Adds basic touch handling to sprites and optimizes transformations

sky/examples/game/lib/game_demo_world.dart

 part of game;
 
 // Random generator
 Math.Random _rand = new Math.Random();
 
 const double _gameSizeWidth = 1024.0;
 const double _gameSizeHeight = 1024.0;
 
 const double _shipRadius = 30.0;
 const double _lrgAsteroidRadius = 40.0;
@@ skipping 52 matching lines @@
     // Add ship
     addShip();
 
     // Add starfield
     _starField = new StarField.withImage(_imgStar, _numStarsInStarField);
     _starField.zPosition = -2.0;
     addChild(_starField);
 
     // Add nebula
     addNebula();
+
+    userInteractionEnabled = true;
+    handleMultiplePointers = true;
   }
 
   // Methods for adding game objects
 
   void addBackground() {
     Sprite sprtBg = new Sprite.withImage(_imgBg);
     sprtBg.size = new Size(_gameSizeWidth, _gameSizeHeight);
     sprtBg.pivot = Point.origin;
     _gameLayer.addChild(sprtBg);
   }
@@ skipping 110 matching lines @@
   // Handling controls
 
   void controlSteering(double x, double y) {
     _joystickX = x;
     _joystickY = y;
   }
 
   void controlFire() {
     addLaser();
   }
+
+  // Handle pointer events
+
+  int _firstPointer = -1;
+  int _secondPointer = -1;
+  Point _firstPointerDownPos;
+
+  bool handleEvent(SpriteBoxEvent event) {
+    Point pointerPos = convertPointToNodeSpace(event.boxPosition);
+    int pointer = event.pointer;
+
+    switch (event.type) {
+      case 'pointerdown':
+        if (_firstPointer == -1) {
+          // Assign the first pointer
+          _firstPointer = pointer;
+          _firstPointerDownPos = pointerPos;
+        }
+        else if (_secondPointer == -1) {
+          // Assign second pointer
+          _secondPointer = pointer;
+          controlFire();
+        }
+        else {
+          // There is a pointer used for steering, let's fire instead
+          controlFire();
+        }
+        break;
+      case 'pointermove':
+        if (pointer == _firstPointer) {
+          // Handle turning control
+          double joystickX = 0.0;
+          double deltaX = pointerPos.x - _firstPointerDownPos.x;
+          if (deltaX > _steeringThreshold || deltaX < -_steeringThreshold) {
+            joystickX = (deltaX - _steeringThreshold)/(_steeringMax - _steeringThreshold);
+            if (joystickX > 1.0) joystickX = 1.0;
+            if (joystickX < -1.0) joystickX = -1.0;
+          }
+
+          double joystickY = 0.0;
+          double deltaY = pointerPos.y - _firstPointerDownPos.y;
+          if (deltaY > _steeringThreshold || deltaY < -_steeringThreshold) {
+            joystickY = (deltaY - _steeringThreshold)/(_steeringMax - _steeringThreshold);
+            if (joystickY > 1.0) joystickY = 1.0;
+            if (joystickY < -1.0) joystickY = -1.0;
+          }
+
+          controlSteering(joystickX, joystickY);
+        }
+        break;
+      case 'pointerup':
+      case 'pointercancel':
+        if (pointer == _firstPointer) {
+          // Un-assign the first pointer
+          _firstPointer = -1;
+          _firstPointerDownPos = null;
+          controlSteering(0.0, 0.0);
+        }
+        else if (pointer == _secondPointer) {
+          _secondPointer = -1;
+        }
+        break;
+      default:
+        break;
+    }
+    return true;
+  }
 }
 
 // Game objects
 
 enum AsteroidSize {
   small,
   medium,
   large,
 }
 
@@ skipping 10 matching lines @@
     return _radius;
   }
 
   Asteroid.withImage(Image img, AsteroidSize this._asteroidSize) : super.withImage(img) {
     size = new Size(radius * 2.0, radius * 2.0);
     position = new Point(_gameSizeWidth * _rand.nextDouble(), _gameSizeHeight * _rand.nextDouble());
     rotation = 360.0 * _rand.nextDouble();
 
     _movementVector = new Point(_rand.nextDouble() * _maxAsteroidSpeed * 2 - _maxAsteroidSpeed,
                                 _rand.nextDouble() * _maxAsteroidSpeed * 2 - _maxAsteroidSpeed);
+
+    userInteractionEnabled = true;
+  }
+
+  bool handleEvent(SpriteBoxEvent event) {
+    if (event.type == "pointerdown") {
+      colorOverlay = new Color(0x99ff0000);
+    }
+    else if (event.type == "pointerup") {
+      colorOverlay = null;
+    }
+    return false;
   }
 }
 
 class Ship extends Sprite {
   Vector2 _movementVector;
   double _rotationTarget;
 
   Ship.withImage(Image img) : super.withImage(img) {
     _movementVector = new Vector2.zero();
     rotation = _rotationTarget = 270.0;
@@ skipping 141 matching lines @@
   double dy = a.y - b.y;
   if (dx < 0.0) dx = -dx;
   if (dy < 0.0) dy = -dy;
   if (dx > dy) {
     return dx + dy/2.0;
   }
   else {
     return dy + dx/2.0;
   }
 }