Tweaked scroll simulation for iOS platform (#6334)

packages/flutter/lib/physics.dart

@@ -11,7 +11,6 @@ library physics;
 export 'src/physics/clamped_simulation.dart';
 export 'src/physics/friction_simulation.dart';
 export 'src/physics/gravity_simulation.dart';
-export 'src/physics/scroll_simulation.dart';
 export 'src/physics/simulation_group.dart';
 export 'src/physics/simulation.dart';
 export 'src/physics/spring_simulation.dart';

packages/flutter/lib/src/widgets/scroll_behavior.dart

@@ -8,16 +8,10 @@ import 'package:flutter/foundation.dart';
 import 'package:flutter/physics.dart';
 import 'package:meta/meta.dart';
 
+import 'scroll_simulation.dart';
+
 export 'package:flutter/foundation.dart' show TargetPlatform;
 
-const double _kScrollDragMountainView = 0.025;
-const double _kScrollDragCupertino = 0.125;
-final SpringDescription _kScrollSpring = new SpringDescription.withDampingRatio(mass: 0.5, springConstant: 100.0, ratio: 1.1);
-
-Simulation _createScrollSimulation(double position, double velocity, double minScrollOffset, double maxScrollOffset, double scrollDrag) {
-  return new ScrollSimulation(position, velocity, minScrollOffset, maxScrollOffset, _kScrollSpring, scrollDrag);
-}
-
 Simulation _createSnapScrollSimulation(double startOffset, double endOffset, double startVelocity, double endVelocity) {
   return new FrictionSimulation.through(startOffset, endOffset, startVelocity, endVelocity);
 }
@@ -69,20 +63,6 @@ abstract class ScrollBehavior<T, U> {
   /// Whether this scroll behavior currently permits scrolling.
   bool get isScrollable => true;
 
-  /// The scroll drag constant to use for physics simulations created by this
-  /// ScrollBehavior.
-  double get scrollDrag {
-    assert(platform != null);
-    switch (platform) {
-      case TargetPlatform.android:
-      case TargetPlatform.fuchsia:
-        return _kScrollDragMountainView;
-      case TargetPlatform.iOS:
-        return _kScrollDragCupertino;
-    }
-    return null;
-  }
-
   @override
   String toString() {
     List<String> description = <String>[];
@@ -220,8 +200,12 @@ class UnboundedBehavior extends ExtentScrollBehavior {
 
   @override
   Simulation createScrollSimulation(double position, double velocity) {
-    return new BoundedFrictionSimulation(
-      scrollDrag, position, velocity, double.NEGATIVE_INFINITY, double.INFINITY
+    return new ScrollSimulation(
+      position: position,
+      velocity: velocity,
+      leadingExtent: double.NEGATIVE_INFINITY,
+      trailingExtent: double.INFINITY,
+      platform: platform,
     );
   }
 
@@ -259,7 +243,13 @@ class OverscrollBehavior extends BoundedBehavior {
 
   @override
   Simulation createScrollSimulation(double position, double velocity) {
-    return _createScrollSimulation(position, velocity, minScrollOffset, maxScrollOffset, scrollDrag);
+    return new ScrollSimulation(
+      position: position,
+      velocity: velocity,
+      leadingExtent: minScrollOffset,
+      trailingExtent: maxScrollOffset,
+      platform: platform,
+    );
   }
 
   @override

packages/flutter/lib/src/widgets/scroll_simulation.dart

@@ -2,10 +2,22 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'friction_simulation.dart';
-import 'simulation_group.dart';
-import 'simulation.dart';
-import 'spring_simulation.dart';
+import 'package:flutter/foundation.dart';
+import 'package:flutter/physics.dart';
+
+final SpringDescription _kScrollSpring = new SpringDescription.withDampingRatio(mass: 0.5, springConstant: 100.0, ratio: 1.1);
+
+class _MountainViewSimulation extends FrictionSimulation {
+  static const double drag = 0.025;
+  _MountainViewSimulation({ double position, double velocity })
+    : super(drag, position, velocity);
+}
+
+class _CupertinoSimulation extends FrictionSimulation {
+  static const double drag = 0.135;
+  _CupertinoSimulation({ double position, double velocity })
+    : super(drag, position, velocity * 0.91);
+}
 
 /// Composite simulation for scrollable interfaces.
 ///
@@ -26,13 +38,22 @@ class ScrollSimulation extends SimulationGroup {
   /// consistent with the other arguments.
   ///
   /// The final argument is the coefficient of friction, which is unitless.
-  ScrollSimulation(
+  ScrollSimulation({
     double position,
     double velocity,
-    this._leadingExtent,
-    this._trailingExtent,
-    this._spring,
-    this._drag) {
+    double leadingExtent,
+    double trailingExtent,
+    SpringDescription spring,
+    double drag,
+    TargetPlatform platform,
+  }) : _leadingExtent = leadingExtent,
+       _trailingExtent = trailingExtent,
+       _spring = spring ?? _kScrollSpring,
+       _drag = drag,
+       _platform = platform {
+    assert(_leadingExtent != null);
+    assert(_trailingExtent != null);
+    assert(_spring != null);
     _chooseSimulation(position, velocity, 0.0);
   }
 
