Add Dismissible.confirmDismiss callback (#26901)

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

@@ -24,6 +24,12 @@ const double _kDismissThreshold = 0.4;
 /// Used by [Dismissible.onDismissed].
 typedef DismissDirectionCallback = void Function(DismissDirection direction);
 
+/// Signature used by [Dismissible] to give the application an opportunity to
+/// confirm or veto a dismiss gesture.
+///
+/// Used by [Dismissible.confirmDismiss].
+typedef ConfirmDismissCallback = Future<bool> Function(DismissDirection direction);
+
 /// The direction in which a [Dismissible] can be dismissed.
 enum DismissDirection {
   /// The [Dismissible] can be dismissed by dragging either up or down.
@@ -77,6 +83,7 @@ class Dismissible extends StatefulWidget {
     @required this.child,
     this.background,
     this.secondaryBackground,
+    this.confirmDismiss,
     this.onResize,
     this.onDismissed,
     this.direction = DismissDirection.horizontal,
@@ -105,6 +112,15 @@ class Dismissible extends StatefulWidget {
   /// has also been specified.
   final Widget secondaryBackground;
 
+  /// Gives the app an opportunity to confirm or veto a pending dismissal.
+  ///
+  /// If the returned Future<bool> completes true, then this widget will be
+  /// dismissed, otherwise it will be moved back to its original location.
+  ///
+  /// If the returned Future<bool> completes to false or null the [onResize]
+  /// and [onDismissed] callbacks will not run.
+  final ConfirmDismissCallback confirmDismiss;
+
   /// Called when the widget changes size (i.e., when contracting before being dismissed).
   final VoidCallback onResize;
 
@@ -386,11 +402,11 @@ class _DismissibleState extends State<Dismissible> with TickerProviderStateMixin
     return _FlingGestureKind.reverse;
   }
 
-  void _handleDragEnd(DragEndDetails details) {
+  Future<void> _handleDragEnd(DragEndDetails details) async {
     if (!_isActive || _moveController.isAnimating)
       return;
     _dragUnderway = false;
-    if (_moveController.isCompleted) {
+    if (_moveController.isCompleted && await _confirmStartResizeAnimation() == true) {
       _startResizeAnimation();
       return;
     }
@@ -424,12 +440,25 @@ class _DismissibleState extends State<Dismissible> with TickerProviderStateMixin
     }
   }
 
-  void _handleDismissStatusChanged(AnimationStatus status) {
+  Future<void> _handleDismissStatusChanged(AnimationStatus status) async {
-    if (status == AnimationStatus.completed && !_dragUnderway)
+    if (status == AnimationStatus.completed && !_dragUnderway) {
-      _startResizeAnimation();
+      if (await _confirmStartResizeAnimation() == true)
+        _startResizeAnimation();
+      else
+        _moveController.reverse();
+    }
     updateKeepAlive();
   }
 
+  Future<bool> _confirmStartResizeAnimation() async {
+    if (widget.confirmDismiss != null) {
+      final DismissDirection direction = _dismissDirection;
+      assert(direction != null);
+      return widget.confirmDismiss(direction);
+    }
+    return true;
+  }
+
   void _startResizeAnimation() {
     assert(_moveController != null);
     assert(_moveController.isCompleted);
@@ -550,4 +579,3 @@ class _DismissibleState extends State<Dismissible> with TickerProviderStateMixin
     );
   }
 }
-

