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

import 'dart:math' as math;
import 'package:flame/box2d/box2d_component.dart';
import 'package:flame/box2d/box2d_game.dart';
import 'package:flame/box2d/contact_callbacks.dart';
import 'package:flame/flame.dart';
import 'package:flame/palette.dart';
import 'package:flutter/material.dart';
import 'package:box2d_flame/box2d.dart';

import 'boundaries.dart';

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Flame.util.fullScreen();
  final MyBox2D box = MyBox2D();
  final MyGame game = MyGame(box);
  runApp(game.widget);
}

class Ball extends BodyComponent {
  Paint originalPaint, currentPaint;
  bool giveNudge = false;

  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(100 + rng.nextInt(155),
            100 + rng.nextInt(155), 100 + rng.nextInt(155), 255))
        .paint;
  }

  void _createBody(double radius, Vector2 position) {
    final CircleShape shape = CircleShape();
    shape.radius = radius;

    final fixtureDef = FixtureDef()
      ..shape = shape
      ..restitution = 1.0
      ..density = 1.0
      ..friction = 0.1;

    final bodyDef = BodyDef()
      // To be able to determine object in collision
      ..setUserData(this)
      ..position = position
      ..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);
  }

  @override
  void update(double t) {
    super.update(t);
    if (giveNudge) {
      body.applyLinearImpulse(Vector2(0, 10000), body.getLocalCenter(), true);
      giveNudge = false;
    }
  }
}

class WhiteBall extends Ball {
  WhiteBall(Vector2 position, Box2DComponent box) : super(position, box) {
    originalPaint = BasicPalette.white.paint;
    currentPaint = originalPaint;
  }
}

class BallContactCallback extends ContactCallback<Ball, Ball> {
  @override
  void begin(Ball ball1, Ball ball2, Contact contact) {
    if (ball1 is WhiteBall || ball2 is WhiteBall) {
      return;
    }
    if (ball1.currentPaint != ball1.originalPaint) {
      ball1.currentPaint = ball2.currentPaint;
    } else {
      ball2.currentPaint = ball1.currentPaint;
    }
  }

  @override
  void end(Ball ball1, Ball ball2, Contact contact) {}
}

class WhiteBallContactCallback extends ContactCallback<Ball, WhiteBall> {
  @override
  void begin(Ball ball, WhiteBall whiteBall, Contact contact) {
    ball.giveNudge = true;
  }

  @override
  void end(Ball ball, WhiteBall whiteBall, Contact contact) {}
}

class BallWallContactCallback extends ContactCallback<Ball, Wall> {
  @override
  void begin(Ball ball, Wall wall, Contact contact) {
    wall.paint = ball.currentPaint;
  }

  @override
  void end(Ball ball1, Wall wall, Contact contact) {}
}

class MyGame extends Box2DGame {
  MyGame(Box2DComponent box) : super(box) {
    final List<BodyComponent> boundaries = createBoundaries(box);
    boundaries.forEach(add);
    addContactCallback(BallContactCallback());
    addContactCallback(BallWallContactCallback());
    addContactCallback(WhiteBallContactCallback());
  }

  @override
  void onTapDown(TapDownDetails details) {
    super.onTapDown(details);
    final Vector2 position =
        Vector2(details.globalPosition.dx, details.globalPosition.dy);
    if (math.Random().nextInt(10) < 2) {
      add(WhiteBall(position, box));
    } else {
      add(Ball(position, box));
    }
  }
}

class MyBox2D extends Box2DComponent {
  MyBox2D() : super(scale: 4.0, gravity: -10.0);

  @override
  void initializeWorld() {}
}