Use announce function in live region

lib/web_ui/lib/src/engine/semantics/accessibility.dart

@@ -29,6 +29,10 @@ AccessibilityAnnouncements get accessibilityAnnouncements {
 }
 AccessibilityAnnouncements? _accessibilityAnnouncements;
 
+void debugOverrideAccessibilityAnnouncements(AccessibilityAnnouncements override) {
+  _accessibilityAnnouncements = override;
+}
+
 /// Initializes the [accessibilityAnnouncements] singleton.
 ///
 /// It is an error to attempt to initialize the singleton more than once. Call

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

@@ -2,41 +2,37 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import '../dom.dart';
+import 'accessibility.dart';
 import 'semantics.dart';
 
 /// Manages semantics configurations that represent live regions.
 ///
-/// "aria-live" attribute is added to communicate the live region to the
-/// assistive technology.
+/// Assistive technologies treat "aria-live" attribute differently. To keep
+/// the behavior consistent, [accessibilityAnnouncements.announce] is used.
 ///
-/// The usage of "aria-live" is browser-dependent.
-///
-/// VoiceOver only supports "aria-live" with "polite" politeness setting. When
-/// the inner html content is changed. It doesn't read the "aria-label".
-///
-/// When there is an aria-live attribute added, assistive technologies read the
+/// When there is an update to [LiveRegion], assistive technologies read the
 /// label of the element. See [LabelAndValue]. If there is no label provided
-/// no content will be read, therefore DOM is cleaned.
+/// no content will be read.
 class LiveRegion extends RoleManager {
   LiveRegion(SemanticsObject semanticsObject)
       : super(Role.labelAndValue, semanticsObject);
 
+  String? _lastAnnouncement;
+
   @override
   void update() {
-    if (semanticsObject.hasLabel) {
-      semanticsObject.element.setAttribute('aria-live', 'polite');
-    } else {
-      _cleanupDom();
+    // Avoid announcing the same message over and over.
+    if (_lastAnnouncement != semanticsObject.label) {
+      _lastAnnouncement = semanticsObject.label;
+      if (semanticsObject.hasLabel) {
+        accessibilityAnnouncements.announce(
+          _lastAnnouncement! , Assertiveness.polite
+        );
+      }
     }
   }
 
-  void _cleanupDom() {
-    semanticsObject.element.removeAttribute('aria-live');
-  }
-
   @override
   void dispose() {
-    _cleanupDom();
   }
 }

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

@@ -1743,12 +1743,38 @@ void _testImage() {
   });
 }
 
+class MockAccessibilityAnnouncements implements AccessibilityAnnouncements {
+  int announceInvoked = 0;
+
+  @override
+  void announce(String message, Assertiveness assertiveness) {
+    announceInvoked += 1;
+  }
+
+  @override
+  DomHTMLElement ariaLiveElementFor(Assertiveness assertiveness) {
+    return createDomHTMLDivElement();
+  }
+
+  @override
+  void dispose() {
+  }
+
+  @override
+  void handleMessage(StandardMessageCodec codec, ByteData? data) {
+  }
+}
+
 void _testLiveRegion() {
-  test('renders a live region if there is a label', () async {
+  test('announces the label after an update', () async {
     semantics()
       ..debugOverrideTimestampFunction(() => _testTime)
       ..semanticsEnabled = true;
 
+    final MockAccessibilityAnnouncements mockAccessibilityAnnouncements =
+        MockAccessibilityAnnouncements();
+    debugOverrideAccessibilityAnnouncements(mockAccessibilityAnnouncements);
+
     final ui.SemanticsUpdateBuilder builder = ui.SemanticsUpdateBuilder();
     updateNode(
       builder,
@@ -1758,19 +1784,20 @@ void _testLiveRegion() {
       rect: const ui.Rect.fromLTRB(0, 0, 100, 50),
     );
     semantics().updateSemantics(builder.build());
-
-    expectSemanticsTree('''
-<sem aria-label="This is a snackbar" aria-live="polite" style="$rootSemanticStyle"></sem>
-''');
+    expect(mockAccessibilityAnnouncements.announceInvoked, 1);
 
     semantics().semanticsEnabled = false;
   });
 
-  test('does not render a live region if there is no label', () async {
+  test('does not announce anything if there is no label', () async {
     semantics()
       ..debugOverrideTimestampFunction(() => _testTime)
       ..semanticsEnabled = true;
 
+    final MockAccessibilityAnnouncements mockAccessibilityAnnouncements =
+        MockAccessibilityAnnouncements();
+    debugOverrideAccessibilityAnnouncements(mockAccessibilityAnnouncements);
+
     final ui.SemanticsUpdateBuilder builder = ui.SemanticsUpdateBuilder();
     updateNode(
       builder,
@@ -1779,10 +1806,40 @@ void _testLiveRegion() {
       rect: const ui.Rect.fromLTRB(0, 0, 100, 50),
     );
     semantics().updateSemantics(builder.build());
+    expect(mockAccessibilityAnnouncements.announceInvoked, 0);
 
-    expectSemanticsTree('''
-<sem style="$rootSemanticStyle"></sem>
-''');
+    semantics().semanticsEnabled = false;
+  });
+
+  test('does not announce the same label over and over', () async {
+    semantics()
+      ..debugOverrideTimestampFunction(() => _testTime)
+      ..semanticsEnabled = true;
+
+    final MockAccessibilityAnnouncements mockAccessibilityAnnouncements =
+        MockAccessibilityAnnouncements();
+    debugOverrideAccessibilityAnnouncements(mockAccessibilityAnnouncements);
+
+    ui.SemanticsUpdateBuilder builder = ui.SemanticsUpdateBuilder();
+    updateNode(
+      builder,
+      label: 'This is a snackbar',
+      flags: 0 | ui.SemanticsFlag.isLiveRegion.index,
+      transform: Matrix4.identity().toFloat64(),
+      rect: const ui.Rect.fromLTRB(0, 0, 100, 50),
+    );
+    semantics().updateSemantics(builder.build());
+
+    builder = ui.SemanticsUpdateBuilder();
+    updateNode(
+      builder,
+      label: 'This is a snackbar',
+      flags: 0 | ui.SemanticsFlag.isLiveRegion.index,
+      transform: Matrix4.identity().toFloat64(),
+      rect: const ui.Rect.fromLTRB(0, 0, 100, 50),
+    );
+    semantics().updateSemantics(builder.build());
+    expect(mockAccessibilityAnnouncements.announceInvoked, 1);
 
     semantics().semanticsEnabled = false;
   });