packages/flutter/test/material/dialog_test.dart

@@ -476,4 +476,135 @@ void main() {
 
     semantics.dispose();
   });
+
+  testWidgets('Dismissable.confirmDismiss defers to an AlertDialog', (WidgetTester tester) async {
+    final GlobalKey<ScaffoldState> _scaffoldKey = GlobalKey<ScaffoldState>();
+    final List<int> dismissedItems = <int>[];
+
+    // Dismiss is confirmed IFF confirmDismiss() returns true.
+    Future<bool> confirmDismiss (DismissDirection dismissDirection) {
+      return showDialog<bool>(
+        context: _scaffoldKey.currentContext,
+        barrierDismissible: true, // showDialog() returns null if tapped outside the dialog
+        builder: (BuildContext context) {
+          return AlertDialog(
+            actions: <Widget>[
+              FlatButton(
+                child: const Text('TRUE'),
+                onPressed: () {
+                  Navigator.pop(context, true); // showDialog() returns true
+                },
+              ),
+              FlatButton(
+                child: const Text('FALSE'),
+                onPressed: () {
+                  Navigator.pop(context, false); // showDialog() returns false
+                },
+              ),
+            ],
+          );
+        },
+      );
+    }
+
+    Widget buildDismissibleItem(int item, StateSetter setState) {
+      return Dismissible(
+        key: ValueKey<int>(item),
+        confirmDismiss: confirmDismiss,
+        onDismissed: (DismissDirection direction) {
+          setState(() {
+            expect(dismissedItems.contains(item), isFalse);
+            dismissedItems.add(item);
+          });
+        },
+        child: SizedBox(
+          height: 100.0,
+          child: Text(item.toString()),
+        ),
+      );
+    }
+
+    Widget buildFrame() {
+      return MaterialApp(
+        home: StatefulBuilder(
+          builder: (BuildContext context, StateSetter setState) {
+            return Scaffold(
+              key: _scaffoldKey,
+              body: Padding(
+                padding: const EdgeInsets.all(16.0),
+                child: ListView(
+                  itemExtent: 100.0,
+                  children: <int>[0, 1, 2, 3, 4]
+                    .where((int i) => !dismissedItems.contains(i))
+                    .map<Widget>((int item) => buildDismissibleItem(item, setState)).toList(),
+                ),
+              ),
+            );
+          },
+        ),
+      );
+    }
+
+    Future<void> dismissItem(WidgetTester tester, int item) async {
+      await tester.fling(find.text(item.toString()), const Offset(300.0, 0.0), 1000.0); // fling to the right
+      await tester.pump(); // start the slide
+      await tester.pump(const Duration(seconds: 1)); // finish the slide and start shrinking...
+      await tester.pump(); // first frame of shrinking animation
+      await tester.pump(const Duration(seconds: 1)); // finish the shrinking and call the callback...
+      await tester.pump(); // rebuild after the callback removes the entry
+    }
+
+    // Dismiss item 0 is confirmed via the AlertDialog
+    await tester.pumpWidget(buildFrame());
+    expect(dismissedItems, isEmpty);
+    await dismissItem(tester, 0); // Causes the AlertDialog to appear per confirmDismiss
+    await tester.pumpAndSettle();
+    await tester.tap(find.text('TRUE')); // AlertDialog action
+    await tester.pumpAndSettle();
+    expect(find.text('TRUE'), findsNothing); // Dialog was dismissed
+    expect(find.text('FALSE'), findsNothing);
+    expect(dismissedItems, <int>[0]);
+    expect(find.text('0'), findsNothing);
+
+    // Dismiss item 1 is not confirmed via the AlertDialog
+    await tester.pumpWidget(buildFrame());
+    expect(dismissedItems, <int>[0]);
+    await dismissItem(tester, 1); // Causes the AlertDialog to appear per confirmDismiss
+    await tester.pumpAndSettle();
+    await tester.tap(find.text('FALSE')); // AlertDialog action
+    await tester.pumpAndSettle();
+    expect(find.text('TRUE'), findsNothing); // Dialog was dismissed
+    expect(find.text('FALSE'), findsNothing);
+    expect(dismissedItems, <int>[0]);
+    expect(find.text('0'), findsNothing);
+    expect(find.text('1'), findsOneWidget);
+
+    // Dismiss item 1 is not confirmed via the AlertDialog
+    await tester.pumpWidget(buildFrame());
+    expect(dismissedItems, <int>[0]);
+    await dismissItem(tester, 1); // Causes the AlertDialog to appear per confirmDismiss
+    await tester.pumpAndSettle();
+    expect(find.text('FALSE'), findsOneWidget);
+    expect(find.text('TRUE'), findsOneWidget);
+    await tester.tapAt(Offset.zero); // Tap outside of the AlertDialog
+    await tester.pumpAndSettle();
+    expect(dismissedItems, <int>[0]);
+    expect(find.text('0'), findsNothing);
+    expect(find.text('1'), findsOneWidget);
+    expect(find.text('TRUE'), findsNothing); // Dialog was dismissed
+    expect(find.text('FALSE'), findsNothing);
+
+    // Dismiss item 1 is confirmed via the AlertDialog
+    await tester.pumpWidget(buildFrame());
+    expect(dismissedItems, <int>[0]);
+    await dismissItem(tester, 1); // Causes the AlertDialog to appear per confirmDismiss
+    await tester.pumpAndSettle();
+    await tester.tap(find.text('TRUE')); // AlertDialog action
+    await tester.pumpAndSettle();
+    expect(find.text('TRUE'), findsNothing); // Dialog was dismissed
+    expect(find.text('FALSE'), findsNothing);
+    expect(dismissedItems, <int>[0, 1]);
+    expect(find.text('0'), findsNothing);
+    expect(find.text('1'), findsNothing);
+  });
 }

packages/flutter/test/widgets/dismissible_test.dart

@@ -15,7 +15,11 @@ List<int> dismissedItems = <int>[];
 Widget background;
 const double crossAxisEndOffset = 0.5;
 
