third_party/polymer/v3_0/components-chromium/iron-a11y-announcer/iron-a11y-announcer.js

/**
@license
Copyright (c) 2015 The Polymer Project Authors. All rights reserved.
This code may only be used under the BSD style license found at
http://polymer.github.io/LICENSE.txt The complete set of authors may be found at
http://polymer.github.io/AUTHORS.txt The complete set of contributors may be
found at http://polymer.github.io/CONTRIBUTORS.txt Code distributed by Google as
part of the polymer project is also subject to an additional IP rights grant
found at http://polymer.github.io/PATENTS.txt
*/
import '../polymer/polymer-legacy.js';

import {Polymer} from '../polymer/lib/legacy/polymer-fn.js';
import {html} from '../polymer/lib/utils/html-tag.js';

/**
`iron-a11y-announcer` is a singleton element that is intended to add a11y
to features that require on-demand announcement from screen readers. In
order to make use of the announcer, it is best to request its availability
in the announcing element.

Example:

    Polymer({

      is: 'x-chatty',

      attached: function() {
        // This will create the singleton element if it has not
        // been created yet:
        Polymer.IronA11yAnnouncer.requestAvailability();
      }
    });

After the `iron-a11y-announcer` has been made available, elements can
make announces by firing bubbling `iron-announce` events.

Example:

    this.fire('iron-announce', {
      text: 'This is an announcement!'
    }, { bubbles: true });

Note: announcements are only audible if you have a screen reader enabled.

@group Iron Elements
@demo demo/index.html
*/
export const IronA11yAnnouncer = Polymer({
  _template: html`
    <style>
      :host {
        display: inline-block;
        position: fixed;
        clip: rect(0px,0px,0px,0px);
      }
    </style>
    <div aria-live$="[[mode]]">[[_text]]</div>
`,

  is: 'iron-a11y-announcer',

  properties: {

    /**
     * The value of mode is used to set the `aria-live` attribute
     * for the element that will be announced. Valid values are: `off`,
     * `polite` and `assertive`.
     */
    mode: {type: String, value: 'polite'},

    _text: {type: String, value: ''}
  },

  created: function() {
    if (!IronA11yAnnouncer.instance) {
      IronA11yAnnouncer.instance = this;
    }

    document.body.addEventListener(
        'iron-announce', this._onIronAnnounce.bind(this));
  },

  /**
   * Cause a text string to be announced by screen readers.
   *
   * @param {string} text The text that should be announced.
   */
  announce: function(text) {
    this._text = '';
    this.async(function() {
      this._text = text;
    }, 100);
  },

  _onIronAnnounce: function(event) {
    if (event.detail && event.detail.text) {
      this.announce(event.detail.text);
    }
  }
});

IronA11yAnnouncer.instance = null;

IronA11yAnnouncer.requestAvailability = function() {
  if (!IronA11yAnnouncer.instance) {
    IronA11yAnnouncer.instance = document.createElement('iron-a11y-announcer');
  }

  document.body.appendChild(IronA11yAnnouncer.instance);
};