Draggable can be accepted when the data is null (#97355)

dev/manual_tests/lib/drag_and_drop.dart

@@ -16,9 +16,9 @@ class ExampleDragTarget extends StatefulWidget {
 class ExampleDragTargetState extends State<ExampleDragTarget> {
   Color _color = Colors.grey;
 
-  void _handleAccept(Color data) {
+  void _handleAccept(Color? data) {
     setState(() {
-      _color = data;
+      _color = data!;
     });
   }
 
@@ -215,7 +215,7 @@ class MovableBall extends StatelessWidget {
       );
     } else {
       return DragTarget<bool>(
-        onAccept: (bool data) { callback(position); },
+        onAccept: (bool? data) { callback(position); },
         builder: (BuildContext context, List<bool?> accepted, List<dynamic> rejected) {
           return dashedBall;
         },

examples/api/lib/widgets/drag_target/draggable.0.dart

@@ -81,9 +81,9 @@ class _MyStatefulWidgetState extends State<MyStatefulWidget> {
               ),
             );
           },
-          onAccept: (int data) {
+          onAccept: (int? data) {
             setState(() {
-              acceptedData += data;
+              acceptedData += data!;
             });
           },
         ),

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

@@ -20,7 +20,7 @@ typedef DragTargetWillAccept<T> = bool Function(T? data);
 /// Signature for causing a [DragTarget] to accept the given data.
 ///
 /// Used by [DragTarget.onAccept].
-typedef DragTargetAccept<T> = void Function(T data);
+typedef DragTargetAccept<T> = void Function(T? data);
 
 /// Signature for determining information about the acceptance by a [DragTarget].
 ///
@@ -638,7 +638,7 @@ class DragTargetDetails<T> {
   DragTargetDetails({required this.data, required this.offset}) : assert(offset != null);
 
   /// The data that was dropped onto this [DragTarget].
-  final T data;
+  final T? data;
 
   /// The global position when the specific pointer event occurred on
   /// the draggable.
@@ -767,14 +767,14 @@ class _DragTargetState<T extends Object> extends State<DragTarget<T>> {
     setState(() {
       _candidateAvatars.remove(avatar);
     });
-    widget.onAccept?.call(avatar.data! as T);
-    widget.onAcceptWithDetails?.call(DragTargetDetails<T>(data: avatar.data! as T, offset: avatar._lastOffset!));
+    widget.onAccept?.call(avatar.data as T?);
+    widget.onAcceptWithDetails?.call(DragTargetDetails<T>(data: avatar.data as T?, offset: avatar._lastOffset!));
   }
 
   void didMove(_DragAvatar<Object> avatar) {
     if (!mounted)
       return;
-    widget.onMove?.call(DragTargetDetails<T>(data: avatar.data! as T, offset: avatar._lastOffset!));
+    widget.onMove?.call(DragTargetDetails<T>(data: avatar.data as T?, offset: avatar._lastOffset!));
   }
 
   @override

packages/flutter/test/widgets/draggable_test.dart

@@ -39,7 +39,9 @@ void main() {
               return const SizedBox(height: 100.0, child: Text('Target'));
             },
             onMove: (_) => moveCount++,
-            onAccept: accepted.add,
+            onAccept: (int? data) {
+              accepted.add(data!);
+            },
             onAcceptWithDetails: acceptedDetails.add,
           ),
         ],
@@ -91,9 +93,60 @@ void main() {
     expect(moveCount, 1);
   });
 
+  testWidgets('Draggable can be accepted when the data is null', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/84816
+    final Map<String, int?> receivedData = <String, int?>{};
+
+    await tester.pumpWidget(MaterialApp(
+      home: Column(
+        children: <Widget>[
+          const Draggable<int>(
+            feedback: Text('Dragging'),
+            child: Text('Source'),
+          ),
+          DragTarget<int>(
+            builder: (BuildContext context, List<int?> data, List<dynamic> rejects) {
+              return const SizedBox(height: 100.0, child: Text('Target 1'));
+            },
+            onWillAccept: (int? data) {
+              receivedData['onWillAccept'] = data;
+              return true;
+            },
+            onAccept: (int? data) {
+              receivedData['onAccept'] = data;
+            },
+            onAcceptWithDetails: (DragTargetDetails<int> details) {
+              receivedData['onAcceptWithDetails'] = details.data;
+            },
+            onMove: (DragTargetDetails<int> details) {
+              receivedData['onMove'] = details.data;
+            },
+          ),
+        ],
+      ),
+    ));
+
+    final Offset firstLocation = tester.getCenter(find.text('Source'));
+    final TestGesture gesture = await tester.startGesture(firstLocation, pointer: 7);
+    await tester.pump();
+
+    final Offset secondLocation = tester.getCenter(find.text('Target 1'));
+    await gesture.moveTo(secondLocation);
+    await tester.pump();
+
+    await gesture.up();
+    await tester.pump();
+
+    expect(receivedData.length, 4);
+    expect(receivedData['onWillAccept'], null);
+    expect(receivedData['onMove'], null);
+    expect(receivedData['onAccept'], null);
+    expect(receivedData['onAcceptWithDetails'], null);
+  });
+
   // Regression test for https://github.com/flutter/flutter/issues/76825
   testWidgets('Drag and drop - onLeave callback fires correctly with generic parameter', (WidgetTester tester) async {
-    final Map<String,int> leftBehind = <String,int>{
+    final Map<String, int> leftBehind = <String, int>{
       'Target 1': 0,
       'Target 2': 0,
     };
@@ -168,7 +221,7 @@ void main() {
   });
 
