Add optional flag to determine assertiveness level in aria announcement for flutter web (#107568)

packages/flutter/lib/src/semantics/semantics_event.dart

@@ -8,6 +8,21 @@ import 'package:flutter/painting.dart';
 
 export 'dart:ui' show TextDirection;
 
+/// Determines the assertiveness level of the accessibility announcement.
+///
+/// It is used by [AnnounceSemanticsEvent] to determine the priority with which
+/// assistive technology should treat announcements.
+enum Assertiveness {
+  /// The assistive technology will speak changes whenever the user is idle.
+  polite,
+
+  /// The assistive technology will interrupt any announcement that it is
+  /// currently making to notify the user about the change.
+  ///
+  /// It should only be used for time-sensitive/critical notifications.
+  assertive,
+}
+
 /// An event sent by the application to notify interested listeners that
 /// something happened to the user interface (e.g. a view scrolled).
 ///
@@ -71,7 +86,7 @@ abstract class SemanticsEvent {
 class AnnounceSemanticsEvent extends SemanticsEvent {
 
   /// Constructs an event that triggers an announcement by the platform.
-  const AnnounceSemanticsEvent(this.message, this.textDirection)
+  const AnnounceSemanticsEvent(this.message, this.textDirection, {this.assertiveness = Assertiveness.polite})
     : assert(message != null),
       assert(textDirection != null),
       super('announce');
@@ -86,11 +101,20 @@ class AnnounceSemanticsEvent extends SemanticsEvent {
   /// This property must not be null.
   final TextDirection textDirection;
 
+  /// Determines whether the announcement should interrupt any existing announcement,
+  /// or queue after it.
+  ///
+  /// On the web this option uses the aria-live level to set the assertiveness
+  /// of the announcement. On iOS, Android, Windows, Linux, macOS, and Fuchsia
+  /// this option currently has no effect.
+  final Assertiveness assertiveness;
+
   @override
   Map<String, dynamic> getDataMap() {
     return <String, dynamic>{
       'message': message,
       'textDirection': textDirection.index,
+      'assertiveness': assertiveness.index,
     };
   }
 }

packages/flutter/lib/src/semantics/semantics_service.dart

@@ -6,7 +6,7 @@ import 'dart:ui' show TextDirection;
 
 import 'package:flutter/services.dart' show SystemChannels;
 
-import 'semantics_event.dart' show AnnounceSemanticsEvent, TooltipSemanticsEvent;
+import 'semantics_event.dart' show AnnounceSemanticsEvent, Assertiveness, TooltipSemanticsEvent;
 
 export 'dart:ui' show TextDirection;
 
@@ -29,8 +29,12 @@ class SemanticsService {
   ///
   /// For example a camera application can use this method to make accessibility
   /// announcements regarding objects in the viewfinder.
-  static Future<void> announce(String message, TextDirection textDirection) async {
-    final AnnounceSemanticsEvent event = AnnounceSemanticsEvent(message, textDirection);
+  ///
+  /// The assertiveness level of the announcement is determined by [assertiveness].
+  /// Currently, this is only supported by the web engine and has no effect on
+  /// other platforms. The default mode is [Assertiveness.polite].
+  static Future<void> announce(String message, TextDirection textDirection, {Assertiveness assertiveness = Assertiveness.polite}) async {
+    final AnnounceSemanticsEvent event = AnnounceSemanticsEvent(message, textDirection, assertiveness: assertiveness);
     await SystemChannels.accessibility.send(event.toMap());
   }
 

packages/flutter/test/semantics/semantics_service_test.dart

@@ -20,11 +20,10 @@ void main() {
     TestDefaultBinaryMessengerBinding.instance!.defaultBinaryMessenger.setMockDecodedMessageHandler<dynamic>(SystemChannels.accessibility, handleMessage);
 
     await SemanticsService.announce('announcement 1', TextDirection.ltr);
-    await SemanticsService.announce('announcement 2', TextDirection.rtl);
-
+    await SemanticsService.announce('announcement 2', TextDirection.rtl, assertiveness: Assertiveness.assertive);
     expect(log, equals(<Map<String, dynamic>>[
-      <String, dynamic>{'type': 'announce', 'data': <String, dynamic>{'message': 'announcement 1', 'textDirection': 1}},
-      <String, dynamic>{'type': 'announce', 'data': <String, dynamic>{'message': 'announcement 2', 'textDirection': 0}},
+      <String, dynamic>{'type': 'announce', 'data': <String, dynamic>{'message': 'announcement 1', 'textDirection': 1, 'assertiveness': 0}},
+      <String, dynamic>{'type': 'announce', 'data': <String, dynamic>{'message': 'announcement 2', 'textDirection': 0, 'assertiveness': 1}},
     ]));
   });
 }