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Reduce iOS scroll damping for lists with differently sized items #59623

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Merged
fluttergithubbot merged 3 commits into from
Aug 8, 2020
Merged

Reduce iOS scroll damping for lists with differently sized items #59623

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217db96
Reduce damping in iOS scrollbehavior for list with differently sized …
passsy Jun 17, 2020
72bb989
Add test
passsy Jun 17, 2020
1123696
Merge branch 'master' into feature/reduce_ios_damping
passsy Jul 14, 2020
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7 changes: 3 additions & 4 deletions packages/flutter/lib/src/widgets/scroll_physics.dart
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Original file line number Diff line number Diff line change
Expand Up @@ -532,7 +532,7 @@ class BouncingScrollPhysics extends ScrollPhysics {
return BouncingScrollSimulation(
spring: spring,
position: position.pixels,
velocity: velocity * 0.91, // TODO(abarth): We should move this constant closer to the drag end.
velocity: velocity,
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@LongCatIsLooong LongCatIsLooong Jun 25, 2020

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👍

leadingExtent: position.minScrollExtent,
trailingExtent: position.maxScrollExtent,
tolerance: tolerance,
Expand All @@ -548,9 +548,8 @@ class BouncingScrollPhysics extends ScrollPhysics {
double get minFlingVelocity => kMinFlingVelocity * 2.0;

// Methodology:
// 1- Use https://github.com/flutter/scroll_overlay to test with Flutter and
// platform scroll views superimposed.
// 2- Record incoming speed and make rapid flings in the test app.
// 1- Use https://github.com/flutter/platform_tests/tree/master/scroll_overlay to test with
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@LongCatIsLooong LongCatIsLooong Jun 25, 2020

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Thank you for updating this!

// Flutter and platform scroll views superimposed.
// 3- If the scrollables stopped overlapping at any moment, adjust the desired
// output value of this function at that input speed.
// 4- Feed new input/output set into a power curve fitter. Change function
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2 changes: 2 additions & 0 deletions packages/flutter/lib/src/widgets/scroll_simulation.dart
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Expand Up @@ -51,6 +51,8 @@ class BouncingScrollSimulation extends Simulation {
_springSimulation = _overscrollSimulation(position, velocity);
_springTime = double.negativeInfinity;
} else {
// Taken from UIScrollView.decelerationRate (.normal = 0.998)
// 0.998^1000 = ~0.135
_frictionSimulation = FrictionSimulation(0.135, position, velocity);
final double finalX = _frictionSimulation.finalX;
if (velocity > 0.0 && finalX > trailingExtent) {
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20 changes: 20 additions & 0 deletions packages/flutter/test/widgets/scroll_physics_test.dart
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Expand Up @@ -97,6 +97,26 @@ void main() {
);
});

test('ScrollPhysics scrolling subclasses - Creating the simulation doesn\'t alter the velocity for time 0', () {
final ScrollMetrics position = FixedScrollMetrics(
minScrollExtent: 0.0,
maxScrollExtent: 100.0,
pixels: 20.0,
viewportDimension: 500.0,
axisDirection: AxisDirection.down,
);

const BouncingScrollPhysics bounce = BouncingScrollPhysics();
const ClampingScrollPhysics clamp = ClampingScrollPhysics();
const PageScrollPhysics page = PageScrollPhysics();

// Calls to createBallisticSimulation may happen on every frame (i.e. when the maxScrollExtent changes)
// Changing velocity for time 0 may cause a sudden, unwanted damping/speedup effect
expect(bounce.createBallisticSimulation(position, 1000).dx(0), moreOrLessEquals(1000));
expect(clamp.createBallisticSimulation(position, 1000).dx(0), moreOrLessEquals(1000));
expect(page.createBallisticSimulation(position, 1000).dx(0), moreOrLessEquals(1000));
});

group('BouncingScrollPhysics test', () {
BouncingScrollPhysics physicsUnderTest;

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