Add support for assertive announcements in aria-live (#34640)

Author: nbayati

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

@@ -5,11 +5,21 @@
 import 'dart:async';
 import 'dart:typed_data';
 
-import '../../engine.dart'  show registerHotRestartListener;
+import '../../engine.dart' show registerHotRestartListener;
 import '../dom.dart';
 import '../services.dart';
 import '../util.dart';
 
+/// Determines the assertiveness level of the accessibility announcement.
+///
+/// It is used to set the priority with which assistive technology should treat announcements.
+///
+/// The order of this enum must match the order of the values in semantics_event.dart in framework.
+enum Assertiveness {
+  polite,
+  assertive,
+}
+
 /// Singleton for accessing accessibility announcements from the platform.
 final AccessibilityAnnouncements accessibilityAnnouncements =
     AccessibilityAnnouncements.instance;
@@ -63,15 +73,19 @@ class AccessibilityAnnouncements {
     final Map<dynamic, dynamic> dataMap = inputMap.readDynamicJson('data');
     final String? message = dataMap.tryString('message');
     if (message != null && message.isNotEmpty) {
-      _initLiveRegion(message);
+      /// The default value for politeness is `polite`.
+      final int ariaLivePolitenessIndex = dataMap.tryInt('assertiveness') ?? 0;
+      final Assertiveness ariaLivePoliteness = Assertiveness.values[ariaLivePolitenessIndex];
+      _initLiveRegion(message, ariaLivePoliteness);
       _removeElementTimer = Timer(durationA11yMessageIsOnDom, () {
         _element!.remove();
       });
     }
   }
 
-  void _initLiveRegion(String message) {
-    _domElement.setAttribute('aria-live', 'polite');
+  void _initLiveRegion(String message, Assertiveness ariaLivePoliteness) {
+    final String assertiveLevel = (ariaLivePoliteness == Assertiveness.assertive) ? 'assertive' : 'polite';
+    _domElement.setAttribute('aria-live', assertiveLevel);
     _domElement.text = message;
     domDocument.body!.append(_domElement);
   }

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

@@ -55,5 +55,37 @@ void testMain() {
           () =>
               expect(domDocument.getElementById('accessibility-element'), isNull));
     });
+
+    test('Default value of aria-live is polite when assertiveness is not specified', () {
+      const Map<dynamic, dynamic> testInput = <dynamic, dynamic>{'data': <dynamic, dynamic>{'message': 'message'}};
+      accessibilityAnnouncements.handleMessage(codec, codec.encodeMessage(testInput));
+      final DomHTMLLabelElement input = domDocument.getElementById('accessibility-element')! as DomHTMLLabelElement;
+
+      expect(input.getAttribute('aria-live'), equals('polite'));
+    });
+
+     test('aria-live is assertive when assertiveness is set to 1', () {
+      const Map<dynamic, dynamic> testInput = <dynamic, dynamic>{'data': <dynamic, dynamic>{'message': 'message', 'assertiveness': 1}};
+      accessibilityAnnouncements.handleMessage(codec, codec.encodeMessage(testInput));
+      final DomHTMLLabelElement input = domDocument.getElementById('accessibility-element')! as DomHTMLLabelElement;
+
+      expect(input.getAttribute('aria-live'), equals('assertive'));
+    });
+
+    test('aria-live is polite when assertiveness is null', () {
+      const Map<dynamic, dynamic> testInput = <dynamic, dynamic>{'data': <dynamic, dynamic>{'message': 'message', 'assertiveness': null}};
+      accessibilityAnnouncements.handleMessage(codec, codec.encodeMessage(testInput));
+      final DomHTMLLabelElement input = domDocument.getElementById('accessibility-element')! as DomHTMLLabelElement;
+
+      expect(input.getAttribute('aria-live'), equals('polite'));
+    });
+
+    test('aria-live is polite when assertiveness is set to 0', () {
+      const Map<dynamic, dynamic> testInput = <dynamic, dynamic>{'data': <dynamic, dynamic>{'message': 'message', 'assertiveness': 0}};
+      accessibilityAnnouncements.handleMessage(codec, codec.encodeMessage(testInput));
+      final DomHTMLLabelElement input = domDocument.getElementById('accessibility-element')! as DomHTMLLabelElement;
+
+      expect(input.getAttribute('aria-live'), equals('polite'));
+    });
   });
 }