Adds support for zOrder and references to parent nodes in sprites

sky/examples/game/lib/game_tests.dart

+part of game;
+
+Math.Random _rand;
+
+class GameTests extends TransformNode{
+
+  Image _imgAsteroid;
+  Image _imgBg;
+  Image _imgShip;
+
+  GameTests(ImageMap images) {
+    // Setup random number generator
+    _rand = new Math.Random();
+
+    // Fetch images
+    _imgBg = images["https://raw.githubusercontent.com/slembcke/GalacticGuardian.spritebuilder/GDC/Packages/SpriteBuilder%20Resources.sbpack/resources-auto/BurnTexture.png"];
+    _imgAsteroid = images["https://raw.githubusercontent.com/slembcke/GalacticGuardian.spritebuilder/GDC/Packages/SpriteBuilder%20Resources.sbpack/Sprites/resources-auto/asteroid_big_002.png"];
+    _imgShip = images["https://raw.githubusercontent.com/slembcke/GalacticGuardian.spritebuilder/GDC/Packages/SpriteBuilder%20Resources.sbpack/Sprites/resources-auto/GG_blueship_Lv3.png"];
+
+    for (int i = 0; i < 100; i++) {
+      addSprite(i/100.0);
+    }
+  }
+
+  void addSprite([double scale = null]) {
+    TestAsteroidSprite sprt = new TestAsteroidSprite.withImage(_imgAsteroid);
+    sprt.width = 64.0;
+    sprt.height = 64.0;
+
+    if (scale == null) {
+      scale = _rand.nextDouble();
+    }
+
+    sprt.zPosition = scale;
+    sprt.scale = scale;
+    sprt.position = new Vector2(_rand.nextDouble()*1024.0, _rand.nextDouble()*1024.0);
+
+    this.addChild(sprt);
+  }
+
+  void update(double dt) {
+    for (TransformNode child in children) {
+      child.update(dt);
+    }
+  }
+}
+
+class TestAsteroidSprite extends SpriteNode {
+
+  Vector2 _movementVector;
+  double _rotationalSpeed;
+
+  TestAsteroidSprite.withImage(Image img) : super.withImage(img) {
+    _movementVector = new Vector2(_rand.nextDouble() * 4.0 - 2.0, _rand.nextDouble() * 4.0 - 2.0);
+    _rotationalSpeed = _rand.nextDouble() * 2.0 - 1.0;
+  }
+
+  void update(double dt) {
+    position = position + _movementVector * scale;
+
+    // Bounce at edges
+    if (position[0] < 0 || position[0] > 1024.0) _movementVector[0] = -_movementVector[0];
+    if (position[1] < 0 || position[1] > 1024.0) _movementVector[1] = -_movementVector[1];
+
+    rotation += _rotationalSpeed;
+  }
+}