Make min/max fling velocity and min fling distance ScrollPhysics properties (#8928)

packages/flutter/lib/src/gestures/drag.dart

@@ -204,12 +204,6 @@ typedef void GestureDragEndCallback(DragEndDetails details);
 /// See [DragGestureRecognizer.onCancel].
 typedef void GestureDragCancelCallback();
 
-bool _isFlingGesture(Velocity velocity) {
-  assert(velocity != null);
-  final double speedSquared = velocity.pixelsPerSecond.distanceSquared;
-  return speedSquared > kMinFlingVelocity * kMinFlingVelocity;
-}
-
 /// Recognizes movement.
 ///
 /// In contrast to [MultiDragGestureRecognizer], [DragGestureRecognizer]
@@ -256,10 +250,30 @@ abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
   /// The pointer that previously triggered [onDown] did not complete.
   GestureDragCancelCallback onCancel;
 
+  /// The minimum distance an input pointer drag must have moved to
+  /// to be considered a fling gesture.
+  ///
+  /// This value is typically compared with the distance traveled along the
+  /// scrolling axis. If null then [kTouchSlop] is used.
+  double minFlingDistance;
+
+  /// The minimum velocity for an input pointer drag to be considered fling.
+  ///
+  /// This value is typically compared with the magnitude of fling gesture's
+  /// velocity along the scrolling axis. If null then [kMinFlingVelocity]
+  /// is used.
+  double minFlingVelocity;
+
+  /// Fling velocity magnitudes will be clamped to this value.
+  ///
+  /// If null then [kMaxFlingVelocity] is used.
+  double maxFlingVelocity;
+
   _DragState _state = _DragState.ready;
   Point _initialPosition;
   Offset _pendingDragOffset;
 
+  bool _isFlingGesture(VelocityEstimate estimate);
   Offset _getDeltaForDetails(Offset delta);
   double _getPrimaryValueFromOffset(Offset value);
   bool get _hasSufficientPendingDragDeltaToAccept;
@@ -345,11 +359,10 @@ abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
       final VelocityTracker tracker = _velocityTrackers[pointer];
       assert(tracker != null);
 
-      Velocity velocity = tracker.getVelocity();
-      if (velocity != null && _isFlingGesture(velocity)) {
-        final Offset pixelsPerSecond = velocity.pixelsPerSecond;
-        if (pixelsPerSecond.distanceSquared > kMaxFlingVelocity * kMaxFlingVelocity)
-          velocity = new Velocity(pixelsPerSecond: (pixelsPerSecond / pixelsPerSecond.distance) * kMaxFlingVelocity);
+      final VelocityEstimate estimate = tracker.getVelocityEstimate();
+      if (estimate != null && _isFlingGesture(estimate)) {
+        final Velocity velocity = new Velocity(pixelsPerSecond: estimate.pixelsPerSecond)
+          .clampMagnitude(minFlingVelocity ?? kMinFlingVelocity, maxFlingVelocity ?? kMaxFlingVelocity);
         invokeCallback<Null>('onEnd', () => onEnd(new DragEndDetails( // ignore: STRONG_MODE_INVALID_CAST_FUNCTION_EXPR, https://github.com/dart-lang/sdk/issues/27504
           velocity: velocity,
           primaryVelocity: _getPrimaryValueFromOffset(velocity.pixelsPerSecond),
@@ -379,6 +392,13 @@ abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
 ///
 ///  * [VerticalMultiDragGestureRecognizer]
 class VerticalDragGestureRecognizer extends DragGestureRecognizer {
+  @override
+  bool _isFlingGesture(VelocityEstimate estimate) {
+    final double minVelocity = minFlingVelocity ?? kMinFlingVelocity;
+    final double minDistance = minFlingDistance ?? kTouchSlop;
+    return estimate.pixelsPerSecond.dy.abs() > minVelocity && estimate.offset.dy.abs() > minDistance;
+  }
+
   @override
   bool get _hasSufficientPendingDragDeltaToAccept => _pendingDragOffset.dy.abs() > kTouchSlop;
 
@@ -400,6 +420,13 @@ class VerticalDragGestureRecognizer extends DragGestureRecognizer {
 ///
 ///  * [HorizontalMultiDragGestureRecognizer]
 class HorizontalDragGestureRecognizer extends DragGestureRecognizer {
+  @override
+  bool _isFlingGesture(VelocityEstimate estimate) {
+    final double minVelocity = minFlingVelocity ?? kMinFlingVelocity;
+    final double minDistance = minFlingDistance ?? kTouchSlop;
+    return estimate.pixelsPerSecond.dx.abs() > minVelocity && estimate.offset.dx.abs() > minDistance;
+  }
+
   @override
   bool get _hasSufficientPendingDragDeltaToAccept => _pendingDragOffset.dx.abs() > kTouchSlop;
 
@@ -420,6 +447,14 @@ class HorizontalDragGestureRecognizer extends DragGestureRecognizer {
 ///  * [ImmediateMultiDragGestureRecognizer]
 ///  * [DelayedMultiDragGestureRecognizer]
 class PanGestureRecognizer extends DragGestureRecognizer {
+  @override
+  bool _isFlingGesture(VelocityEstimate estimate) {
+    final double minVelocity = minFlingVelocity ?? kMinFlingVelocity;
+    final double minDistance = minFlingDistance ?? kTouchSlop;
+    return estimate.pixelsPerSecond.distanceSquared > minVelocity * minVelocity &&
+      estimate.offset.distanceSquared > minDistance * minDistance;
+  }
+
   @override
   bool get _hasSufficientPendingDragDeltaToAccept {
     return _pendingDragOffset.distance > kPanSlop;