-Widget buildTest({ double startToEndThreshold, TextDirection textDirection = TextDirection.ltr }) {
+Widget buildTest({
+  double startToEndThreshold,
+  TextDirection textDirection = TextDirection.ltr,
+  Future<bool> Function(BuildContext context, DismissDirection direction) confirmDismiss,
+}) {
   return Directionality(
     textDirection: textDirection,
     child: StatefulBuilder(
@@ -25,6 +29,9 @@ Widget buildTest({ double startToEndThreshold, TextDirection textDirection = Tex
             dragStartBehavior: DragStartBehavior.down,
             key: ValueKey<int>(item),
             direction: dismissDirection,
+            confirmDismiss: confirmDismiss == null ? null : (DismissDirection direction) {
+              return confirmDismiss(context, direction);
+            },
             onDismissed: (DismissDirection direction) {
               setState(() {
                 reportedDismissDirection = direction;
@@ -660,4 +667,63 @@ void main() {
     expect(find.text('1'), findsOneWidget);
     expect(dismissedItems, isEmpty);
   });
+
+  testWidgets('confirmDismiss returns values: true, false, null', (WidgetTester tester) async {
+    scrollDirection = Axis.vertical;
+    dismissDirection = DismissDirection.horizontal;
+    DismissDirection confirmDismissDirection;
+
+    Widget buildFrame(bool confirmDismissValue) {
+      return buildTest(
+        confirmDismiss: (BuildContext context, DismissDirection dismissDirection) {
+          confirmDismissDirection = dismissDirection;
+          return Future<bool>.value(confirmDismissValue);
+        }
+      );
+    }
+
+    // Dismiss is confirmed IFF confirmDismiss() returns true.
+    await tester.pumpWidget(buildFrame(true));
+    expect(dismissedItems, isEmpty);
+
+    await dismissItem(tester, 0, gestureDirection: AxisDirection.right, mechanism: flingElement);
+    expect(find.text('0'), findsNothing);
+    expect(dismissedItems, equals(<int>[0]));
+    expect(reportedDismissDirection, DismissDirection.startToEnd);
+    expect(confirmDismissDirection, DismissDirection.startToEnd);
+
+    await dismissItem(tester, 1, gestureDirection: AxisDirection.left, mechanism: flingElement);
+    expect(find.text('1'), findsNothing);
+    expect(dismissedItems, equals(<int>[0, 1]));
+    expect(reportedDismissDirection, DismissDirection.endToStart);
+    expect(confirmDismissDirection, DismissDirection.endToStart);
+
+    // Dismiss is not confirmed if confirmDismiss() returns false
+    dismissedItems = <int>[];
+    await tester.pumpWidget(buildFrame(false));
+
+    await dismissItem(tester, 0, gestureDirection: AxisDirection.right, mechanism: flingElement);
+    expect(find.text('0'), findsOneWidget);
+    expect(dismissedItems, isEmpty);
+    expect(confirmDismissDirection, DismissDirection.startToEnd);
+
+    await dismissItem(tester, 1, gestureDirection: AxisDirection.left, mechanism: flingElement);
+    expect(find.text('1'), findsOneWidget);
+    expect(dismissedItems, isEmpty);
+    expect(confirmDismissDirection, DismissDirection.endToStart);
+
+    // Dismiss is not confirmed if confirmDismiss() returns null
+    dismissedItems = <int>[];
+    await tester.pumpWidget(buildFrame(null));
+
+    await dismissItem(tester, 0, gestureDirection: AxisDirection.right, mechanism: flingElement);
+    expect(find.text('0'), findsOneWidget);
+    expect(dismissedItems, isEmpty);
+    expect(confirmDismissDirection, DismissDirection.startToEnd);
+
+    await dismissItem(tester, 1, gestureDirection: AxisDirection.left, mechanism: flingElement);
+    expect(find.text('1'), findsOneWidget);
+    expect(dismissedItems, isEmpty);
+    expect(confirmDismissDirection, DismissDirection.endToStart);
+  });
 }