XUL

XUL (/ˈzuːl/ ZOOL), which stands for XML User Interface Language, is a user interface markup language developed by Mozilla. XUL is implemented as an XML dialect, enabling graphical user interfaces to be written in a similar manner to web pages.

XUL can be used to write cross-platform applications. The most prominent example is the Firefox web browser with its Gecko engine that renders both web pages and the XUL browser interface.^[1]

In the past, Firefox permitted add-ons to extensively alter its user interface via custom XUL code, but this capability was removed in Firefox 57 and replaced with the less-permissive WebExtensions API.^[2][3] (Three forks of Firefox still support the legacy capability: Pale Moon,^[4] Basilisk,^[5] Waterfox.^[6])

Design

XUL relies on multiple existing Web standards and Web 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 which Mozilla tri-licensed under the GNU GPL, GNU LGPL, and MPL^[7] until Gecko changed to GPL-compatible, MPL 2.0.

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

XUL provides a portable definition for common widgets, allowing them to move easily to any platform on which Mozilla applications run.^[8]

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

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, provided a rich interface for searching books at Amazon.com.^[10]

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 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. Mozilla-programmers sometimes refer to XUL applications running locally as chrome^[11]. Not to be confused with the Google Chrome browser.

Other applications using XUL include:

• 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 Nightingale and Songbird music-players and Miro video-player all use built-in XUL.
• 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.
• The Flickr Uploader was built using XUL, and source code is available under GPLv2.^[12]
• Kiwix, the offline Wikipedia-viewer
• BlueGriffon is a WYSIWYG HTML-Editor based on Gecko, XUL and Javascript

With the release of Firefox 4, support for remote XUL was disabled by default, due to security concerns. Loading an XUL page via HTTP would now display an error unless the domain was added to a hidden whitelist.^[13]

Ghostbusters reference

The villain of the 1984 film Ghostbusters was a deity called Zuul who possesses the character Dana Barrett and declares, "There is no Dana. There is only Zuul".^[14] The creators of XUL, which is pronounced the same as Zuul, made the slogan "There is no data. There is only XUL!", part of which became the XML namespace.^[15]

Example

This example shows three buttons stacked on top of each other in a vertical box container:^[16]

<?xml version="1.0"?>
<?xml-stylesheet href="chrome://global/skin/" type="text/css"?>

<window id="vbox example" title="Example 3...."
xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
  <vbox>
    <button id="yes1" label="Yes"/>
    <button id="no1" label="No"/>
    <button id="maybe1" label="Maybe"/>
  </vbox>
</window>

Similar languages

Based on HTML, DHTML, and JavaScript, Microsoft's HTML Application resembles XUL as it provides a stand-alone capability to run applications on Windows, outside a Web browser.

The Microsoft Windows Presentation Foundation (WPF) defines XAML, another declarative UI language similar to XUL — it runs on top of the .NET Framework stack.

JavaFX defines FXML as its own declarative UI language like XUL.

See also

• Extensible User Interface Protocol (XUP)
• User interface markup language
• Layout manager
• Mozilla application framework
• XBL
• XULRunner
• ZK Framework

References

External links

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