This tutorial guides you to learning XUL (XML User-interface Language) which is a cross-platform language for describing user interfaces of applications.

This tutorial will demonstrate creating a simple find file user interface, much like that provided by the Macintosh's Sherlock or the find file dialog in Windows. Note that only the user interface will be created and some limited functionality. The actual finding of files will be not be implemented. A blue line will appear to the left of a paragraph where the find file dialog is being modified. You can follow along at these sections.

What is XUL and why was it created?

XUL (pronounced zool and it rhymes with cool) was created to make development of the Mozilla browser easier and faster. It is an XML language so all features available to XML are also available to XUL.

Most applications need to be developed using features of a specific platform making building cross-platform software time-consuming and costly. This may not be important for some, but users may want to use an application on other devices such as handheld devices or set-top boxes.

A number of cross-platform solutions have been developed in the past. Java, for example, has portability as a main selling point. XUL is one such language designed specifically for building portable user interfaces.

It takes a long time to build an application even for only one platform. The time required to compile and debug can be lengthy. With XUL, an interface can be implemented and modified quickly and easily.

XUL has all the advantages of other XML languages. For example XHTML or other XML languages such as MathML or SVG can be inserted within it. Also, text displayed with XUL is easily localizable, which means that it can be translated into other languages with little effort. Style sheets can be applied to modify the appearance of the user interface (much like the skins or themes feature in WinAmp or some window managers).

What kinds of user-interfaces can be made with XUL?

XUL provides the ability to create most elements found in modern graphical interfaces. It is generic enough that it could be applied to the special needs of certain devices and powerful enough that developers can create sophisticated interfaces with it.

Some elements that can be created are:

The displayed content can be created from the contents of a XUL file or with data from a datasource. In Mozilla, such datasources include a user's mailbox, their bookmarks and search results. The contents of menus, trees and other elements can be populated with this data, or with your own data supplied in an RDF file.

XUL content is normally loaded from a package installed into Mozilla. These packages allow an application to have additional privileges, such as reading local files and modifying user preferences. The XUL files and associated scripts and images of an application would be packaged into a single file and downloaded and installed by the user. Mozilla provides a means of having these packages installed and registered without having to write a lot of complex code. In addition, these packages may overlay the browser or other applications to add features; this is the way in which Firefox extensions work.

It is also possible to open XUL files directly from the file system or from a remote web site, however they will be restricted in the kinds of operations they can do, and some aspects of XUL will not work. However, if you do want to load XUL content from a remote site, the Web server must be set up to send XUL files with the content type 'application/vnd.mozilla.xul+xml'. XUL is usually stored in files with a .xul extension. You can open a XUL file with Mozilla as you would any other file, using the Open File command from the File menu or typing the URL into the address bar.

What do I need to know to use this tutorial?

You should have an understanding of HTML and at least a basic understanding of XML and CSS. Here are some guidelines to keep in mind:

XUL is supported in Mozilla and browsers that are also based upon on the Gecko engine, such as Netscape 6 or later and Mozilla Firefox. Due to various changes in XUL syntax over time, you will want to get the latest version for the examples to work properly. Most examples should work in Mozilla 1.0 or later. XUL is fairly similar in Firefox and to other browsers, although it has some specific differences such as support for customizable toolbars.

This tutorial attempts to cover much of XUL's functionality, however, not all features are discussed. Once you are familiar with XUL, you can use the XUL Element Reference to find out about other features supported by specific elements.

(Next) First, let's look at how XUL files are organized.

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