[web] Refactor a11y announcements out of FlutterViewEmbedder (flutter/engine#47487)

- Remove a11y announcements from `FlutterViewEmbedder`.
- Simplify a11y announcements tests so they don't need `FlutterViewEmbedder` nor `DomManager`.
- Left a few a11y-multi-view TODOs (cc @yjbanov).

Part of https://github.com/flutter/flutter/issues/134443

engine/src/flutter/lib/web_ui/lib/src/engine/embedder.dart

@@ -133,8 +133,8 @@ class FlutterViewEmbedder {
   DomElement get textEditingHostNodeDEPRECATED => _textEditingHostNode;
   late DomElement _textEditingHostNode;
 
-  AccessibilityAnnouncements get accessibilityAnnouncements => _accessibilityAnnouncements;
-  late AccessibilityAnnouncements _accessibilityAnnouncements;
+  DomElement get announcementsHostDEPRECATED => _announcementsHost;
+  late DomElement _announcementsHost;
 
   static const String defaultFontStyle = 'normal';
   static const String defaultFontWeight = 'normal';
@@ -214,12 +214,11 @@ class FlutterViewEmbedder {
         .instance.semanticsHelper
         .prepareAccessibilityPlaceholder();
 
-    final DomElement announcementsElement = createDomElement('flt-announcement-host');
-    _accessibilityAnnouncements = AccessibilityAnnouncements(hostElement: announcementsElement);
+    _announcementsHost = createDomElement('flt-announcement-host');
 
     shadowRoot.append(accessibilityPlaceholder);
     shadowRoot.append(_sceneHostElement);
-    shadowRoot.append(announcementsElement);
+    shadowRoot.append(_announcementsHost);
 
     // The semantic host goes last because hit-test order-wise it must be
     // first. If semantics goes under the scene host, platform views will
@@ -248,11 +247,6 @@ class FlutterViewEmbedder {
     window.onResize.listen(_metricsDidChange);
   }
 
-  /// For tests only.
-  void debugOverrideAccessibilityAnnouncements(AccessibilityAnnouncements override) {
-    _accessibilityAnnouncements = override;
-  }
-
   /// The framework specifies semantics in physical pixels, but CSS uses
   /// logical pixels. To compensate, an inverse scale is injected at the root
   /// level.

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

@@ -601,7 +601,9 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher {
       case 'flutter/accessibility':
         // In widget tests we want to bypass processing of platform messages.
         const StandardMessageCodec codec = StandardMessageCodec();
-        flutterViewEmbedder.accessibilityAnnouncements.handleMessage(codec, data);
+        // TODO(yjbanov): Dispatch the announcement to the correct view?
+        //                https://github.com/flutter/flutter/issues/137445
+        implicitView!.accessibilityAnnouncements.handleMessage(codec, data);
         replyToPlatformMessage(callback, codec.encodeMessage(true));
         return;
 

engine/src/flutter/lib/web_ui/lib/src/engine/semantics/live_region.dart

@@ -2,14 +2,17 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import '../embedder.dart' show flutterViewEmbedder;
+import 'package:meta/meta.dart';
+
+import '../platform_dispatcher.dart';
 import 'accessibility.dart';
+import 'label_and_value.dart';
 import 'semantics.dart';
 
 /// Manages semantics configurations that represent live regions.
 ///
 /// Assistive technologies treat "aria-live" attribute differently. To keep
-/// the behavior consistent, [accessibilityAnnouncements.announce] is used.
+/// the behavior consistent, [AccessibilityAnnouncements.announce] is used.
 ///
 /// When there is an update to [LiveRegion], assistive technologies read the
 /// label of the element. See [LabelAndValue]. If there is no label provided
@@ -20,6 +23,17 @@ class LiveRegion extends RoleManager {
 
   String? _lastAnnouncement;
 
