Actively reject UiKitView gestures. (#25792)

flutter/engine#7307 changes the engine side of embedded UIView to only
reject gestures when the framework sends a `rejectGesture` message, so
that gesture resolution can done after a touch sequence has ended (see
PR description for flutter/engine#7307 for more details).

This change makes the framework send a `rejectGesture` message to the
engine when a UiKitView rejects a gesture.

I'm planning to land this PR before the engine side change, so right now
it swallows the exception thrown if there is no engine implementation
for `rejectGesture` (which keeps us with the current behavior). After
this change lands I'll land the engine PR, and then clean up the part
that swallows the exception.

packages/flutter/lib/src/rendering/platform_view.dart

@@ -412,11 +412,17 @@ class _UiKitViewGestureRecognizer extends OneSequenceGestureRecognizer {
 
   @override
   void rejectGesture(int pointer) {
-    // Currently the engine rejects the gesture when the sequence is done.
+    controller.rejectGesture().catchError((dynamic e) {
-    // This doesn't work well with gesture recognizers that recognize after the sequence
+      if (e is MissingPluginException) {
-    // has ended.
+        // We land the framework part of active gesture rejection before the engine part.
-    // TODO(amirh): trigger an engine gesture reject here.
+        // There will be some commit range where we call rejectGesture from the framework and it
-    // https://github.com/flutter/flutter/issues/24076
+        // isn't implemented in the engine, if that is the case we swallow the MissingPluginException.
+        // Once the engine support lands we will remove the enclosing catchError call.
+        // TODO(amirh): remove this once the engine supports active gesture rejection.
+        return;
+      }
+      throw e;
+    });
   }
 
   void reset() {

packages/flutter/lib/src/services/platform_views.dart

@@ -630,6 +630,18 @@ class UiKitViewController {
     return SystemChannels.platform_views.invokeMethod('acceptGesture', args);
   }
 
+  /// Rejects an active gesture.
+  ///
+  /// When a touch sequence is happening on the embedded UIView all touch events are delayed.
+  /// Calling this method drops the buffered touch events and prevents any future touch events for
+  /// the pointers that are part of the active touch sequence from arriving to the embedded view.
+  Future<void> rejectGesture() {
+    final Map<String, dynamic> args = <String, dynamic> {
+      'id': id,
+    };
+    return SystemChannels.platform_views.invokeMethod('rejectGesture', args);
+  }
+
   /// Disposes the view.
   ///
   /// The [UiKitViewController] object is unusable after calling this.

packages/flutter/test/services/fake_platform_views.dart

@@ -159,9 +159,12 @@ class FakeIosPlatformViewsController {
   // delayed until it completes.
   Completer<void> creationDelay;
 
-  // Maps a view id to the number of gestures it accepted so fat.
+  // Maps a view id to the number of gestures it accepted so far.
   final Map<int, int> gesturesAccepted = <int, int>{};
 
+  // Maps a view id to the number of gestures it rejected so far.
+  final Map<int, int> gesturesRejected = <int, int>{};
+
   void registerViewType(String viewType) {
     _registeredViewTypes.add(viewType);
   }
@@ -174,6 +177,8 @@ class FakeIosPlatformViewsController {
         return _dispose(call);
       case 'acceptGesture':
         return _acceptGesture(call);
+      case 'rejectGesture':
+        return _rejectGesture(call);
     }
     return Future<dynamic>.sync(() => null);
   }
@@ -202,6 +207,7 @@ class FakeIosPlatformViewsController {
 
     _views[id] = FakeUiKitView(id, viewType, creationParams);
     gesturesAccepted[id] = 0;
+    gesturesRejected[id] = 0;
     return Future<int>.sync(() => null);
   }
 
@@ -212,6 +218,13 @@ class FakeIosPlatformViewsController {
     return Future<int>.sync(() => null);
   }
 
+  Future<dynamic> _rejectGesture(MethodCall call) async {
+    final Map<dynamic, dynamic> args = call.arguments;
+    final int id = args['id'];
+    gesturesRejected[id] += 1;
+    return Future<int>.sync(() => null);
+    }
+
   Future<dynamic> _dispose(MethodCall call) {
     final int id = call.arguments;
 

packages/flutter/test/widgets/platform_view_test.dart

@@ -1196,6 +1196,7 @@ void main() {
 
       expect(verticalDragAcceptedByParent, true);
       expect(viewsController.gesturesAccepted[currentViewId + 1], 0);
+      expect(viewsController.gesturesRejected[currentViewId + 1], 1);
     });
 
     testWidgets('UiKitView gesture recognizers', (WidgetTester tester) async {
@@ -1237,6 +1238,7 @@ void main() {
 
       expect(verticalDragAcceptedByParent, false);
       expect(viewsController.gesturesAccepted[currentViewId + 1], 1);
+      expect(viewsController.gesturesRejected[currentViewId + 1], 0);
     });
 
     testWidgets('UiKitView can claim gesture after all pointers are up', (WidgetTester tester) async {
@@ -1276,6 +1278,7 @@ void main() {
       expect(verticalDragAcceptedByParent, false);
 
       expect(viewsController.gesturesAccepted[currentViewId + 1], 1);
+      expect(viewsController.gesturesRejected[currentViewId + 1], 0);
     });
 
     testWidgets('UiKitView rebuilt during gesture', (WidgetTester tester) async {
@@ -1320,6 +1323,7 @@ void main() {
       await gesture.up();
 
       expect(viewsController.gesturesAccepted[currentViewId + 1], 1);
+      expect(viewsController.gesturesRejected[currentViewId + 1], 0);
     });
 
     testWidgets('UiKitView with eager gesture recognizer', (WidgetTester tester) async {
@@ -1359,6 +1363,7 @@ void main() {
       // the Android view). Here we assert that with the eager recognizer in the gesture team the
       // pointer down event is immediately dispatched.
       expect(viewsController.gesturesAccepted[currentViewId + 1], 1);
+      expect(viewsController.gesturesRejected[currentViewId + 1], 0);
     });
 
     testWidgets('AndroidView rebuilt with same gestureRecognizers', (WidgetTester tester) async {