Port sky-drawer to Dart

sky/framework/animation/timer.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 "dart:math" as math;
+import "dart:sky";
+
+abstract class AnimationDelegate {
+  void updateAnimation(double t);
+}
+
+class AnimationTimer {
+  final AnimationDelegate _delegate;
+  double _startTime = 0.0;
+  double _duration = 0.0;
+  int _animationId = 0;
+
+  AnimationTimer(this._delegate);
+
+  void start(double duration) {
+    if (_animationId != 0)
+      stop();
+    _duration = duration;
+    _scheduleTick();
+  }
+
+  void stop() {
+    window.cancelAnimationFrame(_animationId);
+    _startTime = 0.0;
+    _duration = 0.0;
+    _animationId = 0;
+  }
+
+  void _scheduleTick() {
+    assert(_animationId == 0);
+    _animationId = window.requestAnimationFrame(_tick);
+  }
+
+  void _tick(double timeStamp) {
+    _animationId = 0;
+    if (_startTime == 0.0)
+      _startTime = timeStamp;
+    double elapsedTime = timeStamp - _startTime;
+    double t = math.max(0.0, math.min(1.0, elapsedTime / _duration));
+    if (t < 1.0)
+      _scheduleTick();
+    else
+      stop();
+    _delegate.updateAnimation(t);
+  }
+}