+  static AccessibilityAnnouncements? _accessibilityAnnouncementsOverride;
+
+  @visibleForTesting
+  static void debugOverrideAccessibilityAnnouncements(AccessibilityAnnouncements? value) {
+    _accessibilityAnnouncementsOverride = value;
+  }
+
+  AccessibilityAnnouncements get _accessibilityAnnouncements =>
+      _accessibilityAnnouncementsOverride ??
+      EnginePlatformDispatcher.instance.implicitView!.accessibilityAnnouncements;
+
   @override
   void update() {
     if (!semanticsObject.isLiveRegion) {
@@ -30,8 +44,9 @@ class LiveRegion extends RoleManager {
     if (_lastAnnouncement != semanticsObject.label) {
       _lastAnnouncement = semanticsObject.label;
       if (semanticsObject.hasLabel) {
-        flutterViewEmbedder.accessibilityAnnouncements.announce(
-          _lastAnnouncement! , Assertiveness.polite
+        _accessibilityAnnouncements.announce(
+          _lastAnnouncement!,
+          Assertiveness.polite,
         );
       }
     }

engine/src/flutter/lib/web_ui/lib/src/engine/view_embedder/dom_manager.dart

@@ -23,7 +23,7 @@ import '../embedder.dart';
 ///   |    |    |    |
 ///   |    |    |    +- <flt-scene>
 ///   |    |    |
-///   |    |    +- <flt-announcement-host>
+///   |    |    +- [announcementsHost] <flt-announcement-host>
 ///   |    |
 ///   |    +- ...platform views
 ///   |
@@ -63,4 +63,7 @@ class DomManager {
   /// Otherwise, the phone will disable focusing by touch, only by tabbing
   /// around the UI.
   DomElement get semanticsHost => _embedder.semanticsHostElementDEPRECATED;
+
+  /// This is where accessibility announcements are inserted.
+  DomElement get announcementsHost => _embedder.announcementsHostDEPRECATED;
 }

engine/src/flutter/lib/web_ui/lib/src/engine/window.dart

@@ -22,6 +22,7 @@ import 'mouse/cursor.dart';
 import 'navigation/history.dart';
 import 'platform_dispatcher.dart';
 import 'platform_views/message_handler.dart';
+import 'semantics/accessibility.dart';
 import 'services.dart';
 import 'util.dart';
 import 'view_embedder/dom_manager.dart';
@@ -63,6 +64,11 @@ base class EngineFlutterView implements ui.FlutterView {
   @override
   void updateSemantics(ui.SemanticsUpdate update) => platformDispatcher.updateSemantics(update);
 
+  // TODO(yjbanov): How should this look like for multi-view?
+  //                https://github.com/flutter/flutter/issues/137445
+  late final AccessibilityAnnouncements accessibilityAnnouncements =
+      AccessibilityAnnouncements(hostElement: dom.announcementsHost);
+
   late final MouseCursor mouseCursor = MouseCursor(dom.rootElement);
 
   late final ContextMenu contextMenu = ContextMenu(dom.rootElement);

engine/src/flutter/lib/web_ui/test/engine/semantics/accessibility_test.dart

@@ -7,11 +7,7 @@ import 'dart:typed_data';
 
 import 'package:test/bootstrap/browser.dart';
 import 'package:test/test.dart';
-import 'package:ui/src/engine/dom.dart';
-import 'package:ui/src/engine/embedder.dart';
-import 'package:ui/src/engine/semantics.dart';
-import 'package:ui/src/engine/services.dart';
-import 'package:ui/src/engine/view_embedder/dom_manager.dart';
+import 'package:ui/src/engine.dart';
 
 const StandardMessageCodec codec = StandardMessageCodec();
 
