Add callback when dismiss threshold is reached (#88736)

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

@@ -30,6 +30,11 @@ typedef DismissDirectionCallback = void Function(DismissDirection direction);
 /// Used by [Dismissible.confirmDismiss].
 typedef ConfirmDismissCallback = Future<bool?> Function(DismissDirection direction);
 
+/// Signature used by [Dismissible] to indicate that the dismissible has been dragged.
+///
+/// Used by [Dismissible.onUpdate].
+typedef DismissUpdateCallback = void Function(DismissUpdateDetails details);
+
 /// The direction in which a [Dismissible] can be dismissed.
 enum DismissDirection {
   /// The [Dismissible] can be dismissed by dragging either up or down.
@@ -98,6 +103,7 @@ class Dismissible extends StatefulWidget {
     this.secondaryBackground,
     this.confirmDismiss,
     this.onResize,
+    this.onUpdate,
     this.onDismissed,
     this.direction = DismissDirection.horizontal,
     this.resizeDuration = const Duration(milliseconds: 300),
@@ -205,10 +211,44 @@ class Dismissible extends StatefulWidget {
   /// This defaults to [HitTestBehavior.opaque].
   final HitTestBehavior behavior;
 
+  /// Called when the dismissible widget has been dragged.
+  ///
+  /// If [onUpdate] is not null, then it will be invoked for every pointer event
+  /// to dispatch the latest state of the drag. For example, this callback
+  /// can be used to for example change the color of the background widget
+  /// depending on whether the dismiss threshold is currently reached.
+  final DismissUpdateCallback? onUpdate;
+
   @override
   State<Dismissible> createState() => _DismissibleState();
 }
 
+/// Details for [DismissUpdateCallback].
+///
+/// See also:
+///
+///   * [Dismissible.onUpdate], which receives this information.
+class DismissUpdateDetails {
+  /// Create a new instance of [DismissUpdateDetails].
+  DismissUpdateDetails({
+    this.direction = DismissDirection.horizontal,
+    this.reached = false,
+    this.previousReached = false
+  });
+
+  /// The direction that the dismissible is being dragged.
+  final DismissDirection direction;
+
+  /// Whether the dismiss threshold is currently reached.
+  final bool reached;
+
+  /// Whether the dismiss threshold was reached the last time this callback was invoked.
+  ///
+  /// This can be used in conjunction with [DismissUpdateDetails.reached] to catch the moment
+  /// that the [Dismissible] is dragged across the threshold.
+  final bool previousReached;
+}
+
 class _DismissibleClipper extends CustomClipper<Rect> {
   _DismissibleClipper({
     required this.axis,
@@ -254,7 +294,8 @@ class _DismissibleState extends State<Dismissible> with TickerProviderStateMixin
   void initState() {
     super.initState();
     _moveController = AnimationController(duration: widget.movementDuration, vsync: this)
-      ..addStatusListener(_handleDismissStatusChanged);
+      ..addStatusListener(_handleDismissStatusChanged)
+      ..addListener(_handleDismissUpdateValueChanged);
     _updateMoveAnimation();
   }
 
@@ -268,6 +309,7 @@ class _DismissibleState extends State<Dismissible> with TickerProviderStateMixin
   bool _confirming = false;
   bool _dragUnderway = false;
   Size? _sizePriorToCollapse;
+  bool _dismissThresholdReached = false;
 
   @override
   bool get wantKeepAlive => _moveController?.isAnimating == true || _resizeController?.isAnimating == true;
@@ -388,6 +430,19 @@ class _DismissibleState extends State<Dismissible> with TickerProviderStateMixin
     }
   }
 
