The Tor Browser / file revision

 /* This Source Code Form is subject to the terms of the Mozilla Public

  * License, v. 2.0. If a copy of the MPL was not distributed with this

  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

 "use strict";

 module.metadata = {

   "stability": "unstable"

 };

 const { Cc, Ci, Cr } = require("chrome");

 const { emit, on, off } = require("./core");

 const { addObserver } = Cc['@mozilla.org/observer-service;1'].

                         getService(Ci.nsIObserverService);

 // Simple class that can be used to instantiate event channel that

 // implements `nsIObserver` interface. It's will is used by `observe`

 // function as observer + event target. It basically proxies observer

 // notifications as to it's registered listeners.

 function ObserverChannel() {}

 Object.freeze(Object.defineProperties(ObserverChannel.prototype, {

   QueryInterface: {

     value: function(iid) {

       if (!iid.equals(Ci.nsIObserver) &&

           !iid.equals(Ci.nsISupportsWeakReference) &&

           !iid.equals(Ci.nsISupports))

         throw Cr.NS_ERROR_NO_INTERFACE;

       return this;

     }

   },

   observe: {

     value: function(subject, topic, data) {

       emit(this, "data", {

         type: topic,

         target: subject,

         data: data

       });

     }

   }

 }));

 function observe(topic) {

   let observerChannel = new ObserverChannel();

   // Note: `nsIObserverService` will not hold a weak reference to a

   // observerChannel (since third argument is `true`). There for if it

   // will be GC-ed with all it's event listeners once no other references

   // will be held.

   addObserver(observerChannel, topic, true);

   return observerChannel;

 }

 exports.observe = observe;