Deprecate single semantics tree assumption from platform dispatcher

lib/ui/platform_dispatcher.dart

@@ -758,6 +758,14 @@ class PlatformDispatcher {
   ///
   /// In either case, this function disposes the given update, which means the
   /// semantics update cannot be used further.
+  @Deprecated('''
+    In a multi-view world, the platform dispatcher can no longer provide apis
+    to update semantics since each view will host its own semantics tree.
+
+    Semantics updates must be passed to an individual [FlutterView]. To update
+    semantics, use PlatformDispatcher.instance.views to get a [FlutterView] and
+    call `updateSemantics`.
+  ''')
   void updateSemantics(SemanticsUpdate update) => _updateSemantics(update);
 
   @FfiNative<Void Function(Pointer<Void>)>('PlatformConfigurationNativeApi::UpdateSemantics')

lib/ui/window.dart

@@ -266,6 +266,19 @@ abstract class FlutterView {
 
   @FfiNative<Void Function(Pointer<Void>)>('PlatformConfigurationNativeApi::Render')
   external static void _render(Scene scene);
+
+  /// Change the retained semantics data about this [FlutterView].
+  ///
+  /// If [PlatformDispatcher.semanticsEnabled] is true, the user has requested that this function
+  /// be called whenever the semantic content of this [FlutterView]
+  /// changes.
+  ///
+  /// This function disposes the given update, which means the semantics update
+  /// cannot be used further.
+  void updateSemantics(SemanticsUpdate update) => _updateSemantics(update);
+
+  @FfiNative<Void Function(Pointer<Void>)>('PlatformConfigurationNativeApi::UpdateSemantics')
+  external static void _updateSemantics(SemanticsUpdate update);
 }
 
 /// A top-level platform window displaying a Flutter layer tree drawn from a
@@ -721,17 +734,6 @@ class SingletonFlutterWindow extends FlutterWindow {
     platformDispatcher.onAccessibilityFeaturesChanged = callback;
   }
 
-  /// Change the retained semantics data about this window.
-  ///
-  /// {@macro dart.ui.window.functionForwardWarning}
-  ///
-  /// If [semanticsEnabled] is true, the user has requested that this function
-  /// be called whenever the semantic content of this window changes.
-  ///
-  /// In either case, this function disposes the given update, which means the
-  /// semantics update cannot be used further.
-  void updateSemantics(SemanticsUpdate update) => platformDispatcher.updateSemantics(update);
-
   /// Sends a message to a platform-specific plugin.
   ///
   /// {@macro dart.ui.window.functionForwardWarning}

lib/web_ui/lib/platform_dispatcher.dart

@@ -83,6 +83,14 @@ abstract class PlatformDispatcher {
   VoidCallback? get onAccessibilityFeaturesChanged;
   set onAccessibilityFeaturesChanged(VoidCallback? callback);
 
+  @Deprecated('''
+    In a multi-view world, the platform dispatcher can no longer provide apis
+    to update semantics since each view will host its own semantics tree.
+
+    Semantics updates must be passed to an individual [FlutterView]. To update
+    semantics, use PlatformDispatcher.instance.views to get a [FlutterView] and
+    call `updateSemantics`.
+  ''')
   void updateSemantics(SemanticsUpdate update);
 
   Locale get locale;

lib/web_ui/lib/src/engine/platform_dispatcher.dart

@@ -698,6 +698,14 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher {
   /// In either case, this function disposes the given update, which means the
   /// semantics update cannot be used further.
   @override
+  @Deprecated('''
+    In a multi-view world, the platform dispatcher can no longer provide apis
+    to update semantics since each view will host its own semantics tree.
+
+    Semantics updates must be passed to an individual [FlutterView]. To update
+    semantics, use PlatformDispatcher.instance.views to get a [FlutterView] and
+    call `updateSemantics`.
+  ''')
   void updateSemantics(ui.SemanticsUpdate update) {
     EngineSemanticsOwner.instance.updateSemantics(update);
   }

lib/web_ui/lib/window.dart

@@ -16,6 +16,7 @@ abstract class FlutterView {
   WindowPadding get padding => viewConfiguration.padding;
   List<DisplayFeature> get displayFeatures => viewConfiguration.displayFeatures;
   void render(Scene scene) => platformDispatcher.render(scene, this);
+  void updateSemantics(SemanticsUpdate update) => platformDispatcher.updateSemantics(update);
 }
 
 abstract class FlutterWindow extends FlutterView {
@@ -130,8 +131,6 @@ abstract class SingletonFlutterWindow extends FlutterWindow {
     platformDispatcher.onAccessibilityFeaturesChanged = callback;
   }
 
-  void updateSemantics(SemanticsUpdate update) => platformDispatcher.updateSemantics(update);
-
   void sendPlatformMessage(
     String name,
     ByteData? data,

shell/platform/embedder/fixtures/main.dart

@@ -264,7 +264,9 @@ void a11y_main() async {
       label: 'Archive',
       hint: 'archive message',
     );
-  PlatformDispatcher.instance.updateSemantics(builder.build());
+
+  PlatformDispatcher.instance.views.first.updateSemantics(builder.build());
+
   signalNativeTest();
 
   // Await semantics action from embedder.

testing/scenario_app/lib/src/locale_initialization.dart

@@ -43,8 +43,8 @@ class LocaleInitialization extends Scenario {
     // On the first frame, pretend that it drew a text field. Send the
     // corresponding semantics tree comprised of 1 node with the locale data
     // as the label.
-    window.updateSemantics((SemanticsUpdateBuilder()
-      ..updateNode(
+    final SemanticsUpdateBuilder semanticsUpdateBuilder =
+      SemanticsUpdateBuilder()..updateNode(
         id: 0,
         // SemanticsFlag.isTextField.
         flags: 16,
@@ -79,8 +79,11 @@ class LocaleInitialization extends Scenario {
         childrenInTraversalOrder: Int32List(0),
         childrenInHitTestOrder: Int32List(0),
         additionalActions: Int32List(0),
-      )).build()
-    );
+      );
+
+    final SemanticsUpdate semanticsUpdate = semanticsUpdateBuilder.build();
+
+    dispatcher.views.first.updateSemantics(semanticsUpdate);
   }
 
   /// Handle taps.
@@ -98,8 +101,8 @@ class LocaleInitialization extends Scenario {
       // Expand for other test cases.
     }
 
-    window.updateSemantics((SemanticsUpdateBuilder()
-      ..updateNode(
+    final SemanticsUpdateBuilder semanticsUpdateBuilder =
+      SemanticsUpdateBuilder()..updateNode(
         id: 0,
         // SemanticsFlag.isTextField.
         flags: 16,
@@ -134,8 +137,12 @@ class LocaleInitialization extends Scenario {
         childrenInTraversalOrder: Int32List(0),
         childrenInHitTestOrder: Int32List(0),
         additionalActions: Int32List(0),
-      )).build()
-    );
+      );
+
+    final SemanticsUpdate semanticsUpdate = semanticsUpdateBuilder.build();
+
+    dispatcher.views.first.updateSemantics(semanticsUpdate);
+
     _tapCount++;
   }
 }