+  void _handleDismissUpdateValueChanged() {
+    if(widget.onUpdate != null) {
+      final bool oldDismissThresholdReached = _dismissThresholdReached;
+      _dismissThresholdReached = _moveController!.value > (widget.dismissThresholds[_dismissDirection] ?? _kDismissThreshold);
+      final DismissUpdateDetails details = DismissUpdateDetails(
+          direction: _dismissDirection,
+          reached: _dismissThresholdReached,
+          previousReached: oldDismissThresholdReached,
+      );
+      widget.onUpdate!(details);
+    }
+  }
+
   void _updateMoveAnimation() {
     final double end = _dragExtent.sign;
     _moveAnimation = _moveController!.drive(

packages/flutter/test/widgets/dismissible_test.dart

@@ -10,6 +10,9 @@ import 'package:flutter_test/flutter_test.dart';
 
 const DismissDirection defaultDismissDirection = DismissDirection.horizontal;
 const double crossAxisEndOffset = 0.5;
+bool reportedDismissUpdateReached = false;
+bool reportedDismissUpdatePreviousReached = false;
+late DismissDirection reportedDismissUpdateReachedDirection;
 
 DismissDirection reportedDismissDirection = DismissDirection.horizontal;
 List<int> dismissedItems = <int>[];
@@ -46,6 +49,11 @@ Widget buildTest({
             onResize: () {
               expect(dismissedItems.contains(item), isFalse);
             },
+            onUpdate: (DismissUpdateDetails details) {
+              reportedDismissUpdateReachedDirection = details.direction;
+              reportedDismissUpdateReached = details.reached;
+              reportedDismissUpdatePreviousReached = details.previousReached;
+            },
             background: background,
             dismissThresholds: startToEndThreshold == null
                 ? <DismissDirection, double>{}
@@ -1053,4 +1061,50 @@ void main() {
     expect(controller.offset, 100.0);
     controller.dispose();
   });
+
+  testWidgets('onUpdate', (WidgetTester tester) async {
+    await tester.pumpWidget(buildTest(
+      scrollDirection: Axis.horizontal,
+    ));
+    expect(dismissedItems, isEmpty);
+
+    // Successful dismiss therefore threshold has been reached
+    await dismissItem(tester, 0, mechanism: flingElement, gestureDirection: AxisDirection.left);
+    expect(find.text('0'), findsNothing);
+    expect(dismissedItems, equals(<int>[0]));
+    expect(reportedDismissUpdateReachedDirection, DismissDirection.endToStart);
+    expect(reportedDismissUpdateReached, true);
+    expect(reportedDismissUpdatePreviousReached, true);
+
+    // Unsuccessful dismiss, threshold has not been reached
+    await checkFlingItemAfterMovement(tester, 1, gestureDirection: AxisDirection.right);
+    expect(find.text('1'), findsOneWidget);
+    expect(dismissedItems, equals(<int>[0]));
+    expect(reportedDismissUpdateReachedDirection, DismissDirection.startToEnd);
+    expect(reportedDismissUpdateReached, false);
+    expect(reportedDismissUpdatePreviousReached, false);
+
+    // Another successful dismiss from another direction
+    await dismissItem(tester, 1, mechanism: flingElement, gestureDirection: AxisDirection.right);
+    expect(find.text('1'), findsNothing);
+    expect(dismissedItems, equals(<int>[0, 1]));
+    expect(reportedDismissUpdateReachedDirection, DismissDirection.startToEnd);
+    expect(reportedDismissUpdateReached, true);
+    expect(reportedDismissUpdatePreviousReached, true);
+
+    await tester.pumpWidget(buildTest(
+      scrollDirection: Axis.horizontal,
+      confirmDismiss: (BuildContext context, DismissDirection dismissDirection) {
+        return Future<bool>.value(false);
+      },
+    ));
+
+    // Threshold has been reached but dismiss was not confirmed
+    await dismissItem(tester, 2, mechanism: flingElement, gestureDirection: AxisDirection.right);
+    expect(find.text('2'), findsOneWidget);
+    expect(dismissedItems, equals(<int>[0, 1]));
+    expect(reportedDismissUpdateReachedDirection, DismissDirection.startToEnd);
+    expect(reportedDismissUpdateReached, false);
+    expect(reportedDismissUpdatePreviousReached, false);
+  });
 }