fix: detect drag cancelled on desktop while waiting for data (#377)

Fixes #377

Author: knopp

super_drag_and_drop/lib/src/drag_internal.dart

@@ -123,6 +123,23 @@ class _DragContextDelegate implements raw.DragContextDelegate {
   }
 }
 
+class _Drag implements Drag {
+  bool _ended = false;
+
+  @override
+  void cancel() {
+    _ended = true;
+  }
+
+  @override
+  void end(DragEndDetails details) {
+    _ended = true;
+  }
+
+  @override
+  void update(DragUpdateDetails details) {}
+}
+
 abstract class _DragDetector extends StatelessWidget {
   final Widget child;
   final DragConfigurationProvider dragConfiguration;
@@ -158,14 +175,16 @@ abstract class _DragDetector extends StatelessWidget {
           }
         });
       }
+      final drag = _Drag();
       _maybeStartDragWithSession(
         dragContext,
         buildContext,
         position,
         session,
         devicePixelRatio,
+        drag,
       );
-      return null;
+      return drag;
     } else {
       return null;
     }
@@ -177,8 +196,14 @@ abstract class _DragDetector extends StatelessWidget {
     Offset position,
     raw.DragSession session,
     double devicePixelRatio,
+    _Drag drag,
   ) async {
     final dragConfiguration = await this.dragConfiguration(position, session);
+    // User ended the drag gesture before the data is available.
+    if (drag._ended) {
+      _dragContext!.cancelSession(session);
+      return;
+    }
     if (dragConfiguration != null) {
       final rawConfiguration =
           await dragConfiguration.intoRaw(devicePixelRatio);

super_native_extensions/lib/src/drag.dart

@@ -146,6 +146,7 @@ abstract class DragContext {
   }
 
   DragSession newSession({int? pointer});
+  void cancelSession(DragSession session);
 
   Future<void> startDrag({
     required BuildContext buildContext,

super_native_extensions/lib/src/native/drag.dart

@@ -70,6 +70,12 @@ class DragSessionImpl extends DragSession {
     }
   }
 
+  void dispose() {
+    _dragging.dispose();
+    _dragCompleted.dispose();
+    _lastScreenLocation.dispose();
+  }
+
   final _dragging = ValueNotifier<bool>(false);
   final _dragCompleted = ValueNotifier<DropOperation?>(null);
   final _lastScreenLocation = ValueNotifier<ui.Offset?>(null);
@@ -182,9 +188,7 @@ class DragContextImpl extends DragContext {
         if (session != null) {
           session._dragging.value = false;
           session._dragCompleted.value = dropOperation;
-          session._dragging.dispose();
-          session._dragCompleted.dispose();
-          session._lastScreenLocation.dispose();
+          session.dispose();
         }
       }, () => null);
     } else {
@@ -199,6 +203,15 @@ class DragContextImpl extends DragContext {
     return DragSessionImpl(dragContext: this);
   }
 
+  @override
+  void cancelSession(DragSession session) {
+    final sessionImpl = session as DragSessionImpl;
+    assert(sessionImpl.dragCompleted.value == null);
+    assert(sessionImpl.dragging.value == false);
+    sessionImpl._dragCompleted.value = DropOperation.userCancelled;
+    session.dispose();
+  }
+
   Future<List<Object?>?> getLocalData(int sessionId) async {
     return _channel.invokeMethod('getLocalData', {
       'sessionId': sessionId,

super_native_extensions/lib/src/web/drag.dart

@@ -89,9 +89,7 @@ class DragSessionImpl extends DragSession implements DragDriverDelegate {
       _dragging.value = false;
       _state = null;
       Future.microtask(() {
-        _dragCompleted.dispose();
-        _dragging.dispose();
-        _lastScreenLocation.dispose();
+        dispose();
         for (final item in configuration.items) {
           item.dataProvider.dispose();
         }
@@ -102,6 +100,12 @@ class DragSessionImpl extends DragSession implements DragDriverDelegate {
     }
   }
 
+  void dispose() {
+    _dragCompleted.dispose();
+    _dragging.dispose();
+    _lastScreenLocation.dispose();
+  }
+
   _SessionState? _state;
 }
 
@@ -264,6 +268,15 @@ class DragContextImpl extends DragContext {
   DragSession newSession({int? pointer}) =>
       DragSessionImpl(pointer: pointer ?? -1);
 
+  @override
+  void cancelSession(DragSession session) {
+    final sessionImpl = session as DragSessionImpl;
+    assert(sessionImpl.dragCompleted.value == null);
+    assert(sessionImpl.dragging.value == false);
+    sessionImpl._dragCompleted.value = DropOperation.userCancelled;
+    session.dispose();
+  }
+
   @override
   Future<void> startDrag({
     required BuildContext buildContext,