Adds support for spline actions and constraints on nodes

examples/game/lib/action_spline.dart

+part of sprites;
+
+Point _cardinalSplineAt(Point p0, Point p1, Point p2, Point p3, double tension, double t) {
+  double t2 = t * t;
+  double t3 = t2 * t;
+
+  double s = (1.0 - tension) / 2.0;
+
+  double b1 = s * ((-t3 + (2.0 * t2)) - t);
+	double b2 = s * (-t3 + t2) + (2.0 * t3 - 3.0 * t2 + 1.0);
+	double b3 = s * (t3 - 2.0 * t2 + t) + (-2.0 * t3 + 3.0 * t2);
+	double b4 = s * (t3 - t2);
+
+  double x = p0.x * b1 + p1.x * b2 + p2.x * b3 + p3.x * b4;
+	double y = p0.y * b1 + p1.y * b2 + p2.y * b3 + p3.y * b4;
+
+  return new Point(x, y);
+}
+
+class ActionSpline extends ActionInterval {
+  ActionSpline(this.setter, this.points, double duration, [Curve curve]) : super(duration, curve) {
+    _dt = 1.0 / (points.length - 1.0);
+  }
+
+  final Function setter;
+  final List<Point> points;
+
+  double _dt;
+
+  void update(double t) {
+    double tension = 0.5;
+
+    int p;
+    double lt;
+
+    if (t < 0.0) t = 0.0;
+
+    if (t >= 1.0) {
+      p = points.length - 1;
+      lt = 1.0;
+    } else {
+      p = (t / _dt).floor();
+      lt = (t - _dt * p) / _dt;
+    }
+
+    Point p0 = points[(p - 1).clamp(0, points.length - 1)];
+    Point p1 = points[(p + 0).clamp(0, points.length - 1)];
+    Point p2 = points[(p + 1).clamp(0, points.length - 1)];
+    Point p3 = points[(p + 2).clamp(0, points.length - 1)];
+
+    Point newPos = _cardinalSplineAt(p0, p1, p2, p3, tension, lt);
+
+    //print("newPos: $newPos");
+
+    setter(newPos);
+  }
+}

examples/game/lib/constraint.dart

+part of sprites;
+
+abstract class Constraint {
+  void preUpdate(Node node, double dt) {
+  }
+
+  void constrain(Node node, double dt);
+}
+
+double _dampenRotation(double src, double dst, double dampening) {
+  double delta = dst - src;
+  while (delta > 180.0) delta -= 360;
+  while (delta < -180) delta += 360;
+  delta *= dampening;
+
+  return src + delta;
+}
+
+class ConstraintRotationToMovement {
+  ConstraintRotationToMovement([this.dampening]);
+  final double dampening;
+
+  Point _lastPosition;
+
+  void preUpdate(Node node, double dt) {
+    _lastPosition = node.position;
+  }
+
+  void constrain(Node node, double dt) {
+    assert(_lastPosition != null);
+    if (_lastPosition == node.position) return;
+
+    // Get the target angle
+    Offset offset = node.position - _lastPosition;
+    double target = degrees(GameMath.atan2(offset.dy, offset.dx));
+
+    if (dampening == null)
+      node.rotation = target;
+    else
+      node.rotation = _dampenRotation(node.rotation, target, dampening);
+  }
+}

examples/game/lib/node.dart

@@ -76,6 +76,25 @@ class Node {
     return _actions;
   }
 
+  List<Constraint> _constraints;
+
+  List<Constraint> get constraints {
+    return _constraints;
+  }
+
+  set constraints(List<Constraint> constraints) {
+    _constraints = constraints;
+    if (_spriteBox != null) _spriteBox._constrainedNodes = null;
+  }
+
+  void applyConstraints(double dt) {
+    if (_constraints == null) return;
+
+    for (Constraint constraint in _constraints) {
+      constraint.constrain(this, dt);
+    }
+  }
+
   // Constructors
 
   /// Creates a new [Node] without any transformation.

examples/game/lib/sprite_box.dart

@@ -74,6 +74,8 @@ class SpriteBox extends RenderBox {
 
   List<ActionController> _actionControllers;
 
+  List<Node> _constrainedNodes;
+
   Rect _visibleArea;
 
   Rect get visibleArea {
@@ -139,11 +141,13 @@ class SpriteBox extends RenderBox {
   _registerNode(Node node) {
     _actionControllers = null;
     _eventTargets = null;
+    if (node == null || node.constraints != null) _constrainedNodes = null;
   }
 
   _deregisterNode(Node node) {
     _actionControllers = null;
     _eventTargets = null;
+    if (node == null || node.constraints != null) _constrainedNodes = null;
   }
 
   // Event handling
@@ -353,8 +357,10 @@ class SpriteBox extends RenderBox {
 
     _frameRate = 1.0/delta;
 
+    _callConstraintsPreUpdate(delta);
     _runActions(delta);
     _callUpdate(_rootNode, delta);
+    _callConstraintsConstrain(delta);
 
     // Schedule next update
     _scheduleTick();
@@ -392,6 +398,42 @@ class SpriteBox extends RenderBox {
     }
   }
 
+  void _callConstraintsPreUpdate(double dt) {
+    if (_constrainedNodes == null) {
+      _constrainedNodes = [];
+      _addConstrainedNodes(_rootNode, _constrainedNodes);
+    }
+
+    for (Node node in _constrainedNodes) {
+      for (Constraint constraint in node.constraints) {
+        constraint.preUpdate(node, dt);
+      }
+    }
+  }
+
+  void _callConstraintsConstrain(double dt) {
+    if (_constrainedNodes == null) {
+      _constrainedNodes = [];
+      _addConstrainedNodes(_rootNode, _constrainedNodes);
+    }
+
+    for (Node node in _constrainedNodes) {
+      for (Constraint constraint in node.constraints) {
+        constraint.constrain(node, dt);
+      }
+    }
+  }
+
+  void _addConstrainedNodes(Node node, List<Node> nodes) {
+    if (node._constraints != null && node._constraints.length > 0) {
+      nodes.add(node);
+    }
+
+    for (Node child in node.children) {
+      _addConstrainedNodes(child, nodes);
+    }
+  }
+
   void _callSpriteBoxPerformedLayout(Node node) {
     node.spriteBoxPerformedLayout();
     for (Node child in node.children) {

examples/game/lib/sprites.dart

@@ -22,6 +22,8 @@ import 'package:sky_services/media/media.mojom.dart';
 import 'package:vector_math/vector_math.dart';
 
 part 'action.dart';
+part 'constraint.dart';
+part 'action_spline.dart';
 part 'color_secuence.dart';
 part 'image_map.dart';
 part 'layer.dart';