XUL, language for building graphical interfaces

XUL allows to create cross-platform, graphical user interfaces for standalone applications. It is a standard and portable format, working on any operating system, but is rather difficult to use.
XUL means for "XML-based User interface Language". This is a semantic XML extension for building graphical interfaces, created by Mozilla in 1999.

XUL seemed destined to become a universal language for describing graphical interfaces. But Microsoft has created his own version, XAML. In addition, the developers have never sought to share the development with third-party tools, which resulted in a plethora of  user interfaces languages based on XML. Finally  HTML 5, thanks to Canvas, multimedia tags, and frameworks able to add new tags to HTML, has become a valid alternative. The future of XUL so seems confined to Firefox and Mozilla tools.

Based on web standards, but complex

The use of RDF and XPCom in particular makes it a  tool difficult to master.

• The code is interpreted.
• JavaScript code may be embedded into xml to define functions called by events.
• Uses bitmap graphics.
• Events (a mouse click for example) are properties of tags and tags are components of the interface.
• Style properties are defined inside standard CSS files.
• Component (mainly the "widgets", graphical components) may be define by user, in XBL or XPF files.
• The RDF format is user to store and deliver texts, for localization for example.
• XPCom is used for the wrapper between XUL and libraries written in various programming languages.
• It is possible to include HTML code in XUL and then run locally on XULRunner a Web application.

Sample code

Displaying "Hello world!":

<?xml version="1.0">
<windows width="600 height="480">
  <description>
    Hello World!
  </description>
</windows>

Drawing a button:

<button 
   label="Close" 
   oncommand="window.close()"
/>

Tools and documentation

• XULRunner
  Download the runtime.
  
• XUL on Mozilla
  Official website. Mozilla is the creator of the language.
• XUL tutorial
  With examples and source code. Explain How to install a XUL application.

See also XAML, the Microsoft version of XUL.