Update docs and add example

doc/examples/box2d/lib/main.dart

@@ -0,0 +1,96 @@
+import 'dart:math' as math;
+import 'package:flame/box2d/box2d_component.dart';
+import 'package:flame/box2d/box2d_game.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 {
+  await Flame.util.fullScreen();
+  runApp(GameController().widget);
+}
+
+class MyPlanet extends BodyComponent {
+  double totalTime = 0;
+  // Creates a BodyComponent that renders a red circle (with a black moving
+  // pulsating circle on the inside) that can interact with other body
+  // components that are added to the same Box2DGame/Box2DComponent.
+  // After 20 seconds the circle will be removed, to show that we don't get
+  // any concurrent modification exceptions.
+  MyPlanet(Box2DComponent box) : super(box) {
+    Vector2 leftCorner = viewport.getScreenToWorld(Vector2.zero());
+    _createBody(50.0, leftCorner);
+  }
+
+  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 = 0.0;
+    fixtureDef.density = 1.0;
+    fixtureDef.friction = 0.1;
+
+    final bodyDef = BodyDef();
+    bodyDef.position = position;
+    bodyDef.angularVelocity = 4.0;
+    bodyDef.type = BodyType.DYNAMIC;
+
+    this.body = world.createBody(bodyDef)
+      ..createFixtureFromFixtureDef(fixtureDef);
+  }
+
+  @override
+  bool destroy() {
+    // Implement your logic for when the component should be removed
+    return totalTime > 20;
+  }
+
+  @override
+  void renderCircle(Canvas c, Offset p, double radius) {
+    Paint red = PaletteEntry(Colors.red).paint;
+    Paint black = PaletteEntry(Colors.black).paint;
+    Paint blue = PaletteEntry(Colors.blue).paint;
+    c.drawCircle(p, radius, red);
+    double angle = body.getAngle();
+    c.drawCircle(p, math.sin(angle)*radius, black);
+    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);
+    totalTime += t;
+  }
+}
+
+class MyGame extends Box2DGame {
+  MyGame(Box2DComponent box) : super(box) {
+    add(MyPlanet(box));
+  }
+}
+
+class MyBox2D extends Box2DComponent {
+  MyBox2D() : super(scale: 4.0, gravity: 0.0);
+
+  @override
+  void initializeWorld() {}
+}
+
+class GameController {
+  MyGame _game;
+
+  GameController() {
+    final MyBox2D box = MyBox2D();
+    _game = MyGame(box);
+    _game.add(MyPlanet(box));
+  }
+
+  Widget get widget => _game.widget;
+}
+