Make Drawer animate using a spring force instead of a linear curve.

DEPS

@@ -34,7 +34,7 @@ vars = {
   'cassowary_dart_revision': '7e5afc5b3956a18636d5b37b1dcba1705865564b',
   'collection_dart_revision': '79ebc6fc2dae581cb23ad50a5c600c1b7dd132f8',
   'crypto_dart_revision': 'd4558dea1639e5ad2a41d045265b8ece270c2d90',
-  'newton_dart_revision': '26da04f0c441d005a6ecbf62ae047cd02ec9abc5',
+  'newton_dart_revision': '9fbe5fbac809246f7ace4176feca13bdf731e393',
   'path_dart_revision': '2f3dcdec32011f1bc41194ae3640d6d9292a7096',
   'quiver_dart_revision': '6bab7dec34189eee579178eb16d3063c8ae69031',
   'source_span_dart_revision': '5c6c13f62fc111adaace3aeb4a38853d64481d06',

sky/sdk/BUILD.gn

@@ -11,6 +11,7 @@ dart_pkg("sky") {
     "lib/animation/animated_simulation.dart",
     "lib/animation/animation_performance.dart",
     "lib/animation/curves.dart",
+    "lib/animation/forces.dart",
     "lib/animation/scroll_behavior.dart",
     "lib/animation/timeline.dart",
     "lib/assets/.gitignore",

sky/sdk/lib/animation/animation_performance.dart

@@ -2,9 +2,11 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'package:newton/newton.dart';
+import 'dart:async';
+
 import 'package:sky/animation/timeline.dart';
 import 'package:sky/animation/curves.dart';
+import 'package:sky/animation/forces.dart';
 
 abstract class AnimatedVariable {
   void setFraction(double t);
@@ -99,23 +101,20 @@ class AnimationPerformance {
   bool get isCompleted => progress == 1.0;
   bool get isAnimating => timeline.isAnimating;
 
-  void play() {
-    _animateTo(1.0);
-  }
-  void reverse() {
-    _animateTo(0.0);
-  }
+  Future play() => _animateTo(1.0);
+  Future reverse() => _animateTo(0.0);
 
   void stop() {
     timeline.stop();
   }
 
-  // Resume animating in a direction, with the given velocity.
-  // TODO(mpcomplete): Allow user to specify the Simulation.
-  void fling({double velocity: 1.0}) {
-    Simulation simulation =
-        timeline.defaultSpringSimulation(velocity: velocity);
-    timeline.fling(simulation);
+  // Flings the timeline with an optional force (defaults to a critically damped
+  // spring) and initial velocity. Negative velocity causes the timeline to go
+  // in reverse.
+  Future fling({double velocity: 1.0, Force force}) {
+    if (force == null)
+      force = kDefaultSpringForce;
+    return timeline.fling(force.release(progress, velocity));
   }
 
   final List<Function> _listeners = new List<Function>();
@@ -134,11 +133,12 @@ class AnimationPerformance {
       listener();
   }
 
-  void _animateTo(double target) {
+  Future _animateTo(double target) {
     double remainingDistance = (target - timeline.value).abs();
     timeline.stop();
-    if (remainingDistance != 0.0)
-      timeline.animateTo(target, duration: duration * remainingDistance);
+    if (remainingDistance == 0.0)
+      return new Future.value();
+    return timeline.animateTo(target, duration: duration * remainingDistance);
   }
 
   void _tick(double t) {

sky/sdk/lib/animation/scroll_behavior.dart

@@ -5,10 +5,11 @@
 import 'dart:math' as math;
 
 import 'package:newton/newton.dart';
+import 'package:sky/animation/forces.dart';
 
 const double _kSecondsPerMillisecond = 1000.0;
 
-abstract class ScrollBehavior {
+abstract class ScrollBehavior extends Force {
   Simulation release(double position, double velocity) => null;
 
   // Returns the new scroll offset.

sky/sdk/lib/animation/timeline.dart

@@ -7,8 +7,6 @@ import 'dart:async';
 import 'package:newton/newton.dart';
 import 'package:sky/animation/animated_simulation.dart';
 
-const  double _kEpsilon = 0.001;
-
 // Simple simulation that linearly varies from |begin| to |end| over |duration|.
 class TweenSimulation extends Simulation {
   final double _durationInSeconds;
@@ -68,17 +66,6 @@ class Timeline {
     _animation.stop();
   }
 
