Fix bottom sheet rebuilding when tapping (#127526)

This fixes an issue where the bottom sheet would rebuild when `enableDrag` is set to true on every tap. This is because `DragGestureRecognizer` would win the arena by default and dispatch the `drag` callbacks (in `acceptGesture`) even though it had not met the drag threshold. This changes keep the default behavior of `DragGestureRecognizer` the same, but adds a parameter `onlyAcceptDragOnThreshold` that a user can use to stop drag callbacks from being fired when the drag threshold has not been met.

Fixes #126833

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

@@ -77,6 +77,7 @@ abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
     super.debugOwner,
     this.dragStartBehavior = DragStartBehavior.start,
     this.velocityTrackerBuilder = _defaultBuilder,
+    this.onlyAcceptDragOnThreshold = false,
     super.supportedDevices,
     AllowedButtonsFilter? allowedButtonsFilter,
   }) : super(allowedButtonsFilter: allowedButtonsFilter ?? _defaultButtonAcceptBehavior);
@@ -210,6 +211,25 @@ abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
   /// If null then [kMaxFlingVelocity] is used.
   double? maxFlingVelocity;
 
+  /// Whether the drag threshold should be met before dispatching any drag callbacks.
+  ///
+  /// The drag threshold is met when the global distance traveled by a pointer has
+  /// exceeded the defined threshold on the relevant axis, i.e. y-axis for the
+  /// [VerticalDragGestureRecognizer], x-axis for the [HorizontalDragGestureRecognizer],
+  /// and the entire plane for [PanGestureRecognizer]. The threshold for both
+  /// [VerticalDragGestureRecognizer] and [HorizontalDragGestureRecognizer] are
+  /// calculated by [computeHitSlop], while [computePanSlop] is used for
+  /// [PanGestureRecognizer].
+  ///
+  /// If true, the drag callbacks will only be dispatched when this recognizer has
+  /// won the arena and the drag threshold has been met.
+  ///
+  /// If false, the drag callbacks will be dispatched immediately when this recognizer
+  /// has won the arena.
+  ///
+  /// This value defaults to false.
+  bool onlyAcceptDragOnThreshold;
+
   /// Determines the type of velocity estimation method to use for a potential
   /// drag gesture, when a new pointer is added.
   ///
@@ -284,6 +304,7 @@ abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
   Offset _getDeltaForDetails(Offset delta);
   double? _getPrimaryValueFromOffset(Offset value);
   bool _hasSufficientGlobalDistanceToAccept(PointerDeviceKind pointerDeviceKind, double? deviceTouchSlop);
+  bool _hasDragThresholdBeenMet = false;
 
   final Map<int, VelocityTracker> _velocityTrackers = <int, VelocityTracker>{};
 
