Example showcasing how to use the ContactCallback

doc/examples/box2d/contact_callbacks/lib/main.dart

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+import 'dart:math' as math;
+import 'package:box2d_contact_callbacks/boundaries.dart';
+import 'package:flame/box2d/box2d_component.dart';
+import 'package:flame/box2d/box2d_game.dart';
+import 'package:flame/box2d/contact_listeners.dart';
+import 'package:flame/flame.dart';
+import 'package:flame/palette.dart';
+import 'package:flutter/material.dart';
+import 'package:box2d_flame/box2d.dart';
+
+void main() async {
+  WidgetsFlutterBinding.ensureInitialized();
+  await Flame.util.fullScreen();
+  runApp(GameController().widget);
+}
+
+class Ball extends BodyComponent {
+  Paint originalPaint, currentPaint;
+
+  Ball(Vector2 position, Box2DComponent box) : super(box) {
+    originalPaint = _randomPaint();
+    currentPaint = originalPaint;
+    Vector2 worldPosition = viewport.getScreenToWorld(position);
+    _createBody(5.0, worldPosition);
+  }
+
+  Paint _randomPaint() {
+    math.Random rng = math.Random();
+    return PaletteEntry(Color.fromARGB(rng.nextInt(255), rng.nextInt(255),
+        rng.nextInt(255), rng.nextInt(255))).paint;
+  }
+
+  void _createBody(double radius, Vector2 position) {
+    final CircleShape shape = CircleShape();
+    shape.radius = radius;
+
+    final fixtureDef = FixtureDef();
+    // To be able to determine object in collision
+    fixtureDef.setUserData(this);
+    fixtureDef.shape = shape;
+    fixtureDef.restitution = 1.0;
+    fixtureDef.density = 1.0;
+    fixtureDef.friction = 0.1;
+
+    final bodyDef = BodyDef();
+    bodyDef.position = position;
+    bodyDef.type = BodyType.DYNAMIC;
+
+    body = world.createBody(bodyDef)..createFixtureFromFixtureDef(fixtureDef);
+  }
+
+  @override
+  bool destroy() {
+    // Implement your logic for when the component should be removed
+    return false;
+  }
+
+  @override
+  void renderCircle(Canvas c, Offset p, double radius) {
+    Paint blue = PaletteEntry(Colors.blue).paint;
+    c.drawCircle(p, radius, currentPaint);
+    double angle = body.getAngle();
+    Offset lineRotation =
+        Offset(math.sin(angle) * radius, math.cos(angle) * radius);
+    c.drawLine(p, p + lineRotation, blue);
+  }
+}
+
+class BallContactCallback implements ContactCallback {
+  @override
+  List<Type> objects = [Ball, Ball];
+
+  BallContactCallback();
+
+  @override
+  void begin(Object contact1, Object contact2) {
+    Ball ball1 = contact1 as Ball;
+    Ball ball2 = contact2 as Ball;
+    if (ball1.currentPaint != ball1.originalPaint) {
+      ball1.currentPaint = ball2.currentPaint;
+    } else {
+      ball2.currentPaint = ball1.currentPaint;
+    }
+  }
+
+  @override
+  void end(Object ship, Object material) {}
+}
+
+class MyGame extends Box2DGame {
+  MyGame(Box2DComponent box) : super(box) {
+    final List<BodyComponent> boundaries = createBoundaries(box);
+    boundaries.forEach(add);
+    addContactCallback(BallContactCallback());
+  }
+
+  @override
+  void onTapDown(TapDownDetails details) {
+    super.onTapDown(details);
+    final Vector2 position =
+        Vector2(details.globalPosition.dx, details.globalPosition.dy);
+    add(Ball(position, box));
+  }
+}
+
+class MyBox2D extends Box2DComponent {
+  MyBox2D() : super(scale: 4.0, gravity: -10.0);
+
+  @override
+  void initializeWorld() {}
+}
+
+class GameController {
+  MyGame _game;
+
+  GameController() {
+    final MyBox2D box = MyBox2D();
+    _game = MyGame(box);
+  }
+
+  Widget get widget => _game.widget;
+}