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Fix lower DragTarget not being recognized in overlapping targets #188979

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Fix lower DragTarget not being recognized in overlapping targets #188979

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10 changes: 9 additions & 1 deletion packages/flutter/lib/src/widgets/drag_target.dart
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Original file line number Diff line number Diff line change
Expand Up @@ -918,7 +918,15 @@ class _DragAvatar<T extends Object> extends Drag {
}

// If everything's the same, report moves, and bail early.
if (listsMatch) {
//
// The prefix match in `listsMatch` is only sufficient to bail when a target
// has already accepted the drag (`_activeTarget != null`), because deeper
// targets below the active one are correctly ignored. When nothing has
// accepted yet, `_enteredTargets` holds every hit target, so a longer
// `targets` list means a new (lower) target has appeared under the pointer
// and must be given a chance to be entered. Requiring an exact-length match
// in that case avoids missing it.
if (listsMatch && (_activeTarget != null || targets.length == _enteredTargets.length)) {
for (final _DragTargetState<Object> target in _enteredTargets) {
target.didMove(this);
}
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167 changes: 167 additions & 0 deletions packages/flutter/test/widgets/draggable_test.dart
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Original file line number Diff line number Diff line change
Expand Up @@ -96,6 +96,173 @@ void main() {
expect(moveCount, 1);
});

// Regression test for https://github.com/flutter/flutter/issues/187543
testWidgets(
'Drag and drop - a lower DragTarget is recognized after entering an upper rejecting target',
(WidgetTester tester) async {
final accepted = <int>[];
const topKey = ValueKey<String>('upper');
const bottomKey = ValueKey<String>('lower');

await tester.pumpWidget(
TestWidgetsApp(
home: Column(
children: <Widget>[
const Draggable<int>(data: 1, feedback: Text('Dragging'), child: Text('Source')),
Expanded(
child: Stack(
children: <Widget>[
// Lower target, on the right half; it accepts.
Positioned(
left: 200.0,
top: 0.0,
width: 200.0,
height: 400.0,
child: DragTarget<int>(
key: bottomKey,
onWillAcceptWithDetails: (DragTargetDetails<int> _) => true,
onAcceptWithDetails: (DragTargetDetails<int> details) =>
accepted.add(details.data),
builder: (BuildContext context, List<int?> data, List<dynamic> rejects) =>
const SizedBox.expand(),
),
),
// Upper target, overlapping the lower one on its left; it rejects.
Positioned(
left: 0.0,
top: 0.0,
width: 300.0,
height: 400.0,
child: DragTarget<int>(
key: topKey,
onWillAcceptWithDetails: (DragTargetDetails<int> _) => false,
builder: (BuildContext context, List<int?> data, List<dynamic> rejects) =>
const SizedBox.expand(),
),
),
],
),
),
],
),
),
);

final Rect topRect = tester.getRect(find.byKey(topKey));
final Rect bottomRect = tester.getRect(find.byKey(bottomKey));
// Covered only by the upper (rejecting) target.
final onlyUpper = Offset(topRect.left + 20.0, topRect.center.dy);
// In the overlap, covered by both targets.
final overlap = Offset(bottomRect.left + 20.0, bottomRect.center.dy);

final TestGesture gesture = await tester.startGesture(
tester.getCenter(find.text('Source')),
pointer: 7,
);
await tester.pump();

// Enter the upper target first; it rejects, so nothing is accepted yet.
await gesture.moveTo(onlyUpper);
await tester.pump();
expect(accepted, isEmpty);

// Move into the overlap. The lower target must now be recognized even
// though the upper (rejecting) target is still under the pointer.
await gesture.moveTo(overlap);
await tester.pump();

await gesture.up();
await tester.pump();

expect(accepted, equals(<int>[1]));
},
);

// Companion to the regression test above (issue #187543): verifies the fix
// preserves the existing behavior where, once a target accepts, the drag does
// not fall through to a lower overlapping target.
testWidgets(
'Drag and drop - an accepting upper DragTarget is not bypassed by a lower overlapping target',
(WidgetTester tester) async {
final upperAccepted = <int>[];
final lowerAccepted = <int>[];
const topKey = ValueKey<String>('upper');
const bottomKey = ValueKey<String>('lower');

await tester.pumpWidget(
TestWidgetsApp(
home: Column(
children: <Widget>[
const Draggable<int>(data: 1, feedback: Text('Dragging'), child: Text('Source')),
Expanded(
child: Stack(
children: <Widget>[
// Lower target, on the right half; it also accepts.
Positioned(
left: 200.0,
top: 0.0,
width: 200.0,
height: 400.0,
child: DragTarget<int>(
key: bottomKey,
onWillAcceptWithDetails: (DragTargetDetails<int> _) => true,
onAcceptWithDetails: (DragTargetDetails<int> details) =>
lowerAccepted.add(details.data),
builder: (BuildContext context, List<int?> data, List<dynamic> rejects) =>
const SizedBox.expand(),
),
),
// Upper target, overlapping the lower one on its left; it accepts.
Positioned(
left: 0.0,
top: 0.0,
width: 300.0,
height: 400.0,
child: DragTarget<int>(
key: topKey,
onWillAcceptWithDetails: (DragTargetDetails<int> _) => true,
onAcceptWithDetails: (DragTargetDetails<int> details) =>
upperAccepted.add(details.data),
builder: (BuildContext context, List<int?> data, List<dynamic> rejects) =>
const SizedBox.expand(),
),
),
],
),
),
],
),
),
);

final Rect topRect = tester.getRect(find.byKey(topKey));
final Rect bottomRect = tester.getRect(find.byKey(bottomKey));
final onlyUpper = Offset(topRect.left + 20.0, topRect.center.dy);
final overlap = Offset(bottomRect.left + 20.0, bottomRect.center.dy);

final TestGesture gesture = await tester.startGesture(
tester.getCenter(find.text('Source')),
pointer: 7,
);
await tester.pump();

// Enter the upper target first; it accepts, becoming the active target.
await gesture.moveTo(onlyUpper);
await tester.pump();

// Move into the overlap. The drag must not fall through the already
// accepting upper target to the lower one.
await gesture.moveTo(overlap);
await tester.pump();

await gesture.up();
await tester.pump();

expect(upperAccepted, equals(<int>[1]));
expect(lowerAccepted, isEmpty);
},
);

// Regression test for https://github.com/flutter/flutter/issues/76825
testWidgets('Drag and drop - onLeave callback fires correctly with generic parameter', (
WidgetTester tester,
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