@@ -386,7 +407,12 @@ abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
           untransformedEndPosition: localPosition
         ).distance * (_getPrimaryValueFromOffset(movedLocally) ?? 1).sign;
         if (_hasSufficientGlobalDistanceToAccept(event.kind, gestureSettings?.touchSlop)) {
-          resolve(GestureDisposition.accepted);
+          _hasDragThresholdBeenMet = true;
+          if (_acceptedActivePointers.contains(event.pointer)) {
+            _checkDrag(event.pointer);
+          } else {
+            resolve(GestureDisposition.accepted);
+          }
         }
       }
     }
@@ -401,45 +427,8 @@ abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
   void acceptGesture(int pointer) {
     assert(!_acceptedActivePointers.contains(pointer));
     _acceptedActivePointers.add(pointer);
-    if (_state != _DragState.accepted) {
-      _state = _DragState.accepted;
-      final OffsetPair delta = _pendingDragOffset;
-      final Duration? timestamp = _lastPendingEventTimestamp;
-      final Matrix4? transform = _lastTransform;
-      final Offset localUpdateDelta;
-      switch (dragStartBehavior) {
-        case DragStartBehavior.start:
-          _initialPosition = _initialPosition + delta;
-          localUpdateDelta = Offset.zero;
-        case DragStartBehavior.down:
-          localUpdateDelta = _getDeltaForDetails(delta.local);
-      }
-      _pendingDragOffset = OffsetPair.zero;
-      _lastPendingEventTimestamp = null;
-      _lastTransform = null;
-      _checkStart(timestamp, pointer);
-      if (localUpdateDelta != Offset.zero && onUpdate != null) {
-        final Matrix4? localToGlobal = transform != null ? Matrix4.tryInvert(transform) : null;
-        final Offset correctedLocalPosition = _initialPosition.local + localUpdateDelta;
-        final Offset globalUpdateDelta = PointerEvent.transformDeltaViaPositions(
-          untransformedEndPosition: correctedLocalPosition,
-          untransformedDelta: localUpdateDelta,
-          transform: localToGlobal,
-        );
-        final OffsetPair updateDelta = OffsetPair(local: localUpdateDelta, global: globalUpdateDelta);
-        final OffsetPair correctedPosition = _initialPosition + updateDelta; // Only adds delta for down behaviour
-        _checkUpdate(
-          sourceTimeStamp: timestamp,
-          delta: localUpdateDelta,
-          primaryDelta: _getPrimaryValueFromOffset(localUpdateDelta),
-          globalPosition: correctedPosition.global,
-          localPosition: correctedPosition.local,
-        );
-      }
-      // This acceptGesture might have been called only for one pointer, instead
-      // of all pointers. Resolve all pointers to `accepted`. This won't cause
-      // infinite recursion because an accepted pointer won't be accepted again.
-      resolve(GestureDisposition.accepted);
+    if (!onlyAcceptDragOnThreshold || _hasDragThresholdBeenMet) {
+      _checkDrag(pointer);
     }
   }
 
@@ -462,6 +451,7 @@ abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
       case _DragState.accepted:
         _checkEnd(pointer);
     }
+    _hasDragThresholdBeenMet = false;
     _velocityTrackers.clear();
     _initialButtons = null;
     _state = _DragState.ready;
@@ -486,6 +476,50 @@ abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
     }
   }
 
+  void _checkDrag(int pointer) {
+    if (_state == _DragState.accepted) {
+      return;
+    }
+    _state = _DragState.accepted;
+    final OffsetPair delta = _pendingDragOffset;
+    final Duration? timestamp = _lastPendingEventTimestamp;
+    final Matrix4? transform = _lastTransform;
+    final Offset localUpdateDelta;
+    switch (dragStartBehavior) {
+      case DragStartBehavior.start:
+        _initialPosition = _initialPosition + delta;
+        localUpdateDelta = Offset.zero;
+      case DragStartBehavior.down:
+        localUpdateDelta = _getDeltaForDetails(delta.local);
+    }
+    _pendingDragOffset = OffsetPair.zero;
+    _lastPendingEventTimestamp = null;
+    _lastTransform = null;
+    _checkStart(timestamp, pointer);
+    if (localUpdateDelta != Offset.zero && onUpdate != null) {
+      final Matrix4? localToGlobal = transform != null ? Matrix4.tryInvert(transform) : null;
+      final Offset correctedLocalPosition = _initialPosition.local + localUpdateDelta;
+      final Offset globalUpdateDelta = PointerEvent.transformDeltaViaPositions(
+        untransformedEndPosition: correctedLocalPosition,
+        untransformedDelta: localUpdateDelta,
+        transform: localToGlobal,
+      );
+      final OffsetPair updateDelta = OffsetPair(local: localUpdateDelta, global: globalUpdateDelta);
+      final OffsetPair correctedPosition = _initialPosition + updateDelta; // Only adds delta for down behaviour
+      _checkUpdate(
+        sourceTimeStamp: timestamp,
+        delta: localUpdateDelta,
+        primaryDelta: _getPrimaryValueFromOffset(localUpdateDelta),
+        globalPosition: correctedPosition.global,
+        localPosition: correctedPosition.local,
+      );
+    }
+    // This acceptGesture might have been called only for one pointer, instead
+    // of all pointers. Resolve all pointers to `accepted`. This won't cause
+    // infinite recursion because an accepted pointer won't be accepted again.
+    resolve(GestureDisposition.accepted);
+  }
+
   void _checkStart(Duration? timestamp, int pointer) {
     if (onStart != null) {
       final DragStartDetails details = DragStartDetails(

