When animating, use the same curve until it completes.

This ensures we don't run into discontinuities when reversing an
animation halfway through. I refactored AnimationValue to have knowledge
of the reverse curves and intervals.

packages/flutter/lib/animation/animated_value.dart

@@ -5,84 +5,96 @@
 import "dart:sky";
 
 import 'package:sky/animation/curves.dart';
+import 'package:sky/animation/direction.dart';
 import 'package:sky/base/lerp.dart';
 
+export 'package:sky/animation/curves.dart' show Interval;
+
 abstract class AnimatedVariable {
-  void setProgress(double t);
+  void setProgress(double t, Direction direction);
   String toString();
 }
 
-class Interval {
-  final double start;
-  final double end;
+abstract class CurvedVariable implements AnimatedVariable {
+  CurvedVariable({this.interval, this.reverseInterval, this.curve, this.reverseCurve});
+
+  Interval interval;
+  Interval reverseInterval;
+  Curve curve;
+  Curve reverseCurve;
+
+  double _transform(double t, Direction direction) {
+    Interval interval = _getInterval(direction);
+    if (interval != null)
+      t = interval.transform(t);
+    if (t == 1.0) // Or should we support inverse curves?
+      return t;
+    Curve curve = _getCurve(direction);
+    if (curve != null)
+      t = curve.transform(t);
+    return t;
+  }
 
-  double adjustTime(double t) {
-    return ((t - start) / (end - start)).clamp(0.0, 1.0);
+  Interval _getInterval(Direction direction) {
+    if (direction == Direction.forward || reverseInterval == null)
+      return interval;
+    return reverseInterval;
   }
 
-  Interval(this.start, this.end) {
-    assert(start >= 0.0);
-    assert(start <= 1.0);
-    assert(end >= 0.0);
-    assert(end <= 1.0);
+  Curve _getCurve(Direction direction) {
+    if (direction == Direction.forward || reverseCurve == null)
+      return curve;
+    return reverseCurve;
   }
 }
 
-class AnimatedValue<T extends dynamic> extends AnimatedVariable {
-  AnimatedValue(this.begin, { this.end, this.interval, this.curve: linear }) {
+class AnimatedValue<T extends dynamic> extends CurvedVariable {
+  AnimatedValue(this.begin, { this.end, Interval interval, Curve curve, Curve reverseCurve })
+    : super(interval: interval, curve: curve, reverseCurve: reverseCurve) {
     value = begin;
   }
 
   T value;
   T begin;
   T end;
-  Interval interval;
-  Curve curve;
 
-  void setProgress(double t) {
+  T lerp(double t) => begin + (end - begin) * t;
+
+  void setProgress(double t, Direction direction) {
     if (end != null) {
-      double adjustedTime = interval == null ? t : interval.adjustTime(t);
-      if (adjustedTime == 1.0) {
-        value = end;
-      } else {
-        // TODO(mpcomplete): Reverse the timeline and curve.
-        value = begin + (end - begin) * curve.transform(adjustedTime);
-      }
+      t = _transform(t, direction);
+      value = (t == 1.0) ? end : lerp(t);
     }
   }
 
   String toString() => 'AnimatedValue(begin=$begin, end=$end, value=$value)';
 }
 
-class AnimatedList extends AnimatedVariable {
+class AnimatedList extends CurvedVariable {
   List<AnimatedVariable> variables;
-  Interval interval;
 
-  AnimatedList(this.variables, { this.interval });
+  AnimatedList(this.variables, { Interval interval, Curve curve, Curve reverseCurve })
+    : super(interval: interval, curve: curve, reverseCurve: reverseCurve);
 
-  void setProgress(double t) {
-    double adjustedTime = interval == null ? t : interval.adjustTime(t);
+  void setProgress(double t, Direction direction) {
+    double adjustedTime = _transform(t, direction);
     for (AnimatedVariable variable in variables)
-      variable.setProgress(adjustedTime);
+      variable.setProgress(adjustedTime, direction);
   }
 
   String toString() => 'AnimatedList([$variables])';
 }
 
 class AnimatedColorValue extends AnimatedValue<Color> {
-  AnimatedColorValue(Color begin, { Color end, Curve curve: linear })
+  AnimatedColorValue(Color begin, { Color end, Curve curve })
     : super(begin, end: end, curve: curve);
 
-  void setProgress(double t) {
-    value = lerpColor(begin, end, t);
-  }
+  Color lerp(double t) => lerpColor(begin, end, t);
 }
 
 class AnimatedRect extends AnimatedValue<Rect> {
-  AnimatedRect(Rect begin, { Rect end, Curve curve: linear })
+  AnimatedRect(Rect begin, { Rect end, Curve curve })
     : super(begin, end: end, curve: curve);
 
