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Tag Archives: XPCOMUtils

We’ve recently added two new API’s on XPCOMUtils that make a common coding pattern simpler to do. It is quite common to have “smart getters” to store a reference to a service that you use in JavaScript. Normally, folks write code that looks something like this:

this.__defineGetter__("_ioService", function() {
  delete this._ioService;
  return this._ioService = Components.classes[";1"].

That gets long and verbose if you have any more than a few of these getters. The new approach looks like this:

XPCOMUtils.defineLazyServiceGetter(this, "_ioService",

The net result is a lot less boilerplate code, which, in my opinion, is a lot nicer to read. I’ve already converted the toolkit code in the Places module over, and the resulting code is much nicer.

Sometimes, however, you want to have a getter for something that is not a service. That’s where the second API comes in. You can pass in a lambda function that will be called when the value is needed for the first time. From then on, the value is cached. Some old code may look something like this:

this.__defineGetter__("_db", function() {
  delete this._db;
  return this._db = Components.classes[";1"].

With the new API, that code turns into this:

XPCOMUtils.defineLazyGetter(this, "_db", function() {
  return Components.classes[";1"].

Again, much nicer looking! If you are interested, the implementation of these methods can be found here.