text: /**
   * Copyright (c) Facebook, Inc. and its affiliates.
   *
   * This source code is licensed under the MIT license found in the
   * LICENSE file in the root directory of this source tree.
   *
   * @flow
   * @format
   */
  
  'use strict';
  
  const DeviceEventManager = require('../BatchedBridge/NativeModules')
    .DeviceEventManager;
  const RCTDeviceEventEmitter = require('../EventEmitter/RCTDeviceEventEmitter');
  
  const DEVICE_BACK_EVENT = 'hardwareBackPress';
  
  type BackPressEventName = 'backPress' | 'hardwareBackPress';
  
  const _backPressSubscriptions = [];
  
  RCTDeviceEventEmitter.addListener(DEVICE_BACK_EVENT, function() {
    for (let i = _backPressSubscriptions.length - 1; i >= 0; i--) {
      if (_backPressSubscriptions[i]()) {
        return;
      }
    }
  
    BackHandler.exitApp();
  });
  
  /**
   * Detect hardware button presses for back navigation.
   *
   * Android: Detect hardware back button presses, and programmatically invoke the default back button
   * functionality to exit the app if there are no listeners or if none of the listeners return true.
   *
   * tvOS: Detect presses of the menu button on the TV remote.  (Still to be implemented:
   * programmatically disable menu button handling
   * functionality to exit the app if there are no listeners or if none of the listeners return true.)
   *
   * iOS: Not applicable.
   *
   * macOS: Not applicable.
   *
   * The event subscriptions are called in reverse order (i.e. last registered subscription first),
   * and if one subscription returns true then subscriptions registered earlier will not be called.
   *
   * Example:
   *
   * ```javascript
   * BackHandler.addEventListener('hardwareBackPress', function() {
   *  // this.onMainScreen and this.goBack are just examples, you need to use your own implementation here
   *  // Typically you would use the navigator here to go to the last state.
   *
   *  if (!this.onMainScreen()) {
   *    this.goBack();
   *    return true;
   *  }
   *  return false;
   * });
   * ```
   */
  type TBackHandler = {|
    +exitApp: () => void,
    +addEventListener: (
      eventName: BackPressEventName,
      handler: Function,
    ) => {remove: () => void},
    +removeEventListener: (
      eventName: BackPressEventName,
      handler: Function,
    ) => void,
  |};
  const BackHandler: TBackHandler = {
    exitApp: function(): void {
      DeviceEventManager.invokeDefaultBackPressHandler();
    },
  
    /**
     * Adds an event handler. Supported events:
     *
     * - `hardwareBackPress`: Fires when the Android hardware back button is pressed or when the
     * tvOS menu button is pressed.
     */
    addEventListener: function(
      eventName: BackPressEventName,
      handler: Function,
    ): {remove: () => void} {
      if (_backPressSubscriptions.indexOf(handler) === -1) {
        _backPressSubscriptions.push(handler);
      }
      return {
        remove: (): void => BackHandler.removeEventListener(eventName, handler),
      };
    },
  
    /**
     * Removes the event handler.
     */
    removeEventListener: function(
      eventName: BackPressEventName,
      handler: Function,
    ): void {
      if (_backPressSubscriptions.indexOf(handler) !== -1) {
        _backPressSubscriptions.splice(
          _backPressSubscriptions.indexOf(handler),
          1,
        );
      }
    },
  };
  
  module.exports = BackHandler;
  