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

/**
 * @flow
 */

import ExecutionEnvironment from 'fbjs/lib/ExecutionEnvironment';
import findIndex from 'array-find-index';
import invariant from 'fbjs/lib/invariant';

const EVENT_TYPES = ['change'];
const VISIBILITY_CHANGE_EVENT = 'visibilitychange';

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

const listeners = [];

class AppState {
  static isAvailable = ExecutionEnvironment.canUseDOM && document.visibilityState;

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

    switch (document.visibilityState) {
      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
      );
      const callback = () => handler(AppState.currentState);
      listeners.push([handler, callback]);
      document.addEventListener(VISIBILITY_CHANGE_EVENT, callback, false);
    }
  }

  static removeEventListener(type: string, handler: Function) {
    if (AppState.isAvailable) {
      invariant(
        EVENT_TYPES.indexOf(type) !== -1,
        'Trying to remove listener for unknown event: "%s"',
        type
      );
      const listenerIndex = findIndex(listeners, pair => pair[0] === handler);
      invariant(
        listenerIndex !== -1,
        'Trying to remove AppState listener for unregistered handler'
      );
      const callback = listeners[listenerIndex][1];
      document.removeEventListener(VISIBILITY_CHANGE_EVENT, callback, false);
      listeners.splice(listenerIndex, 1);
    }
  }
}

module.exports = AppState;