@@ -20,27 +16,24 @@ void main() {
 }
 
 void testMain() {
-  late DomManager domManager;
   late AccessibilityAnnouncements accessibilityAnnouncements;
 
   setUp(() {
-    final FlutterViewEmbedder embedder = FlutterViewEmbedder();
-    domManager = DomManager.fromFlutterViewEmbedderDEPRECATED(embedder);
-    accessibilityAnnouncements = embedder.accessibilityAnnouncements;
+    final DomElement announcementsHost = createDomElement('flt-announcement-host');
+    accessibilityAnnouncements = AccessibilityAnnouncements(hostElement: announcementsHost);
     setLiveMessageDurationForTest(const Duration(milliseconds: 10));
     expect(
-      domManager.renderingHost.querySelector('flt-announcement-polite'),
+      announcementsHost.querySelector('flt-announcement-polite'),
       accessibilityAnnouncements.ariaLiveElementFor(Assertiveness.polite),
     );
     expect(
-      domManager.renderingHost.querySelector('flt-announcement-assertive'),
+      announcementsHost.querySelector('flt-announcement-assertive'),
       accessibilityAnnouncements.ariaLiveElementFor(Assertiveness.assertive),
     );
   });
 
   tearDown(() async {
     await Future<void>.delayed(liveMessageDuration * 2);
-    domManager.rootElement.remove();
   });
 
   group('$AccessibilityAnnouncements', () {

engine/src/flutter/lib/web_ui/test/engine/semantics/semantics_test.dart

@@ -2331,6 +2331,10 @@ class MockAccessibilityAnnouncements implements AccessibilityAnnouncements {
 }
 
 void _testLiveRegion() {
+  tearDown(() {
+    LiveRegion.debugOverrideAccessibilityAnnouncements(null);
+  });
+
   test('announces the label after an update', () async {
     semantics()
       ..debugOverrideTimestampFunction(() => _testTime)
@@ -2338,7 +2342,7 @@ void _testLiveRegion() {
 
     final MockAccessibilityAnnouncements mockAccessibilityAnnouncements =
         MockAccessibilityAnnouncements();
-    flutterViewEmbedder.debugOverrideAccessibilityAnnouncements(mockAccessibilityAnnouncements);
+    LiveRegion.debugOverrideAccessibilityAnnouncements(mockAccessibilityAnnouncements);
 
     final ui.SemanticsUpdateBuilder builder = ui.SemanticsUpdateBuilder();
     updateNode(
@@ -2361,7 +2365,7 @@ void _testLiveRegion() {
 
     final MockAccessibilityAnnouncements mockAccessibilityAnnouncements =
         MockAccessibilityAnnouncements();
-    flutterViewEmbedder.debugOverrideAccessibilityAnnouncements(mockAccessibilityAnnouncements);
+    LiveRegion.debugOverrideAccessibilityAnnouncements(mockAccessibilityAnnouncements);
 
     final ui.SemanticsUpdateBuilder builder = ui.SemanticsUpdateBuilder();
     updateNode(
@@ -2383,7 +2387,7 @@ void _testLiveRegion() {
 
     final MockAccessibilityAnnouncements mockAccessibilityAnnouncements =
         MockAccessibilityAnnouncements();
-    flutterViewEmbedder.debugOverrideAccessibilityAnnouncements(mockAccessibilityAnnouncements);
+    LiveRegion.debugOverrideAccessibilityAnnouncements(mockAccessibilityAnnouncements);
 
     ui.SemanticsUpdateBuilder builder = ui.SemanticsUpdateBuilder();
     updateNode(

engine/src/flutter/lib/web_ui/test/engine/view_embedder/dom_manager_test.dart

@@ -22,6 +22,7 @@ void doTests() {
       expect(domManager.platformViewsHost, embedder.glassPaneElementDEPRECATED);
       expect(domManager.textEditingHost, embedder.textEditingHostNodeDEPRECATED);
       expect(domManager.semanticsHost, embedder.semanticsHostElementDEPRECATED);
+      expect(domManager.announcementsHost, embedder.announcementsHostDEPRECATED);
     });
   });
 }