   testWidgets('Drag and drop - onLeave callback fires correctly', (WidgetTester tester) async {
-    final Map<String,int> leftBehind = <String,int>{
+    final Map<String, int> leftBehind = <String, int>{
       'Target 1': 0,
       'Target 2': 0,
     };
@@ -244,7 +297,7 @@ void main() {
 
   // Regression test for https://github.com/flutter/flutter/issues/76825
   testWidgets('Drag and drop - onMove callback fires correctly with generic parameter', (WidgetTester tester) async {
-    final Map<String,int> targetMoveCount = <String,int>{
+    final Map<String, int> targetMoveCount = <String, int>{
       'Target 1': 0,
       'Target 2': 0,
     };
@@ -263,7 +316,7 @@ void main() {
             },
             onMove: (DragTargetDetails<int> details) {
               targetMoveCount['Target 1'] =
-                  targetMoveCount['Target 1']! + details.data;
+                  targetMoveCount['Target 1']! + details.data!;
             },
           ),
           DragTarget<int>(
@@ -272,7 +325,7 @@ void main() {
             },
             onMove: (DragTargetDetails<int> details) {
               targetMoveCount['Target 2'] =
-                  targetMoveCount['Target 2']! + details.data;
+                  targetMoveCount['Target 2']! + details.data!;
             },
           ),
         ],
@@ -317,7 +370,7 @@ void main() {
   });
 
