83 lines
3.3 KiB
Dart
83 lines
3.3 KiB
Dart
import 'package:flutter/physics.dart';
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import 'package:flutter/widgets.dart';
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// adapted from Flutter `FixedExtentScrollPhysics` in `/widgets/list_wheel_scroll_view.dart`
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class KnownExtentScrollPhysics extends ScrollPhysics {
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final double Function(int index) indexToScrollOffset;
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final int Function(double offset) scrollOffsetToIndex;
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const KnownExtentScrollPhysics({
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required this.indexToScrollOffset,
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required this.scrollOffsetToIndex,
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super.parent,
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});
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@override
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KnownExtentScrollPhysics applyTo(ScrollPhysics? ancestor) {
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return KnownExtentScrollPhysics(
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indexToScrollOffset: indexToScrollOffset,
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scrollOffsetToIndex: scrollOffsetToIndex,
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parent: buildParent(ancestor),
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);
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}
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@override
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Simulation? createBallisticSimulation(ScrollMetrics position, double velocity) {
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final ScrollMetrics metrics = position;
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// Scenario 1:
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// If we're out of range and not headed back in range, defer to the parent
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// ballistics, which should put us back in range at the scrollable's boundary.
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if ((velocity <= 0.0 && metrics.pixels <= metrics.minScrollExtent) || (velocity >= 0.0 && metrics.pixels >= metrics.maxScrollExtent)) {
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return super.createBallisticSimulation(metrics, velocity);
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}
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// Create a test simulation to see where it would have ballistically fallen
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// naturally without settling onto items.
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final Simulation? testFrictionSimulation = super.createBallisticSimulation(metrics, velocity);
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// Scenario 2:
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// If it was going to end up past the scroll extent, defer back to the
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// parent physics' ballistics again which should put us on the scrollable's
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// boundary.
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if (testFrictionSimulation != null && (testFrictionSimulation.x(double.infinity) == metrics.minScrollExtent || testFrictionSimulation.x(double.infinity) == metrics.maxScrollExtent)) {
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return super.createBallisticSimulation(metrics, velocity);
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}
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// From the natural final position, find the nearest item it should have
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// settled to.
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final offset = (testFrictionSimulation?.x(double.infinity) ?? metrics.pixels).clamp(metrics.minScrollExtent, metrics.maxScrollExtent);
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final int settlingItemIndex = scrollOffsetToIndex(offset);
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final double settlingPixels = indexToScrollOffset(settlingItemIndex);
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// Scenario 3:
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// If there's no velocity and we're already at where we intend to land,
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// do nothing.
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if (velocity.abs() < toleranceFor(position).velocity && (settlingPixels - metrics.pixels).abs() < toleranceFor(position).distance) {
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return null;
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}
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// Scenario 4:
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// If we're going to end back at the same item because initial velocity
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// is too low to break past it, use a spring simulation to get back.
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if (settlingItemIndex == scrollOffsetToIndex(metrics.pixels)) {
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return SpringSimulation(
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spring,
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metrics.pixels,
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settlingPixels,
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velocity,
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tolerance: toleranceFor(position),
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);
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}
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// Scenario 5:
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// Create a new friction simulation except the drag will be tweaked to land
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// exactly on the item closest to the natural stopping point.
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return FrictionSimulation.through(
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metrics.pixels,
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settlingPixels,
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velocity,
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toleranceFor(position).velocity * velocity.sign,
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);
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}
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}
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