@@ -40,6 +61,7 @@ class ScrollSimulation extends SimulationGroup {
   final double _trailingExtent;
   final SpringDescription _spring;
   final double _drag;
+  final TargetPlatform _platform;
 
   bool _isSpringing = false;
   Simulation _currentSimulation;
@@ -76,7 +98,24 @@ class ScrollSimulation extends SimulationGroup {
     }
 
     if (_currentSimulation == null) {
-      _currentSimulation = new FrictionSimulation(_drag, position, velocity);
+      switch (_platform) {
+        case TargetPlatform.android:
+        case TargetPlatform.fuchsia:
+          _currentSimulation = new _MountainViewSimulation(
+            position: position,
+            velocity: velocity
+          );
+          break;
+        case TargetPlatform.iOS:
+          _currentSimulation = new _CupertinoSimulation(
+            position: position,
+            velocity: velocity,
+          );
+          break;
+      }
+      // No platform specified
+      _currentSimulation ??= new FrictionSimulation(_drag, position, velocity);
+
       return true;
     }
 

packages/flutter/lib/widgets.dart

@@ -48,6 +48,7 @@ export 'src/widgets/raw_keyboard_listener.dart';
 export 'src/widgets/routes.dart';
 export 'src/widgets/scroll_behavior.dart';
 export 'src/widgets/scroll_configuration.dart';
+export 'src/widgets/scroll_simulation.dart';
 export 'src/widgets/scrollable.dart';
 export 'src/widgets/scrollable_grid.dart';
 export 'src/widgets/scrollable_list.dart';

packages/flutter/test/physics/newton_test.dart

@@ -5,6 +5,7 @@
 import 'package:test/test.dart';
 
 import 'package:flutter/physics.dart';
+import 'package:flutter/widgets.dart';
 
 void main() {
   test('test_friction', () {
@@ -191,13 +192,27 @@ void main() {
     SpringDescription spring = new SpringDescription.withDampingRatio(
         mass: 1.0, springConstant: 50.0, ratio: 0.5);
 
-    ScrollSimulation scroll = new ScrollSimulation(100.0, 800.0, 0.0, 300.0, spring, 0.3);
+    ScrollSimulation scroll = new ScrollSimulation(
+      position: 100.0,
+      velocity: 800.0,
+      leadingExtent: 0.0,
+      trailingExtent: 300.0,
+      spring: spring,
+      drag: 0.3
+    );
     scroll.tolerance = const Tolerance(velocity: 0.5, distance: 0.1);
     expect(scroll.isDone(0.0), false);
     expect(scroll.isDone(0.5), false); // switch from friction to spring
     expect(scroll.isDone(3.5), true);
 
-    ScrollSimulation scroll2 = new ScrollSimulation(100.0, -800.0, 0.0, 300.0, spring, 0.3);
+    ScrollSimulation scroll2 = new ScrollSimulation(
+      position: 100.0,
+      velocity: -800.0,
+      leadingExtent: 0.0,
+      trailingExtent: 300.0,
+      spring: spring,
+      drag: 0.3
+    );
     scroll2.tolerance = const Tolerance(velocity: 0.5, distance: 0.1);
     expect(scroll2.isDone(0.0), false);
     expect(scroll2.isDone(0.5), false); // switch from friction to spring
@@ -208,8 +223,14 @@ void main() {
     SpringDescription spring = new SpringDescription.withDampingRatio(
         mass: 1.0, springConstant: 50.0, ratio: 0.5);
 
-    ScrollSimulation scroll =
-        new ScrollSimulation(100.0, 400.0, 0.0, double.INFINITY, spring, 0.3);
+    ScrollSimulation scroll = new ScrollSimulation(
+      position: 100.0,
+      velocity: 400.0,
+      leadingExtent: 0.0,
+      trailingExtent: double.INFINITY,
+      spring: spring,
+      drag: 0.3,
+    );
     scroll.tolerance = const Tolerance(velocity: 1.0);
 
     expect(scroll.isDone(0.0), false);
@@ -232,7 +253,14 @@ void main() {
 
   test('over/under scroll spring', () {
     SpringDescription spring = new SpringDescription.withDampingRatio(mass: 1.0, springConstant: 170.0, ratio: 1.1);
-    ScrollSimulation scroll = new ScrollSimulation(500.0, -7500.0, 0.0, 1000.0, spring, 0.025);
+    ScrollSimulation scroll = new ScrollSimulation(
+      position: 500.0,
+      velocity: -7500.0,
+      leadingExtent: 0.0,
+      trailingExtent: 1000.0,
+      spring: spring,
+      drag: 0.025,
+    );
     scroll.tolerance = new Tolerance(velocity: 45.0, distance: 1.5);
 
     expect(scroll.isDone(0.0), false);