XUL (pronounced zool ()), the XML User Interface Language, an XML user interface markup language developed by the Mozilla project, operates in Mozilla cross-platform applications such as Firefox and Flock. The Gecko layout engine provides the only complete implementation of XUL.

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 and web-developer documentation is available online at the Mozilla Developer Center http://developer.mozilla.org/en/docs/XUL.

XUL has no formal specification and is not interoperable with non-Gecko implementations. However, its implementation (Gecko) is open source, tri-licensed under the GPL, LGPL, and MPL.^[1]

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

As its main benefit, XUL provides a simple and portable definition of common widgets. This reduces the software development effort in a way analogous to the savings offered by 4GL tools.

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 elements

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

Top-level elements

e.g., window, page, dialog, wizard, etc.

Widgets

e.g., label, button, text box, list box, combo box, radio button, check box, tree, menu, toolbar, group box, tab box, color picker, spacer, splitter, etc.

Box model

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

Events and Scripts

e.g., 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), 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.^[2]

XUL applications

While XUL serves primarily for creating the Mozilla applications and their extensions, it may also feature in web-applications transferred over HTTP. A former XUL application of this type, well-known when it was operating, the Mozilla Amazon Browser, 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 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.

ActiveState's Komodo IDE uses XUL as well as the recently announced Open Komodo Project http://openkomodo.com .

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

Mozilla-programmers sometimes refer to XUL applications running locally as "chrome".^[3]

The name

The XUL name 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

When viewed with a XUL-capable application, the document pointed to displays the slogan in large letters in the center of the screen (similar to that to the right).

"Keymaster" and "gatekeeper" also refer to the same film plotline. 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.

Example code

This piece of code shows 3 buttons stacked on top of each other in a vertical box container:^[4]

<?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>

See also

• XBL
• Mozilla application framework
• XULRunner
• Layout manager
• List of user interface markup languages
• Comparison of user interface markup languages
• OpenLaszlo

References

External links

• XUL documentation on developer.mozilla.org
• XULPlanet ? A developer site with tutorials, references and forums
• XUL Periodic Table ? Visual demonstration of XUL capabilities (Requires an XUL-enabled browser such as Mozilla Firefox)
• XUL and XML ? How to use XUL

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