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.

There is a simple example game here that can be used as reference of how to use box2d on Flame, although it is a little bit outdated and doesn't use Box2DGame.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, 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.

Contact callbacks

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

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:

final bodyDef = BodyDef()
  // To be able to determine object in collision

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 BodyComponents named Ball and Wall and you want to do something when they come in contact you would do like this:

class BallWallCallback implements ContactCallback<Ball, Wall> {

  void begin(Ball ball, Wall wall, Contact contact) {

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

and then you simply add BallWallCallback to your Box2DGame:

class MyGame extends Box2DGame {
  MyGame(Box2DComponent box) : super(box) {

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.