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
   */
  
  // On Apple TV, this implements back navigation using the TV remote's menu button.
  // On iOS, this just implements a stub.
  
  'use strict';
  
  const Platform = require('./Platform');
  const TVEventHandler = require('../Components/AppleTV/TVEventHandler');
  
  type BackPressEventName = 'backPress' | 'hardwareBackPress';
  
  function emptyFunction(): void {}
  
  /**
   * 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.
   *
   * 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,
  |};
  
  let BackHandler: TBackHandler;
  
  if (Platform.isTV) {
    const _tvEventHandler = new TVEventHandler();
    const _backPressSubscriptions = new Set();
  
    _tvEventHandler.enable(this, function(cmp, evt) {
      if (evt && evt.eventType === 'menu') {
        let invokeDefault = true;
        const subscriptions = Array.from(
          _backPressSubscriptions.values(),
        ).reverse();
  
        for (let i = 0; i < subscriptions.length; ++i) {
          if (subscriptions[i]()) {
            invokeDefault = false;
            break;
          }
        }
  
        if (invokeDefault) {
          BackHandler.exitApp();
        }
      }
    });
  
    BackHandler = {
      exitApp: emptyFunction,
  
      addEventListener: function(
        eventName: BackPressEventName,
        handler: Function,
      ): {remove: () => void} {
        _backPressSubscriptions.add(handler);
        return {
          remove: () => BackHandler.removeEventListener(eventName, handler),
        };
      },
  
      removeEventListener: function(
        eventName: BackPressEventName,
        handler: Function,
      ): void {
        _backPressSubscriptions.delete(handler);
      },
    };
  } else {
    BackHandler = {
      exitApp: emptyFunction,
      addEventListener(_eventName: BackPressEventName, _handler: Function) {
        return {
          remove: emptyFunction,
        };
      },
      removeEventListener(_eventName: BackPressEventName, _handler: Function) {},
    };
  }
  
  module.exports = BackHandler;
  