goo2d_physics_box2d

Box2D v3 physics for goo2d. Bodies and colliders are declared on an entity like any other component, and the world is stepped inside the game isolate in step with the fixed tick.

flutter pub add goo2d_physics_box2d

A falling crate

import 'package:goo2d_physics_box2d/goo2d_physics_box2d.dart';

class Crate extends EntityStruct
    with Transform2D, WorldTransform2D, Renderable2D, Collider2D, RigidBody2D {
  late final BoxBody box;
  late final Sprite sprite;

  @override
  void describeCollider(ColliderDescriptor descriptor) {
    super.describeCollider(descriptor);
    box = descriptor.hasBoxCollider(
      halfWidth: 0.5, halfHeight: 0.5, friction: 0.4,
    );
  }

  @override
  void describeSprites(SpriteDescriptor descriptor) {
    super.describeSprites(descriptor);
    sprite = descriptor.has(width: 1, height: 1, color: 0xFFCC8844);
  }
}

Nothing here says the crate is dynamic, because that is where RigidBody2D starts. A prefab configures its body by moving the column defaults it wants different, and only those:

class Ground extends EntityStruct with Transform2D, Collider2D, RigidBody2D {
  @override
  void describeStruct(DataDescriptor data) {
    super.describeStruct(data);
    bodyType.defaultValue = BodyType2D.staticBody;
  }
}

Declare the system once, and that is the whole opt-in. Bodies appear as entities spawn and go away as they despawn:

@override
void describeSystems(SystemDescriptor descriptor) {
  super.describeSystems(descriptor);
  descriptor.has(Box2DPhysicsSystem(gravityY: 10));
}

Push a body around, or change what kind of body it is:

final body = entity.get<RigidBody2D>();
body.applyImpulse(entity, 5, 0);
body.setVelocity(entity, 3, 0);
body.bodyType[entity] = BodyType2D.staticBody;

Work in metres. Box2D is tuned for objects roughly 0.1 m to 10 m. Treating one world unit as one pixel gives a 32-pixel crate the mass of a 32-metre building, which is usually reported as "the physics feels floaty". Apply the pixels-per-metre scale once, at the rendering edge.

Next

• Physics covers the nine joints, effectors, raycasts and overlap queries.

Bodies, colliders, all nine joints, effectors and queries work today, with the solver spread across worker threads.