packages/flutter/lib/src/material/bottom_sheet.dart

@@ -357,15 +357,14 @@ class _BottomSheetState extends State<BottomSheet> {
         dragHandleColor: widget.dragHandleColor,
         dragHandleSize: widget.dragHandleSize,
       );
-      // Only add [GestureDetector] to the drag handle when the rest of the
+      // Only add [_BottomSheetGestureDetector] to the drag handle when the rest of the
       // bottom sheet is not draggable. If the whole bottom sheet is draggable,
       // no need to add it.
       if (!widget.enableDrag) {
-        dragHandle = GestureDetector(
+        dragHandle = _BottomSheetGestureDetector(
           onVerticalDragStart: _handleDragStart,
           onVerticalDragUpdate: _handleDragUpdate,
           onVerticalDragEnd: _handleDragEnd,
-          excludeFromSemantics: true,
           child: dragHandle,
         );
       }
@@ -407,11 +406,10 @@ class _BottomSheetState extends State<BottomSheet> {
       );
     }
 
-    return !widget.enableDrag ? bottomSheet : GestureDetector(
+    return !widget.enableDrag ? bottomSheet : _BottomSheetGestureDetector(
       onVerticalDragStart: _handleDragStart,
       onVerticalDragUpdate: _handleDragUpdate,
       onVerticalDragEnd: _handleDragEnd,
-      excludeFromSemantics: true,
       child: bottomSheet,
     );
   }
@@ -1300,7 +1298,39 @@ PersistentBottomSheetController<T> showBottomSheet<T>({
   );
 }
 
+class _BottomSheetGestureDetector extends StatelessWidget {
+  const _BottomSheetGestureDetector({
+    required this.child,
+    required this.onVerticalDragStart,
+    required this.onVerticalDragUpdate,
+    required this.onVerticalDragEnd,
+  });
+
+  final Widget child;
+  final GestureDragStartCallback onVerticalDragStart;
+  final GestureDragUpdateCallback onVerticalDragUpdate;
+  final GestureDragEndCallback onVerticalDragEnd;
 
+  @override
+  Widget build(BuildContext context) {
+    return RawGestureDetector(
+      excludeFromSemantics: true,
+      gestures: <Type, GestureRecognizerFactory<GestureRecognizer>>{
+        VerticalDragGestureRecognizer : GestureRecognizerFactoryWithHandlers<VerticalDragGestureRecognizer>(
+          () => VerticalDragGestureRecognizer(debugOwner: this),
+          (VerticalDragGestureRecognizer instance) {
+            instance
+              ..onStart = onVerticalDragStart
+              ..onUpdate = onVerticalDragUpdate
+              ..onEnd = onVerticalDragEnd
+              ..onlyAcceptDragOnThreshold = true;
+          },
+        ),
+      },
+      child: child,
+    );
+  }
+}
 
 // BEGIN GENERATED TOKEN PROPERTIES - BottomSheet
 

packages/flutter/test/gestures/monodrag_test.dart

@@ -74,6 +74,75 @@ void main() {
     GestureBinding.instance.handleEvent(up91, HitTestEntry(MockHitTestTarget()));
   });
 
