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

// Copyright 2013 The Flutter Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

// @dart = 2.6
part of engine;

/// Singleton for accessing accessibility announcements from the platform.
final AccessibilityAnnouncements accessibilityAnnouncements =
    AccessibilityAnnouncements.instance;

/// Attaches accessibility announcements coming from the 'flutter/accessibility'
/// channel as temporary elements to the DOM.
class AccessibilityAnnouncements {
  /// Initializes the [AccessibilityAnnouncements] singleton if it is not
  /// already initialized.
  static AccessibilityAnnouncements get instance {
    return _instance ??= AccessibilityAnnouncements._();
  }

  static AccessibilityAnnouncements _instance;

  AccessibilityAnnouncements._() {
    registerHotRestartListener(() {
      _removeElementTimer?.cancel();
    });
  }

  /// Timer that times when the accessibility element should be removed from the
  /// DOM.
  ///
  /// The element is added to the DOM temporarily for announcing the
  /// message to the assistive technology.
  Timer _removeElementTimer;

  /// The duration the accessibility announcements stay on the DOM.
  ///
  /// It is removed after this time expired.
  Duration durationA11yMessageIsOnDom = const Duration(seconds: 5);

  /// Element which is used to communicate the message from the
  /// 'flutter/accessibility' to the assistive technologies.
  ///
  /// This element gets attached to the DOM temporarily. It gets removed
  /// after a duration. See [durationA11yMessageIsOnDom].
  ///
  /// This element has aria-live attribute.
  ///
  /// It also has id 'accessibility-element' for testing purposes.
  html.HtmlElement _element;

  html.HtmlElement get _domElement => _element ??= _createElement();

  /// Decodes the message coming from the 'flutter/accessibility' channel.
  void handleMessage(StandardMessageCodec codec, ByteData data) {
    final Map<dynamic, dynamic> inputMap =
        codec.decodeMessage(data);
    final Map<dynamic, dynamic> dataMap = inputMap['data'];
    final String message = dataMap['message'];
    if (message != null && message.isNotEmpty) {
      _initLiveRegion(message);
      _removeElementTimer = Timer(durationA11yMessageIsOnDom, () {
        _element.remove();
      });
    }
  }

  void _initLiveRegion(String message) {
    _domElement.setAttribute('aria-live', 'polite');
    _domElement.text = message;
    html.document.body.append(_domElement);
  }

  html.LabelElement _createElement() {
    final html.LabelElement liveRegion = html.LabelElement();
    liveRegion.setAttribute('id', 'accessibility-element');
    liveRegion.style
      ..position = 'fixed'
      ..overflow = 'hidden'
      ..transform = 'translate(-99999px, -99999px)'
      ..width = '1px'
      ..height = '1px';
    return liveRegion;
  }
}