Update docs and add example

doc/box2d.md

@@ -1,7 +1,16 @@
-# Box2d
+# Box2D
 
 Although Flame does not implements Box2d itself, it bundles a forked port of the Java Box2d to Dart by Google.
 
 The source of the bundled box2d on Flame can be found [here](https://github.com/feroult/box2d.dart).
 
 There is also a simple example game that can be used as reference of how to use box2d on Flame [here](https://github.com/feroult/haunt).
+
+## BaseGame extension
+
+If you are going to use Box2D in your project it can be a good idea to use the Box2D specific
+extension of the `BaseGame` class. It is called `Box2DGame` and it will control the adding and
+removal of Box2D's BodyComponents from your root `Box2DComponent`.
+
+A simple `Box2DGame` implementation example can be seen in the [examples folder](./examples/box2d).
+

doc/examples/box2d/.gitignore

@@ -0,0 +1,70 @@
+# Miscellaneous
+*.class
+*.log
+*.pyc
+*.swp
+.DS_Store
+.atom/
+.buildlog/
+.history
+.svn/
+
+# IntelliJ related
+*.iml
+*.ipr
+*.iws
+.idea/
+
+# Visual Studio Code related
+.vscode/
+
+# Flutter/Dart/Pub related
+**/doc/api/
+.dart_tool/
+.flutter-plugins
+.packages
+.pub-cache/
+.pub/
+/build/
+
+# Android related
+**/android/**/gradle-wrapper.jar
+**/android/.gradle
+**/android/captures/
+**/android/gradlew
+**/android/gradlew.bat
+**/android/local.properties
+**/android/**/GeneratedPluginRegistrant.java
+
+# iOS/XCode related
+**/ios/**/*.mode1v3
+**/ios/**/*.mode2v3
+**/ios/**/*.moved-aside
+**/ios/**/*.pbxuser
+**/ios/**/*.perspectivev3
+**/ios/**/*sync/
+**/ios/**/.sconsign.dblite
+**/ios/**/.tags*
+**/ios/**/.vagrant/
+**/ios/**/DerivedData/
+**/ios/**/Icon?
+**/ios/**/Pods/
+**/ios/**/.symlinks/
+**/ios/**/profile
+**/ios/**/xcuserdata
+**/ios/.generated/
+**/ios/Flutter/App.framework
+**/ios/Flutter/Flutter.framework
+**/ios/Flutter/Generated.xcconfig
+**/ios/Flutter/app.flx
+**/ios/Flutter/app.zip
+**/ios/Flutter/flutter_assets/
+**/ios/ServiceDefinitions.json
+**/ios/Runner/GeneratedPluginRegistrant.*
+
+# Exceptions to above rules.
+!**/ios/**/default.mode1v3
+!**/ios/**/default.mode2v3
+!**/ios/**/default.pbxuser
+!**/ios/**/default.perspectivev3
+!/packages/flutter_tools/test/data/dart_dependencies_test/**/.packages

doc/examples/box2d/.metadata

@@ -0,0 +1,10 @@
+# This file tracks properties of this Flutter project.
+# Used by Flutter tool to assess capabilities and perform upgrades etc.
+#
+# This file should be version controlled and should not be manually edited.
+
+version:
+  revision: 7fc14a55af64462763d28abfb4e610086c6e0f39
+  channel: dev
+
+project_type: app

doc/examples/box2d/README.md

@@ -0,0 +1,3 @@
+# box2d
+
+A Flame game showcasing how to use box2d together with flame.

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;
+}
+

doc/examples/box2d/pubspec.yaml

@@ -0,0 +1,17 @@
+name: box2d
+description: Flame sample game showcasing the structure for box2d games
+
+version: 1.0.0+1
+
+environment:
+  sdk: ">=2.1.0 <3.0.0"
+
+dependencies:
+  flutter:
+    sdk: flutter
+  flame:
+    path: ../../../
+
+dev_dependencies:
+  flutter_test:
+    sdk: flutter

lib/box2d/box2d_game.dart

@@ -1,3 +1,5 @@
+import 'dart:ui';
+
 import 'package:flame/box2d/box2d_component.dart';
 import 'package:flame/components/component.dart';
 import 'package:flame/game.dart';
@@ -10,27 +12,36 @@ class Box2DGame extends BaseGame {
 
   @override
   void add(Component c) {
-    super.add(c);
     if(c is BodyComponent) {
       box.add(c);
+    } else {
+      super.add(c);
     }
   }
 
   @override
   void addLater(Component c) {
-    super.addLater(c);
     if(c is BodyComponent) {
       _addLater.add(c);
+    } else {
+      super.addLater(c);
     }
   }
 
   @override
   void update(double t) {
     super.update(t);
-    components
+    box.update(t);
-        .where((c) => c is BodyComponent && c.destroy())
+    box.components
-        .forEach((body) => box.remove(body));
+        .where((c) => c.destroy()).toList()
+        .forEach((c) => box.remove(c));
     box.addAll(_addLater);
     _addLater.clear();
   }
+
+  @override
+  void render(Canvas c) {
+    super.render(c);
+    box.render(c);
+  }
 }