Dart by Google.

The source of the bundled box2d on Flame can be found [here](https://github.com/flame-engine/box2d.dart).

There is a simple example game [here](https://github.com/feroult/haunt) that can be used as reference of how to use box2d on Flame, although it is a little bit outdated and doesn't use `Box2DGame`.

There are some updated examples of how to use it [here](/doc/examples/box2d), but they are not full game implementations.

## BaseGame extension

If you are going to use Box2D in your project it can be a good idea to use the Box2D specific extension of the `BaseGame` class.

It is called `Box2DGame` and it will control the adding and removal of Box2D's BodyComponents from your root `Box2DComponent`.

A simple `Box2DGame` implementation example can be seen in the [examples folder](/doc/examples/box2d/simple).

## Contact callbacks

If you are using `Box2DGame` you can take advantage of its way of handling contacts between two `BodyComponent`s.

When creating the body definition for your `BodyComponent` make sure that you set the userdata to the current object, otherwise it will not be possible to detect collisions.
Like this:
```dart
final bodyDef = BodyDef()
  // To be able to determine object in collision
  ..setUserData(this);
```

Now you have to make an implementation of `ContactCallback` where you set which two types that it should react when they come in contact.
If you have two `BodyComponent`s named Ball and Wall and you want to do something when they come in contact you would do like this:

```dart
class BallWallCallback implements ContactCallback<Ball, Wall> {
  BallWallCallback();

  @override
  void begin(Ball ball, Wall wall, Contact contact) {
    wall.remove();
  }

  @override
  void end(Ball ball, Wall wall, Contact contact) {}
}
```

and then you simply add `BallWallCallback` to your `Box2DGame`:

```dart
class MyGame extends Box2DGame {
  MyGame(Box2DComponent box) : super(box) {
    addContactCallback(BallWallCallback());
  }
}
```

Every time `Ball` and `Wall` gets in contact `begin` will be called, and once the objects stop being in contact `end` will be called.

If you want an object to interact with all other bodies, put `BodyComponent` as the one of the parameters of your `ContactCallback` like this:

`class BallAnythingCallback implements ContactCallback<Ball, BodyComponent> ...`

An implementation example can be seen in the [examples folder](/doc/examples/box2d/contact_callbacks).