refactor: Don't use static for Distance and Collision (#92)

# Description

This removes the use of `static` for `Collision` and `Distance` so that
multiple worlds can be used simultaneously and old stuff is GCed
properly.

## Checklist

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- [x] I have read the [Contributor Guide] and followed the process
outlined for submitting PRs.
- [x] I have updated/added tests for ALL new/updated/fixed
functionality.
- [x] I have updated/added relevant documentation in `docs` and added
dartdoc comments with `///`.
- [x] I have updated/added relevant examples in `examples`.

## Breaking Change

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packages/forge2d/example/web/ball_cage.dart

@@ -16,14 +16,6 @@ class BallCage extends Demo {
   /// Constructs a new BallCage.
   BallCage() : super('Ball cage');
 
-  /// Entrypoint.
-  static void main() {
-    final cage = BallCage();
-    cage.initialize();
-    cage.initializeAnimation();
-    cage.runAnimation();
-  }
-
   @override
   void initialize() {
     // Define the circle shape.
@@ -91,5 +83,8 @@ class BallCage extends Demo {
 }
 
 void main() {
-  BallCage.main();
+  final cage = BallCage();
+  cage.initialize();
+  cage.initializeAnimation();
+  cage.runAnimation();
 }

packages/forge2d/example/web/blob_test.dart

@@ -8,14 +8,6 @@ class BlobTest extends Demo {
   /// Constructs a new BlobTest.
   BlobTest() : super('Blob test');
 
-  /// Entrypoint.
-  static void main() {
-    final blob = BlobTest();
-    blob.initialize();
-    blob.initializeAnimation();
-    blob.runAnimation();
-  }
-
   @override
   void initialize() {
     Body ground;
@@ -79,5 +71,8 @@ class BlobTest extends Demo {
 }
 
 void main() {
-  BlobTest.main();
+  final blob = BlobTest();
+  blob.initialize();
+  blob.initializeAnimation();
+  blob.runAnimation();
 }

packages/forge2d/example/web/box_test.dart

@@ -8,14 +8,6 @@ class BoxTest extends Demo {
   /// Constructs a new BoxTest.
   BoxTest() : super('Box test');
 
-  /// Entrypoint.
-  static void main() {
-    final boxTest = BoxTest();
-    boxTest.initialize();
-    boxTest.initializeAnimation();
-    boxTest.runAnimation();
-  }
-
   @override
   void initialize() {
     _createGround();
@@ -69,5 +61,8 @@ class BoxTest extends Demo {
 }
 
 void main() {
-  BoxTest.main();
+  final boxTest = BoxTest();
+  boxTest.initialize();
+  boxTest.initializeAnimation();
+  boxTest.runAnimation();
 }

packages/forge2d/example/web/domino_tower.dart

@@ -17,14 +17,6 @@ class DominoTower extends Demo {
   /// Construct a DominoTower.
   DominoTower() : super('Domino tower');
 
-  /// Entrypoint.
-  static void main() {
-    final tower = DominoTower();
-    tower.initialize();
-    tower.initializeAnimation();
-    tower.runAnimation();
-  }
-
   void makeDomino(double x, double y, {required bool horizontal}) {
     final shape = PolygonShape()
       ..setAsBoxXY(.5 * dominoWidth, .5 * dominoHeight);
@@ -142,5 +134,8 @@ class DominoTower extends Demo {
 }
 
 void main() {
-  DominoTower.main();
+  final tower = DominoTower();
+  tower.initialize();
+  tower.initializeAnimation();
+  tower.runAnimation();
 }

packages/forge2d/example/web/friction_joint_test.dart

@@ -7,15 +7,6 @@ import 'demo.dart';
 class FrictionJointTest extends Demo {
   FrictionJointTest() : super('FrictionJoint test');
 
-  /// Entrypoint.
-  static void main() {
-    final test = FrictionJointTest();
-    test.initialize();
-    test.initializeAnimation();
-    test.debugDraw.appendFlags(DebugDraw.jointBit);
-    test.runAnimation();
-  }
-
   late Body _ground;
   late FixtureDef _boxFixture;
 
@@ -100,5 +91,9 @@ class FrictionJointTest extends Demo {
 }
 
 void main() {
-  FrictionJointTest.main();
+  final test = FrictionJointTest();
+  test.initialize();
+  test.initializeAnimation();
+  test.debugDraw.appendFlags(DebugDraw.jointBit);
+  test.runAnimation();
 }

packages/forge2d/example/web/particles.dart

@@ -15,14 +15,6 @@ class Particles extends Demo {
 
   /// Constructs a new Particles example.
   Particles() : super('Particles');
-
-  static void main() {
-    Particles()
-      ..initialize()
-      ..initializeAnimation()
-      ..runAnimation();
-  }
-
   @override
   void initialize() {
     // Define the circle shape.
@@ -103,5 +95,8 @@ class Particles extends Demo {
 }
 
 void main() {
-  Particles.main();
+  Particles()
+    ..initialize()
+    ..initializeAnimation()
+    ..runAnimation();
 }

packages/forge2d/example/web/racer.dart

@@ -10,16 +10,6 @@ import 'racer/ground_area.dart';
 import 'racer/tire.dart';
 
 class Racer extends Demo implements ContactListener {
-  static void main() {
-    final racer = Racer();
-    racer.initialize();
-    racer.initializeAnimation();
-    final paragraph = ParagraphElement()
-      ..innerText = 'Use the arrow keys to drive the car';
-    document.body?.nodes.add(paragraph);
-    racer.runAnimation();
-  }
-
   Racer() : super('Racer', Vector2.zero(), 2.5);
 