   testWidgets('Drag and drop - onMove callback fires correctly', (WidgetTester tester) async {
-    final Map<String,int> targetMoveCount = <String,int>{
+    final Map<String, int> targetMoveCount = <String, int>{
       'Target 1': 0,
       'Target 2': 0,
     };
@@ -1167,7 +1220,9 @@ void main() {
             builder: (BuildContext context, List<int?> data, List<dynamic> rejects) {
               return const SizedBox(height: 100.0, child: Text('Target'));
             },
-            onAccept: accepted.add,
+            onAccept: (int? data) {
+              accepted.add(data!);
+            },
             onAcceptWithDetails: acceptedDetails.add,
           ),
         ],
@@ -1243,7 +1298,9 @@ void main() {
               );
             },
             onWillAccept: (int? data) => false,
-            onAccept: accepted.add,
+            onAccept: (int? data) {
+              accepted.add(data!);
+            },
             onAcceptWithDetails: acceptedDetails.add,
           ),
         ],
@@ -1316,7 +1373,9 @@ void main() {
             return const SizedBox(height: 100.0, child: Text('Target'));
           },
           onWillAccept: (int? data) => false,
-          onAccept: accepted.add,
+          onAccept: (int? data) {
+            accepted.add(data!);
+          },
           onAcceptWithDetails: acceptedDetails.add,
         ),
       ]),
@@ -1366,7 +1425,9 @@ void main() {
               return const SizedBox(height: 100.0, child: Text('Target'));
             },
             onWillAccept: (int? data) => false,
-            onAccept: accepted.add,
+            onAccept: (int? data) {
+              accepted.add(data!);
+            },
             onAcceptWithDetails: acceptedDetails.add,
           ),
         ],
@@ -1580,7 +1641,9 @@ void main() {
               );
             },
             onWillAccept: (int? data) => false,
-            onAccept: accepted.add,
+            onAccept: (int? data) {
+              accepted.add(data!);
+            },
             onAcceptWithDetails: acceptedDetails.add,
           ),
         ],
@@ -1648,7 +1711,9 @@ void main() {
             builder: (BuildContext context, List<int?> data, List<dynamic> rejects) {
               return const SizedBox(height: 100.0, child: Text('Target'));
             },
-            onAccept: accepted.add,
+            onAccept: (int? data) {
+              accepted.add(data!);
+            },
             onAcceptWithDetails: acceptedDetails.add,
           ),
         ],
@@ -1789,7 +1854,9 @@ void main() {
             builder: (BuildContext context, List<int?> data, List<dynamic> rejects) {
               return const SizedBox(height: 100.0, child: Text('Target'));
             },
-            onAccept: accepted.add,
+            onAccept: (int? data) {
+              accepted.add(data!);
+            },
             onAcceptWithDetails: acceptedDetails.add,
           ),
         ],
@@ -1867,7 +1934,9 @@ void main() {
                     ),
                   );
                 },
-                onAccept: acceptedInts.add,
+                onAccept: (int? data) {
+                  acceptedInts.add(data!);
+                },
                 onAcceptWithDetails: acceptedIntsDetails.add,
               ),
               DragTarget<double>(
@@ -1879,7 +1948,9 @@ void main() {
                     ),
                   );
                 },
-                onAccept: acceptedDoubles.add,
+                onAccept: (double? data) {
+                  acceptedDoubles.add(data!);
+                },
                 onAcceptWithDetails: acceptedDoublesDetails.add,
               ),
             ],
@@ -1995,7 +2066,9 @@ void main() {
                       child: Text('Target1'),
                     ),
                   );
-                }, onAccept: acceptedDragTargetDatas.add,
+                }, onAccept: (DragTargetData? data) {
+                acceptedDragTargetDatas.add(data!);
+              },
                 onAcceptWithDetails: acceptedDragTargetDataDetails.add,
               ),
               DragTarget<ExtendedDragTargetData>(
@@ -2007,7 +2080,9 @@ void main() {
                     ),
                   );
                 },
-                onAccept: acceptedExtendedDragTargetDatas.add,
+                onAccept: (ExtendedDragTargetData? data) {
+                  acceptedExtendedDragTargetDatas.add(data!);
+                },
                 onAcceptWithDetails: acceptedExtendedDragTargetDataDetails.add,
               ),
             ],
@@ -2057,7 +2132,9 @@ void main() {
               builder: (BuildContext context, List<int?> data, List<dynamic> rejects) {
                 return const SizedBox(height: 100.0, child: Text('Target'));
               },
-              onAccept: accepted.add,
+              onAccept: (int? data) {
+                accepted.add(data!);
+              },
               onAcceptWithDetails: acceptedDetails.add,
             ),
           ],
@@ -2316,7 +2393,9 @@ void main() {
             builder: (BuildContext context, List<int?> data, List<dynamic> rejects) {
               return const SizedBox(height: 100.0, child: Text('Target'));
             },
-            onAccept: accepted.add,
+            onAccept: (int? data) {
+              accepted.add(data!);
+            },
             onAcceptWithDetails: acceptedDetails.add,
           ),
         ],
@@ -2346,7 +2425,9 @@ void main() {
             builder: (BuildContext context, List<int?> data, List<dynamic> rejects) {
               return const SizedBox(height: 100.0, child: Text('Target'));
             },
-            onAccept: accepted.add,
+            onAccept: (int? data) {
+              accepted.add(data!);
+            },
             onAcceptWithDetails: acceptedDetails.add,
           ),
         ],
@@ -2426,7 +2507,9 @@ void main() {
             builder: (BuildContext context, List<int?> data, List<dynamic> rejects) {
               return const SizedBox(height: 100.0, child: Text('Target'));
             },
-            onAccept: accepted.add,
+            onAccept: (int? data) {
+              accepted.add(data!);
+            },
             onAcceptWithDetails: acceptedDetails.add,
           ),
         ],
@@ -2511,7 +2594,9 @@ void main() {
             builder: (BuildContext context, List<int?> data, List<dynamic> rejects) {
               return const SizedBox(height: 100.0, child: Text('Target'));
             },
-            onAccept: accepted.add,
+            onAccept: (int? data) {
+              accepted.add(data!);
+            },
             onAcceptWithDetails: acceptedDetails.add,
           ),
         ],
@@ -2765,7 +2850,9 @@ void main() {
             builder: (BuildContext context, List<Object?> data, List<dynamic> rejects) {
               return const SizedBox(height: 100.0, child: Text('Target'));
             },
-            onAccept: accepted.add,
+            onAccept: (Object? data) {
+              accepted.add(data!);
+            },
           ),
         ],
       ),
@@ -2801,7 +2888,9 @@ void main() {
             builder: (BuildContext context, List<int?> data, List<dynamic> rejects) {
               return const SizedBox(height: 100.0, child: Text('Target'));
             },
-            onAccept: accepted.add,
+            onAccept: (int? data) {
+              accepted.add(data!);
+            },
           ),
         ],
       ),
@@ -2837,7 +2926,9 @@ void main() {
             builder: (BuildContext context, List<int?> data, List<dynamic> rejects) {
               return const SizedBox(height: 100.0, child: Text('Target'));
             },
-            onAccept: accepted.add,
+            onAccept: (int? data) {
+              accepted.add(data!);
+            },
             onWillAccept: (int? data) {
               if (data == null)
                 isReceiveNullDataForCheck = true;
@@ -3180,7 +3271,9 @@ Future<void> _testChildAnchorFeedbackPosition({ required WidgetTester tester, do
                   builder: (BuildContext context, List<int?> data, List<dynamic> rejects) {
                     return const SizedBox(height: 100.0, child: Text('Target'));
                   },
-                  onAccept: accepted.add,
+                  onAccept: (int? data) {
+                    accepted.add(data!);
+                  },
                   onAcceptWithDetails: acceptedDetails.add,
                 ),
               ],