-  void setProgress(double t) {
-    value = lerpRect(begin, end, t);
-  }
+  Rect lerp(double t) => lerpRect(begin, end, t);
 }

packages/flutter/lib/animation/animation_performance.dart

@@ -5,10 +5,11 @@
 import 'dart:async';
 
 import 'package:sky/animation/animated_value.dart';
+import 'package:sky/animation/direction.dart';
 import 'package:sky/animation/forces.dart';
 import 'package:sky/animation/timeline.dart';
 
-export 'package:sky/animation/forces.dart' show Direction;
+export 'package:sky/animation/direction.dart' show Direction;
 
 enum AnimationStatus {
   dismissed, // stoped at 0
@@ -39,6 +40,12 @@ class AnimationPerformance {
   Direction _direction;
   Direction get direction => _direction;
 
+  // This controls which curve we use for variables with different curves in
+  // the forward/reverse directions. Curve direction is only reset when we hit
+  // 0 or 1, to avoid discontinuities.
+  Direction _curveDirection;
+  Direction get curveDirection => _curveDirection;
+
   // If non-null, animate with this force instead of a tween animation.
   Force attachedForce;
 
@@ -74,6 +81,10 @@ class AnimationPerformance {
         AnimationStatus.reverse;
   }
 
+  void updateVariable(AnimatedVariable variable) {
+    variable.setProgress(progress, curveDirection);
+  }
+
   Future play([Direction direction = Direction.forward]) {
     _direction = direction;
     return resume();
@@ -136,6 +147,13 @@ class AnimationPerformance {
     _lastStatus = currentStatus;
   }
 
+  void _updateCurveDirection() {
+    if (status != _lastStatus) {
+      if (_lastStatus == AnimationStatus.dismissed || _lastStatus == AnimationStatus.completed)
+        _curveDirection = direction;
+    }
+  }
+
   Future _animateTo(double target) {
     Duration remainingDuration = duration * (target - timeline.value).abs();
     timeline.stop();
@@ -145,8 +163,9 @@ class AnimationPerformance {
   }
 
   void _tick(double t) {
+    _updateCurveDirection();
     if (variable != null)
-      variable.setProgress(t);
+      variable.setProgress(t, curveDirection);
     _notifyListeners();
     _checkStatusChanged();
   }

packages/flutter/lib/animation/curves.dart

@@ -21,6 +21,22 @@ class Linear implements Curve {
   }
 }
 
+class Interval implements Curve {
+  final double start;
+  final double end;
+
+  Interval(this.start, this.end) {
+    assert(start >= 0.0);
+    assert(start <= 1.0);
+    assert(end >= 0.0);
+    assert(end <= 1.0);
+  }
+
+  double transform(double t) {
+    return ((t - start) / (end - start)).clamp(0.0, 1.0);
+  }
+}
+
 class ParabolicFall implements Curve {
   const ParabolicFall();
 

packages/flutter/lib/animation/direction.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.
+
+enum Direction {
+  forward,
+  reverse
+}

packages/flutter/lib/animation/forces.dart

@@ -3,12 +3,7 @@
 // found in the LICENSE file.
 
 import 'package:newton/newton.dart';
-
-// TODO(mpcomplete): This doesn't belong here.
-enum Direction {
-  forward,
-  reverse
-}
+import 'package:sky/animation/direction.dart';
 
 // Base class for creating Simulations for the animation Timeline.
 abstract class Force {

packages/flutter/lib/rendering/toggleable.dart

@@ -44,15 +44,7 @@ abstract class RenderToggleable extends RenderConstrainedBox {
   void set value(bool value) {
     if (value == _value) return;
     _value = value;
-    // TODO(abarth): Setting the curve on the position means there's a
-    // discontinuity when we reverse the timeline.
-    if (value) {
-      _position.curve = easeIn;
-      _performance.play();
-    } else {
-      _position.curve = easeOut;
-      _performance.reverse();
-    }
+    performance.play(value ? Direction.forward : Direction.reverse);
   }
 
   ValueChanged _onChanged;
@@ -63,7 +55,7 @@ abstract class RenderToggleable extends RenderConstrainedBox {
   }
 
   final AnimatedValue<double> _position =
-      new AnimatedValue<double>(0.0, end: 1.0);
+      new AnimatedValue<double>(0.0, end: 1.0, curve: easeIn, reverseCurve: easeOut);
   AnimatedValue<double> get position => _position;
 
   AnimationPerformance _performance;

packages/flutter/lib/widgets/animated_container.dart

@@ -16,35 +16,23 @@ class AnimatedBoxConstraintsValue extends AnimatedValue<BoxConstraints> {
   AnimatedBoxConstraintsValue(BoxConstraints begin, { BoxConstraints end, Curve curve: linear })
     : super(begin, end: end, curve: curve);
 
-  void setProgress(double t) {
-    // TODO(abarth): We should lerp the BoxConstraints.
-    value = end;
-  }
+  // TODO(abarth): We should lerp the BoxConstraints.
+  BoxConstraints lerp(double t) => end;
 }
 
 class AnimatedBoxDecorationValue extends AnimatedValue<BoxDecoration> {
   AnimatedBoxDecorationValue(BoxDecoration begin, { BoxDecoration end, Curve curve: linear })
     : super(begin, end: end, curve: curve);
 
