Improve Sky's Scrollable physics

We now use a physics simulationt to drive the scroll fling. We're also closer
to integrating fling into overscroll.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1004363002

framework/animation/mechanics.dart

@@ -5,7 +5,7 @@
 import 'dart:math' as math;
 
 const double kGravity = -0.980;
-const double _kMinVelocity = 0.1;
+const double _kMinVelocity = 0.01;
 
 abstract class System {
   void update(double deltaT);
@@ -69,6 +69,23 @@ class ParticleInBox extends System {
   }
 }
 
+class ParticleInBoxWithFriction extends ParticleInBox {
+  final double friction;
+  final double _sign;
+
+  ParticleInBoxWithFriction({Particle particle, Box box, this.friction})
+      : super(particle: particle, box: box),
+        _sign = particle.velocity.sign;
+
+  void update(double deltaT) {
+    double force = -_sign * friction;
+    particle.applyImpluse(force * deltaT);
+    if (particle.velocity.sign != _sign)
+      particle.velocity = 0.0;
+    super.update(deltaT);
+  }
+}
+
 class Spring {
   final double k;
   double displacement;

framework/animation/scroll_behavior.dart

@@ -6,7 +6,8 @@ import 'dart:math' as math;
 import 'mechanics.dart';
 import 'generators.dart';
 
-const double _kSlope = 0.01;
+const double _kSlope = 0.02;
+const double _kFriction = 0.004;
 
 abstract class ScrollBehavior {
   Simulation release(Particle particle) => null;
@@ -33,8 +34,16 @@ class BoundedScrollBehavior extends ScrollBehavior {
 
 class OverscrollBehavior extends ScrollBehavior {
   Simulation release(Particle particle) {
-    if (particle.position >= 0.0)
-      return null;
+    if (particle.position >= 0.0) {
+      if (particle.velocity == 0.0)
+        return null;
+      System system = new ParticleInBoxWithFriction(
+        particle: particle,
+        box: new Box(min: 0.0),
+        friction: _kFriction);
+      return new Simulation(system,
+        terminationCondition: () => particle.velocity == 0.0);
+    }
     System system = new ParticleClimbingRamp(
         particle: particle,
         box: new Box(max: 0.0),

framework/components/fixed_height_scrollable.dart

@@ -66,7 +66,7 @@ abstract class FixedHeightScrollable extends Scrollable {
           alignmentDelta -= _itemHeight;
 
         double drawStart = scrollOffset + alignmentDelta;
-        itemNumber = (drawStart / _itemHeight).floor();
+        itemNumber = math.max(0, (drawStart / _itemHeight).floor());
 
         transformStyle =
             'transform: translateY(${(alignmentDelta).toStringAsFixed(2)}px)';

framework/components/scrollable.dart

@@ -2,21 +2,26 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import '../animation/curves.dart';
-import '../animation/fling_curve.dart';
 import '../animation/generators.dart';
-import '../animation/scroll_behavior.dart';
 import '../animation/mechanics.dart';
+import '../animation/scroll_behavior.dart';
 import '../fn.dart';
+import '../theme/view-configuration.dart' as config;
+import 'dart:math' as math;
 import 'dart:sky' as sky;
 
+const double _kMillisecondsPerSecond = 1000.0;
+
+double _velocityForFlingGesture(sky.GestureEvent event) {
+  return math.max(-config.kMaxFlingVelocity, math.min(config.kMaxFlingVelocity,
+      -event.velocityY)) / _kMillisecondsPerSecond;
+}
+
 abstract class Scrollable extends Component {
   ScrollBehavior scrollBehavior;
   double get scrollOffset => _scrollOffset;
 
   double _scrollOffset = 0.0;
-  FlingCurve _flingCurve;
-  int _flingAnimationId;
   Simulation _simulation;
 
   Scrollable({Object key, this.scrollBehavior}) : super(key: key) {
@@ -31,7 +36,6 @@ abstract class Scrollable extends Component {
 
   void didUnmount() {
     super.didUnmount();
-    _stopFling();
     _stopSimulation();
   }
 
@@ -45,18 +49,6 @@ abstract class Scrollable extends Component {
     return true;
   }
 
-  void _scheduleFlingUpdate() {
-    _flingAnimationId = sky.window.requestAnimationFrame(_updateFling);
-  }
-
-  void _stopFling() {
-    if (_flingAnimationId == null)
-      return;
-    sky.window.cancelAnimationFrame(_flingAnimationId);
-    _flingCurve = null;
-    _flingAnimationId = null;
-  }
-
   void _stopSimulation() {
     if (_simulation == null)
       return;
@@ -64,16 +56,8 @@ abstract class Scrollable extends Component {
     _simulation = null;
   }
 
-  void _updateFling(double timeStamp) {
-    double scrollDelta = _flingCurve.update(timeStamp);
-    if (!scrollBy(scrollDelta))
-      return _settle();
-    _scheduleFlingUpdate();
-  }
-
-  void _settle() {
-    _stopFling();
-    Particle particle = new Particle(position: scrollOffset);
+  void _startSimulation(Particle particle) {
+    _stopSimulation();
     _simulation = scrollBehavior.release(particle);
     if (_simulation == null)
       return;
@@ -84,14 +68,17 @@ abstract class Scrollable extends Component {
     });
   }
 
+  Particle _createParticle([double velocity = 0.0]) {
+    return new Particle(position: _scrollOffset, velocity: velocity);
+  }
+
   void _handlePointerDown(_) {
-    _stopFling();
     _stopSimulation();
   }
 
   void _handlePointerUpOrCancel(_) {
-    if (_flingCurve == null)
-      _settle();
+    if (_simulation == null)
+      _startSimulation(_createParticle());
   }
 
   void _handleScrollUpdate(sky.GestureEvent event) {
@@ -99,13 +86,11 @@ abstract class Scrollable extends Component {
   }
 
   void _handleFlingStart(sky.GestureEvent event) {
-    _stopSimulation();
-    _flingCurve = new FlingCurve(-event.velocityY, event.timeStamp);
-    _scheduleFlingUpdate();
+    _startSimulation(_createParticle(_velocityForFlingGesture(event)));
   }
 
   void _handleFlingCancel(sky.GestureEvent event) {
-    _settle();
+    _startSimulation(_createParticle());
   }
 
   void _handleWheel(sky.WheelEvent event) {

framework/theme/view-configuration.dart

@@ -5,6 +5,8 @@
 // Modeled after Android's ViewConfiguration:
 // https://github.com/android/platform_frameworks_base/blob/master/core/java/android/view/ViewConfiguration.java
 
-const int kScrollbarSize = 10;
+const int kMaxFlingVelocity = 8000;
+const int kMinFlingVelocity = 50;
 const int kScrollbarFadeDelay = 300;
 const int kScrollbarFadeDuration = 250;
+const int kScrollbarSize = 10;