Baby steps towards an odeon-like animation system. First victim: Drawer.

sky/sdk/lib/animation/animation_performance.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 'animated_value.dart';
+import 'curves.dart';
+
+// TODO(mpcomplete): make this an interface.
+// TODO(mpcomplete): find a better name, or reconcile this with AnimatedValue.
+class AnimationVariable {
+  double value;
+  double begin, end;
+  Curve curve = linear;
+
+  AnimationVariable(this.begin, this.end, {this.curve}) {
+    value = begin;
+  }
+
+  void setPercentage(double t) {
+    // TODO(mpcomplete): Reverse the timeline and curve.
+    value = begin + (end - begin) * curve.transform(t);
+  }
+}
+
+// This class manages a "performance" - a collection of values that change
+// based on a timeline. For example, a performance may handle an animation
+// of a menu opening by sliding and fading in (changing Y value and opacity)
+// over .5 seconds. The performance can move forwards (present) or backwards
+// (dismiss). A consumer may also take direct control of the timeline by
+// manipulating |percentage|, or |fling| the timeline causing a physics-based
+// simulation to take over the progression.
+class AnimationPerformance {
+  // TODO(mpcomplete): make this a list, or composable somehow.
+  AnimationVariable variable;
+  // Advances from 0 to 1. On each tick, we'll update our variable's values.
+  AnimatedValue timeline = new AnimatedValue(0.0);
+  // TODO(mpcomplete): duration should be on a director.
+  // TODO(mpcomplete): maybe this should be in seconds?
+  double durationMS;
+
+  AnimationPerformance() {
+    timeline.onValueChanged.listen((double t) {
+      variable.setPercentage(t);
+    });
+  }
+
+  double get percentage => timeline.value;
+  void set percentage(double t) {
+    stop();
+    timeline.value = t.clamp(0.0, 1.0);
+  }
+
+  bool get isDismissed => timeline.value == 0;
+  bool get isPresented => timeline.value == 1;
+  bool get isAnimating => timeline.isAnimating;
+
+  void present() {
+    _animateTo(1.0);
+  }
+  void dismiss() {
+    _animateTo(0.0);
+  }
+
+  void stop() {
+    timeline.stop();
+  }
+
+  // Resume animating in a direction, with the given velocity.
+  // TODO(mpcomplete): this should be a force with friction so it slows over
+  // time.
+  void fling({double velocity: 1.0}) {
+    double target = velocity.sign < 0.0 ? 0.0 : 1.0;
+    double distance = (target - timeline.value).abs();
+    double duration = distance / velocity.abs();
+
+    if (distance > 0)
+      timeline.animateTo(target, duration, curve: linear);
+  }
+
+  // TODO(mpcomplete): fix animateTo to better handle resuming an animation,
+  // with timelines. Also need to support forces.
+  void _animateTo(double target) {
+    double remainingDistance = (target - timeline.value).abs();
+    timeline.stop();
+    if (remainingDistance != 0)
+      timeline.animateTo(target, remainingDistance * durationMS);
+  }
+}
+
+