   late int _controlState;
@@ -186,5 +176,11 @@ class Racer extends Demo implements ContactListener {
 }
 
 void main() {
-  Racer.main();
+  final racer = Racer();
+  racer.initialize();
+  racer.initializeAnimation();
+  final paragraph = ParagraphElement()
+    ..innerText = 'Use the arrow keys to drive the car';
+  document.body?.nodes.add(paragraph);
+  racer.runAnimation();
 }

packages/forge2d/lib/src/collision/collision.dart

@@ -98,6 +98,7 @@ class Collision {
     int indexB,
     Transform xfA,
     Transform xfB,
+    Distance distance,
   ) {
     _input.proxyA.set(shapeA, indexA);
     _input.proxyB.set(shapeB, indexB);
@@ -107,7 +108,7 @@ class Collision {
 
     _cache.count = 0;
 
-    World.distance.compute(_output, _cache, _input);
+    distance.compute(_output, _cache, _input);
     // djm note: anything significant about 10.0f?
     return _output.distance < 10.0 * settings.epsilon;
   }

packages/forge2d/lib/src/collision/time_of_impact.dart

@@ -53,7 +53,7 @@ class TimeOfImpact {
   /// If you change the time interval, you should call this function again.
   /// Note: use Distance to compute the contact point and normal at the time of
   /// impact.
-  void timeOfImpact(TOIOutput output, TOIInput input) {
+  void timeOfImpact(TOIOutput output, TOIInput input, Distance distance) {
     // CCD via the local separating axis method. This seeks progression
     // by computing the largest time at which separation is maintained.
 
@@ -100,7 +100,7 @@ class TimeOfImpact {
       // to get a separating axis
       _distanceInput.transformA = _xfA;
       _distanceInput.transformB = _xfB;
-      World.distance.compute(_distanceOutput, _cache, _distanceInput);
+      distance.compute(_distanceOutput, _cache, _distanceInput);
 
       // If the shapes are overlapped, we give up on continuous collision.
       if (_distanceOutput.distance <= 0.0) {

packages/forge2d/lib/src/dynamics/contact_manager.dart

@@ -3,11 +3,13 @@ import 'package:forge2d/forge2d.dart';
 /// Delegate of World.
 class ContactManager implements PairCallback {
   BroadPhase broadPhase;
+  final Collision collision;
+  final Distance distance;
   final List<Contact> contacts = [];
   ContactFilter? contactFilter;
   ContactListener? contactListener;
 
-  ContactManager(this.broadPhase) {
+  ContactManager(this.broadPhase, this.collision, this.distance) {
     contactFilter = ContactFilter();
   }
 
@@ -51,7 +53,14 @@ class ContactManager implements PairCallback {
       return;
     }
 
-    final contact = Contact.init(fixtureA, indexA, fixtureB, indexB);
+    final contact = Contact.fromPair(
+      fixtureA,
+      indexA,
+      fixtureB,
+      indexB,
+      collision,
+      distance,
+    );
 
     // Insert into the world.
     contacts.add(contact);

packages/forge2d/lib/src/dynamics/contacts/chain_and_circle_contact.dart

@@ -6,14 +6,16 @@ class ChainAndCircleContact extends Contact {
     super.indexA,
     super.fixtureB,
     super.indexB,
+    super.collision,
+    super.distance,
   )   : assert(fixtureA.type == ShapeType.chain),
         assert(fixtureB.type == ShapeType.circle);
 
   @override
   void evaluate(Manifold manifold, Transform xfA, Transform xfB) {
     final chain = fixtureA.shape as ChainShape;
     final edge = chain.childEdge(indexA);
-    World.collision.collideEdgeAndCircle(
+    collision.collideEdgeAndCircle(
       manifold,
       edge,
       xfA,

packages/forge2d/lib/src/dynamics/contacts/chain_and_polygon_contact.dart

@@ -6,14 +6,16 @@ class ChainAndPolygonContact extends Contact {
     super.indexA,
     super.fixtureB,
     super.indexB,
+    super.collision,
+    super.distance,
   )   : assert(fixtureA.type == ShapeType.chain),
         assert(fixtureB.type == ShapeType.polygon);
 
   @override
   void evaluate(Manifold manifold, Transform xfA, Transform xfB) {
     final chain = fixtureA.shape as ChainShape;
     final edge = chain.childEdge(indexA);
-    World.collision.collideEdgeAndPolygon(
+    collision.collideEdgeAndPolygon(
       manifold,
       edge,
       xfA,

packages/forge2d/lib/src/dynamics/contacts/circle_contact.dart

@@ -1,14 +1,18 @@
 import 'package:forge2d/forge2d.dart';
 
 class CircleContact extends Contact {
-  CircleContact(Fixture fixtureA, Fixture fixtureB)
-      : assert(fixtureA.type == ShapeType.circle),
+  CircleContact(
+    Fixture fixtureA,
+    Fixture fixtureB,
+    Collision collision,
+    Distance distance,
+  )   : assert(fixtureA.type == ShapeType.circle),
         assert(fixtureB.type == ShapeType.circle),
-        super(fixtureA, 0, fixtureB, 0);
+        super(fixtureA, 0, fixtureB, 0, collision, distance);
 
   @override
   void evaluate(Manifold manifold, Transform xfA, Transform xfB) {
-    World.collision.collideCircles(
+    collision.collideCircles(
       manifold,
       fixtureA.shape as CircleShape,
       xfA,