Add an accessibility service with an announce method. (#12594)

packages/flutter/lib/semantics.dart

@@ -14,3 +14,4 @@
 library semantics;
 
 export 'src/semantics/semantics.dart';
+export 'src/semantics/semantics_service.dart';

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

@@ -14,7 +14,7 @@ abstract class SemanticsEvent {
   /// Initializes internal fields.
   ///
   /// [type] is a string that identifies this class of [SemanticsEvent]s.
-  SemanticsEvent(this.type);
+  const SemanticsEvent(this.type);
 
   /// The type of this event.
   ///
@@ -121,3 +121,40 @@ class ScrollCompletedSemanticsEvent extends SemanticsEvent {
     return map;
   }
 }
+
+/// An event for a semantic announcement.
+///
+/// This should be used for announcement that are not seamlessly announced by
+/// the system as a result of a UI state change.
+///
+/// For example a camera application can use this method to make accessibility
+/// announcements regarding objects in the viewfinder.
+///
+/// When possible, prefer using mechanisms like [Semantics] to implicitly
+/// trigger announcements over using this event.
+class AnnounceSemanticsEvent extends SemanticsEvent {
+
+  /// Constructs an event that triggers an announcement by the platform.
+  const AnnounceSemanticsEvent(this.message, this.textDirection) :
+    assert(message != null),
+    assert(textDirection != null),
+    super('announce');
+
+  /// The message to announce.
+  ///
+  /// This property must not be null.
+  final String message;
+
+  /// Text direction for [message].
+  ///
+  /// This property must not be null.
+  final TextDirection textDirection;
+
+  @override
+  Map<String, dynamic> getDataMap() {
+    return <String, dynamic>{
+      'message': message,
+      'textDirection': textDirection.index,
+    };
+  }
+}

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

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:async';
+import 'dart:ui' show TextDirection;
+
+import 'package:flutter/services.dart' show SystemChannels;
+
+import 'semantics_event.dart' show AnnounceSemanticsEvent;
+
+
+/// Allows access to the platform's accessibility services.
+///
+/// Events sent by this service are handled by the platform-specific
+/// accessibility bridge in Flutter's engine.
+///
+/// When possible, prefer using mechanisms like [Semantics] to implicitly
+/// trigger announcements over using this event.
+class SemanticsService {
+  SemanticsService._();
+
+  /// Sends a semantic announcement.
+  ///
+  /// This should be used for announcement that are not seamlessly announced by
+  /// the system as a result of a UI state change.
+  ///
+  /// For example a camera application can use this method to make accessibility
+  /// announcements regarding objects in the viewfinder.
+  static Future<Null> announce(String message, TextDirection textDirection) async {
+    final AnnounceSemanticsEvent event = new AnnounceSemanticsEvent(message, textDirection);
+    await SystemChannels.accessibility.send(event.toMap());
+  }
+}

packages/flutter/test/semantics/semantics_service_test.dart

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:ui' show TextDirection;
+
+import 'package:flutter/semantics.dart';
+import 'package:flutter/services.dart' show SystemChannels;
+import 'package:test/test.dart';
+
+void main() {
+  test('Semantic announcement', () async {
+    final List<Map<String, dynamic>> log = <Map<String, dynamic>>[];
+
+    SystemChannels.accessibility.setMockMessageHandler((Map<String, dynamic> message) async {
+      log.add(message);
+    });
+
+    await SemanticsService.announce('announcement 1', TextDirection.ltr);
+    await SemanticsService.announce('announcement 2', TextDirection.rtl);
+
+    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}},
+    ]));
+  });
+}