In computer programming, XUL (pronounced zool ([zu:l])), the XML User Interface Language, is an XML user interface markup language developed by the Mozilla project. XUL operates in Mozilla cross-platform applications such as Firefox and Flock. The Mozilla Gecko layout engine provides an implementation of XUL used in the Firefox browser.^[1]

[edit] Design

XUL relies on multiple existing web standards and technologies, including CSS, JavaScript, and DOM. Such reliance makes XUL relatively easy to learn for people with a background in web-programming and design.

XUL has no formal specification and does not inter-operate with non-Gecko implementations. However, it uses an open source implementation of Gecko, tri-licensed under the GPL, LGPL, and MPL.^[2]

Mozilla provides experimental XULRunner builds to let developers build their applications on top of the Mozilla application framework and XUL in particular.

XUL provides a portable definition for common widgets, allowing them to move easily to any platform on which Mozilla applications run.^[3]

[edit] XUL documents

Programmers typically define a XUL interface as three discrete sets of components:

1. content: the XUL document(s), whose elements define the layout of the user interface
2. skin: the CSS and image files, which define the appearance of an application
3. locale: the files containing user-visible strings for easy software localization

[edit] XUL elements

XUL defines a wide range of elements, which roughly belong to the following types:

top-level elements, such as

window, page, dialog, wizard, etc.

widgets, like

label, button, text box, list box, combo box, radio button, check box, tree, menu, toolbar, group box, tab box, colorpicker, spacer, splitter, etc.

box model

e.g., box, grid, stack, deck, etc.

events and scripts

for instance: script, command, key, broadcaster, observer, etc.

data source

e.g., template, rule, etc.

others

e.g., overlay (analogous to SSI, but client-side and more powerful, with higher performance), iframe, browser, editor, etc.

One can use elements from other applications of XML within XUL documents, such as XHTML, SVG, and MathML.

Mozilla added some common widgets — <scale/> (sometimes called "slider"), <textbox type="number"/> (spinbox), time and date pickers — during the Gecko 1.9 development-cycle.^[4]

[edit] XUL applications

While XUL serves primarily for constructing Mozilla applications and their extensions, it may also feature in web applications transferred over HTTP. The Mozilla Amazon Browser, a former XUL application of this type (and well-known in its day^[5]), provided a rich interface for searching books at Amazon.com.

However, many of the powerful features of Mozilla (such as privileged XPCOM objects) remain unavailable to unprivileged XUL documents unless the script has a digital signature, and unless the user obtains grants of certain^[which?] privileges to the application. Such documents also suffer from various limitations of the browser, including the inability to load remote XUL, DTD, and RDF documents.

As Gecko provides the only full implementation of XUL, such applications remain inaccessible to users of browsers not based on Mozilla.

The Ample SDK Open-Source GUI Framework provides a cross-browser implementation of XUL in JavaScript.

The ActiveState Komodo IDE uses XUL as well as the Open Komodo project announced in 2007.

The Songbird music-player and Miro video-player both use built-in XUL.

The Cyclone3 CMS uses XUL as an add-on in Mozilla Firefox.

The Elixon WCMS/XUL Content management system uses exclusively remote XUL, thus overcoming some of the aforementioned limits of remote unprivileged XUL documents.

The developers of the Celtx media pre-production application used XUL.

Mozilla-programmers sometimes refer to XUL applications running locally as "chrome".^[6]

[edit] The name

The name "XUL" references the film Ghostbusters (1984), in which the ghost of an ancient Sumerian deity called Zuul possesses the character Dana Barrett (played by Sigourney Weaver) and declares, "There is no Dana, only Zuul". Since XUL, unusually, uses XML to define an interface rather than a document, its developers adopted the slogan: "There is no data, there is only XUL". Hence the XML namespace URI at the beginning of every XUL document:

 http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul 

The linked document displays the slogan in large letters in the center of the screen.

"Keymaster" and "gatekeeper" also refer to the plot-line of the Ghostbusters film . (Other references to Ghostbusters also occur within Mozilla products: for instance, the JavaScript has a debugger component called Venkman, after one of the main characters in the film.)

[edit] Example

This example shows 3 buttons stacked on top of each other in a vertical box container:^[7]

 <?xml version="1.0"?> <?xml-stylesheet href="chrome://global/skin/" type="text/css"?>   <window id="vbox example" title="Example" xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">   <vbox>     <button id="yes" label="Yes"/>     <button id="no" label="No"/>     <button id="maybe" label="Maybe"/>   </vbox> </window> 

[edit] Similar languages

HTML Application resembles XUL in that it provides a stand-alone capability to run applications based on HTML, DHTML, and Javascript outside of a web browser for the Microsoft Windows environment developed by Microsoft.

The Microsoft Windows Presentation Foundation (WPF), another declarative UI language (it runs on top of the .NET Framework 2.0 stack as part of the .NET Framework 3.0 extensions), also resembles XUL.

[edit] See also

Free software portal

• Layout manager
• Mozilla application framework
• XBL
• XULRunner

[edit] References

[edit] External links

• XUL documentation on developer.mozilla.org
• XUL Tutorial
• XUL Periodic Table — Visual demonstration of XUL capabilities (requires a Gecko-based (XUL-enabled) browser such as Mozilla Firefox)