| Filename extension | .xul |
|---|---|
| Internet media type | application/ |
| Developed by | Mozilla Foundation |
| Type of format | markup language |
In computer programming, XUL (pronounced /ˈzuːl/ zuul), 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. The Mozilla Gecko layout engine provides the implementation of XUL used in the Firefox browser.[1]
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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, 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 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.[3]
Programmers typically define a XUL interface as three discrete sets of components:
XUL defines a wide range of elements, which roughly belong to the following types:
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]
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 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.[6]
Other applications using XUL are:
With the release of Firefox 4, support for remote XUL was disabled by default, due to security concerns.[7] Loading an XUL page via HTTP would now display an error unless the domain was added to a hidden whitelist.
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, like MXML, 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 debugger component is called Venkman, after one of the main characters in the film.)
This example shows 3 buttons stacked on top of each other in a vertical box container:[8]
<?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="yes" label="Yes"/> <button id="no" label="No"/> <button id="maybe" label="Maybe"/> </vbox> </window>
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) defines XAML, another declarative UI language similar to XUL — it runs on top of the .NET Framework stack.
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