react-native-web/src/exports/AppState/index.js

React Native for Web

/**
 * Copyright (c) Nicolas Gallagher.
 * Copyright (c) Meta Platforms, Inc. and affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 *
 * @noflow
 */

'use client';

import invariant from 'fbjs/lib/invariant';
import EventEmitter from '../../vendor/react-native/vendor/emitter/EventEmitter';
import canUseDOM from '../../modules/canUseDom';

// Android 4.4 browser
const isPrefixed =
  canUseDOM &&
  !document.hasOwnProperty('hidden') &&
  document.hasOwnProperty('webkitHidden');

const EVENT_TYPES = ['change', 'memoryWarning'];
const VISIBILITY_CHANGE_EVENT = isPrefixed
  ? 'webkitvisibilitychange'
  : 'visibilitychange';
const VISIBILITY_STATE_PROPERTY = isPrefixed
  ? 'webkitVisibilityState'
  : 'visibilityState';

const AppStates = {
  BACKGROUND: 'background',
  ACTIVE: 'active'
};

let changeEmitter = null;

export default class AppState {
  static isAvailable = canUseDOM && !!document[VISIBILITY_STATE_PROPERTY];

  static get currentState() {
    if (!AppState.isAvailable) {
      return AppStates.ACTIVE;
    }

    switch (document[VISIBILITY_STATE_PROPERTY]) {
      case 'hidden':
      case 'prerender':
      case 'unloaded':
        return AppStates.BACKGROUND;
      default:
        return AppStates.ACTIVE;
    }
  }

  static addEventListener(type: string, handler: Function) {
    if (AppState.isAvailable) {
      invariant(
        EVENT_TYPES.indexOf(type) !== -1,
        'Trying to subscribe to unknown event: "%s"',
        type
      );
      if (type === 'change') {
        if (!changeEmitter) {
          changeEmitter = new EventEmitter();

          document.addEventListener(
            VISIBILITY_CHANGE_EVENT,
            () => {
              if (changeEmitter) {
                changeEmitter.emit('change', AppState.currentState);
              }
            },
            false
          );
        }
        return changeEmitter.addListener(type, handler);
      }
    }
  }
}