FireTray/src/modules/PrefListener.jsm

/* -*- Mode: js2; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
// https://developer.mozilla.org/en/Code_snippets/Preferences

var EXPORTED_SYMBOLS = [ "PrefListener" ];

const Cc = Components.classes;
const Ci = Components.interfaces;
const Cu = Components.utils;


/**
 * @constructor
 *
 * @param {string} branch_name
 * @param {Function} callback must have the following arguments:
 *   branch, pref_leaf_name
 */
function PrefListener(branch_name, callback) {
  // Keeping a reference to the observed preference branch or it will get
  // garbage collected.
  var prefService = Components.classes["@mozilla.org/preferences-service;1"]
        .getService(Components.interfaces.nsIPrefService);
  this._branch = prefService.getBranch(branch_name);
  this._branch.QueryInterface(Ci.nsIPrefBranch);
  this._callback = callback;
}

PrefListener.prototype.observe = function(subject, topic, data) {
  if (topic == 'nsPref:changed')
    this._callback(this._branch, data);
};

/**
 * @param {boolean=} trigger if true triggers the registered function
 *   on registration, that is, when this method is called.
 */
PrefListener.prototype.register = function(trigger) {
  this._branch.addObserver('', this, false);
  if (trigger) {
    let that = this;
    this._branch.getChildList('', {}).
      forEach(function (pref_leaf_name)
              { that._callback(that._branch, pref_leaf_name); });
  }
};

PrefListener.prototype.unregister = function() {
  if (this._branch)
    this._branch.removeObserver('', this);
};