Issue 3826 - Filter preference change events

lib/prefs.js

Index: lib/prefs.js
===================================================================
--- a/lib/prefs.js
+++ b/lib/prefs.js
@@ -17,11 +17,15 @@
 
 /** @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!
@@ -178,12 +182,27 @@
   */
 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.
@@ -290,7 +309,7 @@
           else
             delete overrides[pref];
 
-          Prefs.onChanged._dispatch(pref);
+          eventEmitter.emit(pref);
         }
       }
     });