-  static final SpringDescription _kDefaultSpringDesc =
-      new SpringDescription.withDampingRatio(
-          mass: 1.0, springConstant: 500.0, ratio: 1.0);
-
-  Simulation defaultSpringSimulation({double velocity: 0.0}) {
-    // Target just past the 0 or 1 endpoint, because the animation will stop
-    // once the Spring gets within the epsilon, and we want to stop at 0 or 1.
-    double target = velocity < 0.0 ? -_kEpsilon : 1.0 + _kEpsilon;
-    return new SpringSimulation(_kDefaultSpringDesc, value, target, velocity);
-  }
-
   // Give |simulation| control over the timeline.
   Future fling(Simulation simulation) {
     stop();

sky/sdk/lib/widgets/dismissable.dart

@@ -13,6 +13,7 @@ import 'package:vector_math/vector_math.dart';
 const Duration _kCardDismissFadeout = const Duration(milliseconds: 300);
 const double _kMinFlingVelocity = 700.0;
 const double _kMinFlingVelocityDelta = 400.0;
+const double _kFlingVelocityScale = 1.0/300.0;
 const double _kDismissCardThreshold = 0.6;
 
 typedef void DismissedCallback();
@@ -129,7 +130,7 @@ class Dismissable extends AnimatedComponent {
       _dragUnderway = false;
       _dragX = event.velocityX.sign;
       _position.end = _activeCardDragEndPoint;
-      _performance.fling(velocity: event.velocityX.abs() / _width);
+      _performance.fling(velocity: event.velocityX.abs() * _kFlingVelocityScale);
     }
   }
 

sky/sdk/lib/widgets/drawer.dart

@@ -5,7 +5,6 @@
 import 'dart:sky' as sky;
 
 import 'package:sky/animation/animation_performance.dart';
-import 'package:sky/animation/curves.dart';
 import 'package:sky/theme/shadows.dart';
 import 'package:sky/theme/colors.dart' as colors;
 import 'package:sky/widgets/animated_component.dart';
@@ -30,14 +29,11 @@ import 'package:vector_math/vector_math.dart';
 // The right nav can vary depending on content.
 
 const double _kWidth = 304.0;
-const double _kMinFlingVelocity = 1.2;
+const double _kMinFlingVelocity = 365.0;
+const double _kFlingVelocityScale = 1.0/300.0;
 const Duration _kBaseSettleDuration = const Duration(milliseconds: 246);
-// TODO(mpcomplete): The curve must be linear if we want the drawer to track
-// the user's finger. Odeon remedies this by attaching spring forces to the
-// initial timeline when animating (so it doesn't look linear).
 const Point _kOpenPosition = Point.origin;
 const Point _kClosedPosition = const Point(-_kWidth, 0.0);
-const Curve _kAnimationCurve = linear;
 
 typedef void DrawerStatusChangeHandler (bool showing);
 
@@ -69,7 +65,7 @@ class Drawer extends AnimatedComponent {
   AnimationPerformance _performance;
 
   void initState() {
-    _position = new AnimatedType<Point>(_kClosedPosition, end: _kOpenPosition, curve: _kAnimationCurve);
+    _position = new AnimatedType<Point>(_kClosedPosition, end: _kOpenPosition);
     _maskColor = new AnimatedColor(colors.transparent, end: const Color(0x7F000000));
     _performance = new AnimationPerformance()
       ..duration = _kBaseSettleDuration
@@ -95,11 +91,18 @@ class Drawer extends AnimatedComponent {
   void _show() {
     if (navigator != null)
       navigator.pushState(this, (_) => _performance.reverse());
-    _performance.play();
+    _fling(1.0);
   }
 
   void _hide() {
-    _performance.reverse();
+    _fling(-1.0);
+  }
+
+  // We fling the performance timeline instead of animating it to give it a
+  // nice spring effect. We can't use curves for this because we need a linear
+  // curve in order to track the user's finger while dragging.
+  void _fling(double direction) {
+    _performance.fling(velocity: direction.sign);
   }
 
   Widget build() {
@@ -151,9 +154,9 @@ class Drawer extends AnimatedComponent {
   DrawerStatus get _status => _performance.isDismissed ? DrawerStatus.inactive : DrawerStatus.active;
   bool get _isMostlyClosed => xPosition <= -_kWidth/2;
 
-  void _settle() => _isMostlyClosed ? _performance.reverse() : _performance.play();
+  void _settle() => _fling(_isMostlyClosed ? -1.0 : 1.0);
 
-  void handleMaskTap(_) => _performance.reverse();
+  void handleMaskTap(_) => _fling(-1.0);
 
   // TODO(mpcomplete): Figure out how to generalize these handlers on a
   // "PannableThingy" interface.
@@ -176,8 +179,7 @@ class Drawer extends AnimatedComponent {
   }
 
   void handleFlingStart(event) {
-    double velocityX = event.velocityX / _kWidth;
-    if (velocityX.abs() >= _kMinFlingVelocity)
-      _performance.fling(velocity: velocityX);
+    if (event.velocityX.abs() >= _kMinFlingVelocity)
+      _performance.fling(velocity: event.velocityX * _kFlingVelocityScale);
   }
 }