Issue 3826 - Filter preference change events

lib/prefs.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-2016 Eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  * GNU General Public License for more details.
  *
  * You should have received a copy of the GNU General Public License
  * along with Adblock Plus.  If not, see <http://www.gnu.org/licenses/>.
  */
 
 /** @module prefs */
 
+let {EventEmitter} = require("events");
+
 const keyPrefix = "pref:";
 
+let eventEmitter = new EventEmitter();
+let overrides = Object.create(null);
+
 /** @lends module:prefs.Prefs */
 let defaults = Object.create(null);
-let overrides = Object.create(null);
 
 /**
  * Only for compatibility with core code. Please do not change!
  *
  * @type {boolean}
  */
 defaults.enabled = true;
 /**
  * The application version as set during initialization. Used to detect updates.
  *
@@ skipping 136 matching lines @@
  * @type {string[]}
  */
 defaults.additional_subscriptions = [];
 
 /**
   * @namespace
   * @static
   */
 let Prefs = exports.Prefs = {
   /**
-   * Fired when the value of a preference changes.
+   * Adds a callback that is called when the
+   * value of a specified preference changed.
    *
-   * @event
-   * @property {string} pref The name of the preference that changed.
+   * @param {string}   preference
+   * @param {function} callback
    */
-  onChanged: new ext._EventTarget(),
+  on: function(preference, callback)
+  {
+    eventEmitter.on(preference, callback);
+  },
+
+  /**
+   * Removes a callback for the specified preference.
+   *
+   * @param {string}   preference
+   * @param {function} callback
+   */
+  off: function(preference, callback)
+  {
+    eventEmitter.off(preference, callback);
+  },
 
   /**
    * A promise that is fullfilled when all preferences have been loaded.
    * Wait for this promise to be fulfilled before using preferences during
    * extension initialization.
    *
    * @type {Promise}
    */
   untilLoaded: null
 };
@@ skipping 86 matching lines @@
       {
         let pref = keyToPref(key);
         if (pref && pref in defaults)
         {
           let change = changes[key];
           if ("newValue" in change && change.newValue != defaults[pref])
             overrides[pref] = change.newValue;
           else
             delete overrides[pref];
 
-          Prefs.onChanged._dispatch(pref);
+          eventEmitter.emit(pref);
         }
       }
     });
   }
 
   Prefs.untilLoaded = Promise.all([localLoaded, managedLoaded]).then(onLoaded);
 }
 
 init();