Port sky-drawer to Dart

sky/framework/animation/controller.dart

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import "curves.dart";
+import "timer.dart";
+
+class AnimationController extends AnimationDelegate {
+  final AnimationDelegate _delegate;
+  AnimationTimer _timer;
+  double _begin = 0.0;
+  double _end = 0.0;
+  Curve _curve;
+  bool _isAnimating = false;
+
+  AnimationController(this._delegate) {
+    _timer = new AnimationTimer(this);
+  }
+
+  bool get isAnimating => _isAnimating;
+
+  void start({double begin: 0.0, double end: 0.0, Curve curve: linear,
+              double duration: 0.0}) {
+    _begin = begin;
+    _end = end;
+    _curve = curve;
+    _isAnimating = true;
+    _timer.start(duration);
+  }
+
+  void stop() {
+    _isAnimating = false;
+    _timer.stop();
+  }
+
+  double _positionForTime(double t) {
+    // Explicitly finish animations at |_end| in case the curve isn't an
+    // exact numerical transform.
+    if (t == 1)
+      return _end;
+    double curvedTime = _curve.transform(t);
+    return _begin + (_end - _begin) * curvedTime;
+  }
+
+  void updateAnimation(double t) {
+    _delegate.updateAnimation(_positionForTime(t));
+  }
+}