-  void setProgress(double t) {
-    if (t == 1.0) {
-      value = end;
-      return;
-    }
-    value = lerpBoxDecoration(begin, end, t);
-  }
+  BoxDecoration lerp(double t) => lerpBoxDecoration(begin, end, t);
 }
 
 class AnimatedEdgeDimsValue extends AnimatedValue<EdgeDims> {
   AnimatedEdgeDimsValue(EdgeDims begin, { EdgeDims end, Curve curve: linear })
     : super(begin, end: end, curve: curve);
 
-  void setProgress(double t) {
-    if (t == 1.0) {
-      value = end;
-      return;
-    }
-    value = new EdgeDims(
+  EdgeDims lerp(double t) {
+    return new EdgeDims(
       lerpNum(begin.top, end.top, t),
       lerpNum(begin.right, end.right, t),
       lerpNum(begin.bottom, end.bottom, t),
@@ -57,17 +45,13 @@ class AnimatedMatrix4Value extends AnimatedValue<Matrix4> {
   AnimatedMatrix4Value(Matrix4 begin, { Matrix4 end, Curve curve: linear })
     : super(begin, end: end, curve: curve);
 
-  void setProgress(double t) {
-    if (t == 1.0) {
-      value = end;
-      return;
-    }
+  Matrix4 lerp(double t) {
     // TODO(mpcomplete): Animate the full matrix. Will animating the cells
     // separately work?
     Vector3 beginT = begin.getTranslation();
     Vector3 endT = end.getTranslation();
     Vector3 lerpT = beginT*(1.0-t) + endT*t;
-    value = new Matrix4.identity()..translate(lerpT);
+    return new Matrix4.identity()..translate(lerpT);
   }
 }
 

packages/flutter/lib/widgets/navigator.dart

@@ -7,7 +7,6 @@ import 'dart:async';
 import 'package:sky/animation/animated_value.dart';
 import 'package:sky/animation/animation_performance.dart';
 import 'package:sky/animation/curves.dart';
-import 'package:sky/animation/forces.dart';
 import 'package:sky/widgets/basic.dart';
 import 'package:sky/widgets/focus.dart';
 import 'package:sky/widgets/transitions.dart';

packages/flutter/lib/widgets/popup_menu.dart

@@ -50,7 +50,6 @@ class PopupMenu extends AnimatedComponent {
   AnimatedValue<double> _width;
   AnimatedValue<double> _height;
   List<AnimatedValue<double>> _itemOpacities;
-  AnimatedList _animationList;
   AnimationPerformance _performance;
 
   void initState() {
@@ -101,8 +100,9 @@ class PopupMenu extends AnimatedComponent {
       ..add(_width)
       ..add(_height)
       ..addAll(_itemOpacities);
-    _animationList = new AnimatedList(variables);
-    _performance.variable = _animationList;
+    AnimatedList list = new AnimatedList(variables)
+      ..reverseInterval = new Interval(0.0, _kMenuCloseIntervalEnd);
+    _performance.variable = list;
   }
 
   void _updateBoxPainter() {
@@ -124,14 +124,12 @@ class PopupMenu extends AnimatedComponent {
 
 
   void _open() {
-    _animationList.interval = null;
     _performance.play();
     if (navigator != null)
       navigator.pushState(this, (_) => _close());
   }
 
   void _close() {
-    _animationList.interval = new Interval(0.0, _kMenuCloseIntervalEnd);
     _performance.reverse();
   }
 

packages/flutter/lib/widgets/transitions.dart

@@ -98,7 +98,7 @@ class SlideTransition extends TransitionBase {
   }
 
   Widget build() {
-    position.setProgress(performance.progress);
+    performance.updateVariable(position);
     Matrix4 transform = new Matrix4.identity()
       ..translate(position.value.x, position.value.y);
     return new Transform(transform: transform, child: child);
@@ -131,7 +131,7 @@ class FadeTransition extends TransitionBase {
   }
 
   Widget build() {
-    opacity.setProgress(performance.progress);
+    performance.updateVariable(opacity);
     return new Opacity(opacity: opacity.value, child: child);
   }
 }
@@ -162,7 +162,7 @@ class ColorTransition extends TransitionBase {
   }
 
   Widget build() {
-    color.setProgress(performance.progress);
+    performance.updateVariable(color);
     return new DecoratedBox(
       decoration: new BoxDecoration(backgroundColor: color.value),
       child: child
@@ -200,9 +200,9 @@ class SquashTransition extends TransitionBase {
 
   Widget build() {
     if (width != null)
-      width.setProgress(performance.progress);
+      performance.updateVariable(width);
     if (height != null)
-      height.setProgress(performance.progress);
+      performance.updateVariable(height);
     return new SizedBox(width: _maybe(width), height: _maybe(height), child: child);
   }
 }