packages/node_modules/pouchdb-utils/src/changesHandler.js

import EventEmitter from 'events';
import hasLocalStorage from './env/hasLocalStorage';
import pick from './pick';
import nextTick from './nextTick';

export default class Changes extends EventEmitter {
  constructor() {
    super();

    this._listeners = {};

    if (hasLocalStorage()) {
      addEventListener("storage", (e) => {
        this.emit(e.key);
      });
    }
  }

  addListener(dbName, id, db, opts) {
    if (this._listeners[id]) {
      return;
    }
    var inprogress = false;
    var self = this;
    function eventFunction() {
      if (!self._listeners[id]) {
        return;
      }
      if (inprogress) {
        inprogress = 'waiting';
        return;
      }
      inprogress = true;
      var changesOpts = pick(opts, [
        'style', 'include_docs', 'attachments', 'conflicts', 'filter',
        'doc_ids', 'view', 'since', 'query_params', 'binary', 'return_docs'
      ]);

      function onError() {
        inprogress = false;
      }

      db.changes(changesOpts).on('change', function (c) {
        if (c.seq > opts.since && !opts.cancelled) {
          opts.since = c.seq;
          opts.onChange(c);
        }
      }).on('complete', function () {
        if (inprogress === 'waiting') {
          nextTick(eventFunction);
        }
        inprogress = false;
      }).on('error', onError);
    }
    this._listeners[id] = eventFunction;
    this.on(dbName, eventFunction);
  }

  removeListener(dbName, id) {
    if (!(id in this._listeners)) {
      return;
    }
    super.removeListener(dbName, this._listeners[id]);
    delete this._listeners[id];
  }

  notifyLocalWindows(dbName) {
    //do a useless change on a storage thing
    //in order to get other windows's listeners to activate
    if (hasLocalStorage()) {
      localStorage[dbName] = (localStorage[dbName] === "a") ? "b" : "a";
    }
  }

  notify(dbName) {
    this.emit(dbName);
    this.notifyLocalWindows(dbName);
  }
}