/**
 * Copyright (c) 2015-present, Facebook, Inc.
 * All rights reserved.
 *
 * This source code is licensed under the BSD-style license found in the
 * LICENSE file in the root directory of this source tree. An additional grant
 * of patent rights can be found in the PATENTS file in the same directory.
 *
 * @providesModule AccessibilityInfo
 * @flow
 */
'use strict';

var NativeModules = require('NativeModules');
var Promise = require('Promise');
var RCTDeviceEventEmitter = require('RCTDeviceEventEmitter');

var AccessibilityManager = NativeModules.AccessibilityManager;

var VOICE_OVER_EVENT = 'voiceOverDidChange';
var ANNOUNCEMENT_DID_FINISH_EVENT = 'announcementDidFinish';

type ChangeEventName = $Enum<{
  change: string,
  announcementFinished: string
}>;

var _subscriptions = new Map();

/**
 * Sometimes it's useful to know whether or not the device has a screen reader
 * that is currently active. The `AccessibilityInfo` API is designed for this
 * purpose. You can use it to query the current state of the screen reader as 
 * well as to register to be notified when the state of the screen reader 
 * changes.
 *
 * See http://facebook.github.io/react-native/docs/accessibilityinfo.html
 */
var AccessibilityInfo = {

  /**
   * Query whether a screen reader is currently enabled. 
   * 
   * Returns a promise which resolves to a boolean. 
   * The result is `true` when a screen reader is enabledand `false` otherwise.
   * 
   * See http://facebook.github.io/react-native/docs/accessibilityinfo.html#fetch
   */
  fetch: function(): Promise {
    return new Promise((resolve, reject) => {
      AccessibilityManager.getCurrentVoiceOverState(
        resolve,
        reject
      );
    });
  },

  /**
   * Add an event handler. Supported events:
   *
   * - `change`: Fires when the state of the screen reader changes. The argument
   *   to the event handler is a boolean. The boolean is `true` when a screen
   *   reader is enabled and `false` otherwise.
   * - `announcementFinished`: iOS-only event. Fires when the screen reader has
   *   finished making an announcement. The argument to the event handler is a
   *   dictionary with these keys:
   *     - `announcement`: The string announced by the screen reader.
   *     - `success`: A boolean indicating whether the announcement was
   *       successfully made.
   * 
   * See http://facebook.github.io/react-native/docs/accessibilityinfo.html#addeventlistener
   */
  addEventListener: function (
    eventName: ChangeEventName,
    handler: Function
  ): Object {
    var listener;

    if (eventName === 'change') {
      listener = RCTDeviceEventEmitter.addListener(
        VOICE_OVER_EVENT,
        handler
      );
    } else if (eventName === 'announcementFinished') {
      listener = RCTDeviceEventEmitter.addListener(
        ANNOUNCEMENT_DID_FINISH_EVENT,
        handler
      );
    }

    _subscriptions.set(handler, listener);
    return {
      remove: AccessibilityInfo.removeEventListener.bind(null, eventName, handler),
    };
  },

  /**
   * Set accessibility focus to a react component.
   * 
   * @platform ios
   * 
   * See http://facebook.github.io/react-native/docs/accessibilityinfo.html#setaccessibilityfocus
   */
  setAccessibilityFocus: function(
    reactTag: number
  ): void {
    AccessibilityManager.setAccessibilityFocus(reactTag);
  },

  /**
   * Post a string to be announced by the screen reader.
   *
   * @platform ios
   * 
   * See http://facebook.github.io/react-native/docs/accessibilityinfo.html#announceforaccessibility
   */
  announceForAccessibility: function(
    announcement: string
  ): void {
    AccessibilityManager.announceForAccessibility(announcement);
  },

  /**
   * Remove an event handler.
   * 
   * See http://facebook.github.io/react-native/docs/accessibilityinfo.html#removeeventlistener
   */
  removeEventListener: function(
    eventName: ChangeEventName,
    handler: Function
  ): void {
    var listener = _subscriptions.get(handler);
    if (!listener) {
      return;
    }
    listener.remove();
    _subscriptions.delete(handler);
  },

};

module.exports = AccessibilityInfo;