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

@@ -4,6 +4,7 @@
 
 import 'dart:math' as math;
 
+import 'package:flutter/gestures.dart' show kMinFlingVelocity;
 import 'package:flutter/physics.dart';
 
 import 'overscroll_indicator.dart';
@@ -81,6 +82,12 @@ class BouncingScrollPhysics extends ScrollPhysics {
     }
     return null;
   }
+
+  // The ballistic simulation here decelerates more slowly than the one for
+  // ClampingScrollPhysics so we require a more deliberate input gesture
+  // to trigger a fling.
+  @override
+  double get minFlingVelocity => kMinFlingVelocity * 2.0;
 }
 
 /// Scroll physics for environments that prevent the scroll offset from reaching

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

@@ -106,6 +106,33 @@ abstract class ScrollPhysics {
 
   Tolerance get tolerance => parent?.tolerance ?? _kDefaultTolerance;
 
+  /// The minimum distance an input pointer drag must have moved to
+  /// to be considered a scroll fling gesture.
+  ///
+  /// This value is typically compared with the distance traveled along the
+  /// scrolling axis.
+  ///
+  /// See also:
+  ///
+  ///  * [VelocityTracker.getVelocityEstimate], which computes the velocity
+  ///    of a press-drag-release gesture.
+  double get minFlingDistance => parent?.minFlingDistance ?? kTouchSlop;
+
+  /// The minimum velocity for an input pointer drag to be considered a
+  /// scroll fling.
+  ///
+  /// This value is typically compared with the magnitude of fling gesture's
+  /// velocity along the scrolling axis.
+  ///
+  /// See also:
+  ///
+  ///  * [VelocityTracker.getVelocityEstimate], which computes the velocity
+  ///    of a press-drag-release gesture.
+  double get minFlingVelocity => parent?.minFlingVelocity ?? kMinFlingVelocity;
+
+  /// Scroll fling velocity magnitudes will be clamped to this value.
+  double get maxFlingVelocity => parent?.maxFlingVelocity ?? kMaxFlingVelocity;
+
   @override
   String toString() {
     if (parent == null)

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

@@ -118,13 +118,14 @@ class ScrollableState extends State<Scrollable> with TickerProviderStateMixin
   ScrollPosition _position;
 
   ScrollBehavior _configuration;
+  ScrollPhysics _physics;
 
-  // only call this from places that will definitely trigger a rebuild
+  // Only call this from places that will definitely trigger a rebuild.
   void _updatePosition() {
     _configuration = ScrollConfiguration.of(context);
-    ScrollPhysics physics = _configuration.getScrollPhysics(context);
+    _physics = _configuration.getScrollPhysics(context);
     if (config.physics != null)
-      physics = config.physics.applyTo(physics);
+      _physics = config.physics.applyTo(_physics);
     final ScrollController controller = config.controller;
     final ScrollPosition oldPosition = position;
     if (oldPosition != null) {
@@ -135,8 +136,8 @@ class ScrollableState extends State<Scrollable> with TickerProviderStateMixin
       scheduleMicrotask(oldPosition.dispose);
     }
 
-    _position = controller?.createScrollPosition(physics, this, oldPosition)
-      ?? ScrollController.createDefaultScrollPosition(physics, this, oldPosition);
+    _position = controller?.createScrollPosition(_physics, this, oldPosition)
+      ?? ScrollController.createDefaultScrollPosition(_physics, this, oldPosition);
     assert(position != null);
     controller?.attach(position);
   }
@@ -201,7 +202,10 @@ class ScrollableState extends State<Scrollable> with TickerProviderStateMixin
                 ..onDown = _handleDragDown
                 ..onStart = _handleDragStart
                 ..onUpdate = _handleDragUpdate
-                ..onEnd = _handleDragEnd;
+                ..onEnd = _handleDragEnd
+                ..minFlingDistance = _physics?.minFlingDistance
+                ..minFlingVelocity = _physics?.minFlingVelocity
+                ..maxFlingVelocity = _physics?.maxFlingVelocity;
             }
           };
           break;
@@ -212,7 +216,10 @@ class ScrollableState extends State<Scrollable> with TickerProviderStateMixin
                 ..onDown = _handleDragDown
                 ..onStart = _handleDragStart
                 ..onUpdate = _handleDragUpdate
-                ..onEnd = _handleDragEnd;
+                ..onEnd = _handleDragEnd
+                ..minFlingDistance = _physics?.minFlingDistance
+                ..minFlingVelocity = _physics?.minFlingVelocity
+                ..maxFlingVelocity = _physics?.maxFlingVelocity;
             }
           };
           break;

packages/flutter/test/material/modal_bottom_sheet_test.dart

@@ -102,7 +102,7 @@ void main() {
     expect(showBottomSheetThenCalled, isFalse);
     expect(find.text('BottomSheet'), findsOneWidget);
 
-    await tester.fling(find.text('BottomSheet'), const Offset(0.0, 20.0), 1000.0);
+    await tester.fling(find.text('BottomSheet'), const Offset(0.0, 30.0), 1000.0);
     await tester.pump(); // drain the microtask queue (Future completion callback)
 
     expect(showBottomSheetThenCalled, isTrue);