XUL

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

XUL
ParadigmDeclarative (markup language)
DeveloperMozilla Foundation
Implementation languageC++
PlatformGecko
OSCross-platform
LicenseMPL
Filename extensions.xul
MIME type: application/vnd.mozilla.xul+xml
WebsiteOfficial documentation
Major implementations
Mozilla
Influenced by
HTML, XML

XUL applications rely on the Mozilla codebase (or a fork of it); the most prominent example is the Firefox web browser. However, in recent years, Mozilla has been reducing the usage of XUL in Firefox.[1][2] The most notable example is the removal of add-on customization. Firefox originally 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.[3][4] Several forks of Firefox, such as Pale Moon,[5] Basilisk,[6] and Waterfox,[7] retain support for XUL add-ons.

History

XUL was devised at Netscape in 1997 as part of the development effort that eventually became the Mozilla codebase.[8] It never gained much traction outside of Mozilla or its forks. In the early 2000s there was some interest in using XUL by other parties, including Amazon,[9] but that dried up with the advent of HTML5.

With the release of Firefox 57 in 2017, Mozilla removed support for legacy add-ons, including the use of custom XUL code.[3][4] This was a key step in the organization's long-term goal of reducing XUL usage in Firefox and replacing it with HTML5 alternatives.[1][2] However, the UXP fork of the codebase maintains the traditional XUL capabilities.[10][11]

Usage

XUL can only be used with the Mozilla codebase (or a fork of it) because the Gecko engine does the XUL rendering.[12]

Application programmers need to 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 defines a wide range of elements, which roughly belong to the following types:

The default behavior of XUL widgets can be altered with XBL bindings.

Example

This example shows three buttons stacked on top of each other in a vertical box container:[13]

<?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">
  <layout>
    <button id="yes1" label="Yes"/>
    <button id="no1" label="No"/>
    <button id="maybe1" label="Maybe"/>
  </layout>
</window>

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]

References
  1. "Life After XUL". Mozilla. Retrieved 28 November 2018.
  2. "Problems with XUL". mozilla.github.io. Retrieved 2019-06-07.
  3. Firefox 57 release notes
  4. Kev Needham (2015-08-21). "The Future of Developing Firefox Add-ons". blog.mozilla.org. Retrieved 2018-04-02.
  5. "Pale Moon future roadmap". Pale Moon. Retrieved 2018-04-02.
  6. "Pale Moon team releases first version of Basilisk browser". ghacks.net. 2017-11-17. Retrieved 2018-04-02.
  7. "Waterfox, Its Legacy and Looking to the Future". Waterfox blog. 2018-04-28. Retrieved 2018-06-20.
  8. Jorge O. Castro (2004-06-15). "Ars Technica sits down with Scott Collins from Mozilla.org". Ars Technica. Retrieved 2018-11-28.
  9. "Remote Application Development with Mozilla, Part 2: A Case Study of the Mozilla Amazon Browser (MAB)". Oreillynet. 2003-02-05.
  10. "UXP vs goanna".
  11. "There is only XUL". Retrieved 18 September 2018.
  12. "Gecko FAQ | MDN". developer.mozilla.org. Retrieved 2021-01-05.
  13. "The Box Model - Mozilla | MDN". web.archive.org. 2017-12-09. Retrieved 2021-01-05.
  14. Ghostbusters clip
  15. Mozilla XML Namespace
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