XUL (pronounced zool ({{IPA|[zu:l]}})), the XML User Interface Language, an XMLuser 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{{Fact|date=September 2008}} 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 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.(1)Mozilla provides experimental XULRunner (embedded in firefox 3) 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.{{Fact|date=March 2008}}
XUL documents
Programmers typically define a XUL interface as three discrete sets of components:
content: the XUL document(s), whose elements define the layout of the user interface
skin: the CSS and image files, which define the appearance of an application
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 — (sometimes called "slider"), (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. The Mozilla Amazon Browser, a former XUL application of this type and well-known in its day(3), 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.The ActiveState Komodo IDE uses XUL as well as the recently-announced Open Komodo Project.The Songbird music-player and Miro video-player both use built-in XUL.The developers of the Celtx media pre-production application used XUL.Mozilla-programmers sometimes refer to XUL applications running locally as "chrome".(4)
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 namespaceURI at the beginning of every XUL document:
weblink
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 the image here)."Keymaster" and "gatekeeper" also refer to the same film plot-line. 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:(5)