+  testGesture('DragGestureRecognizer should not dispatch drag callbacks when it wins the arena if onlyAcceptDragOnThreshold is true and the threshold has not been met', (GestureTester tester) {
+    final VerticalDragGestureRecognizer verticalDrag = VerticalDragGestureRecognizer();
+    final List<String> dragCallbacks = <String>[];
+    verticalDrag
+      ..onlyAcceptDragOnThreshold = true
+      ..onStart = (DragStartDetails details) {
+        dragCallbacks.add('onStart');
+      }
+      ..onUpdate = (DragUpdateDetails details) {
+        dragCallbacks.add('onUpdate');
+      }
+      ..onEnd = (DragEndDetails details) {
+        dragCallbacks.add('onEnd');
+      };
+
+    const PointerDownEvent down1 = PointerDownEvent(
+      pointer: 6,
+      position: Offset(10.0, 10.0),
+    );
+
+    const PointerUpEvent up1 = PointerUpEvent(
+      pointer: 6,
+      position: Offset(10.0, 10.0),
+    );
+
+    verticalDrag.addPointer(down1);
+    tester.closeArena(down1.pointer);
+    tester.route(down1);
+    tester.route(up1);
+    expect(dragCallbacks.isEmpty, true);
+    verticalDrag.dispose();
+    dragCallbacks.clear();
+  });
+
+  testGesture('DragGestureRecognizer should dispatch drag callbacks when it wins the arena if onlyAcceptDragOnThreshold is false and the threshold has not been met', (GestureTester tester) {
+    final VerticalDragGestureRecognizer verticalDrag = VerticalDragGestureRecognizer();
+    final List<String> dragCallbacks = <String>[];
+    verticalDrag
+      ..onlyAcceptDragOnThreshold = false
+      ..onStart = (DragStartDetails details) {
+        dragCallbacks.add('onStart');
+      }
+      ..onUpdate = (DragUpdateDetails details) {
+        dragCallbacks.add('onUpdate');
+      }
+      ..onEnd = (DragEndDetails details) {
+        dragCallbacks.add('onEnd');
+      };
+
+    const PointerDownEvent down1 = PointerDownEvent(
+      pointer: 6,
+      position: Offset(10.0, 10.0),
+    );
+
+    const PointerUpEvent up1 = PointerUpEvent(
+      pointer: 6,
+      position: Offset(10.0, 10.0),
+    );
+
+    verticalDrag.addPointer(down1);
+    tester.closeArena(down1.pointer);
+    tester.route(down1);
+    tester.route(up1);
+    expect(dragCallbacks.isEmpty, false);
+    expect(dragCallbacks, <String>['onStart', 'onEnd']);
+    verticalDrag.dispose();
+    dragCallbacks.clear();
+  });
+
   group('Recognizers on different button filters:', () {
     final List<String> recognized = <String>[];
     late HorizontalDragGestureRecognizer primaryRecognizer;

packages/flutter/test/material/bottom_sheet_test.dart

@@ -199,6 +199,43 @@ void main() {
     expect(find.text('BottomSheet'), findsNothing);
   });
 
+  testWidgets('Tapping on a BottomSheet should not trigger a rebuild when enableDrag is true', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/126833.
+    final GlobalKey<ScaffoldState> scaffoldKey = GlobalKey<ScaffoldState>();
+    int buildCount = 0;
+
+    await tester.pumpWidget(MaterialApp(
+      home: Scaffold(
+        key: scaffoldKey,
+        body: const Center(child: Text('body')),
+      ),
+    ));
+
+    await tester.pump();
+    expect(buildCount, 0);
+    expect(find.text('BottomSheet'), findsNothing);
+
+    scaffoldKey.currentState!.showBottomSheet<void>((BuildContext context) {
+      buildCount++;
+      return const SizedBox(
+        height: 200.0,
+        child: Text('BottomSheet'),
+      );
+    },
+     enableDrag: true,
+    );
+
+    await tester.pumpAndSettle();
+    expect(buildCount, 1);
+    expect(find.text('BottomSheet'), findsOneWidget);
+
+    // Tap on bottom sheet should not trigger a rebuild.
+    await tester.tap(find.text('BottomSheet'));
+    await tester.pumpAndSettle();
+    expect(buildCount, 1);
+    expect(find.text('BottomSheet'), findsOneWidget);
+  });
+
   testWidgets('Modal BottomSheet builder should only be called once', (WidgetTester tester) async {
     late BuildContext savedContext;