Adds virtual joystick to games library

examples/game/lib/sprites.dart

@@ -39,3 +39,4 @@ part 'sprite_box.dart';
 part 'sprite_widget.dart';
 part 'texture.dart';
 part 'util.dart';
+part 'virtual_joystick.dart';

examples/game/lib/virtual_joystick.dart

+part of sprites;
+
+class VirtualJoystick extends NodeWithSize {
+  VirtualJoystick() : super(new Size(160.0, 160.0)) {
+    userInteractionEnabled = true;
+    handleMultiplePointers = false;
+    position = new Point(160.0, -20.0);
+    pivot = new Point(0.5, 1.0);
+    _center = new Point(size.width / 2.0, size.height / 2.0);
+    _handlePos = _center;
+
+    _paintHandle = new Paint()
+      ..color=new Color(0xffffffff);
+    _paintControl = new Paint()
+      ..color=new Color(0xffffffff)
+      ..strokeWidth = 1.0
+      ..setStyle(PaintingStyle.stroke);
+  }
+
+  Point _value = Point.origin;
+  Point get value => _value;
+
+  bool _isDown = false;
+  bool get isDown => _isDown;
+
+  Point _pointerDownAt;
+  Point _center;
+  Point _handlePos;
+
+  Paint _paintHandle;
+  Paint _paintControl;
+
+  bool handleEvent(SpriteBoxEvent event) {
+    if (event.type == "pointerdown") {
+      _pointerDownAt = event.boxPosition;
+      actions.stopAll();
+      _isDown = true;
+    }
+    else if (event.type == "pointerup" || event.type == "pointercancel") {
+      _pointerDownAt = null;
+      _value = Point.origin;
+      ActionTween moveToCenter = new ActionTween((a) => _handlePos = a, _handlePos, _center, 0.4, elasticOut);
+      actions.run(moveToCenter);
+      _isDown = false;
+    } else if (event.type == "pointermove") {
+      Offset movedDist = event.boxPosition - _pointerDownAt;
+
+      _value = new Point(
+        (movedDist.dx / 80.0).clamp(-1.0, 1.0),
+        (movedDist.dy / 80.0).clamp(-1.0, 1.0));
+
+        _handlePos = _center + new Offset(_value.x * 40.0, _value.y * 40.0);
+    }
+    return true;
+  }
+
+  void paint(PaintingCanvas canvas) {
+    applyTransformForPivot(canvas);
+    canvas.drawCircle(_handlePos, 25.0, _paintHandle);
+    canvas.drawCircle(_center, 40.0, _paintControl);
+  }
+}