View source for XUL

{{short description|User interface markup language}}
{{infobox programming language
| logo = 
| logo caption = 
| screenshot = 
| screenshot caption = 
| file ext = <code>.xul</code><br />[[MIME type]]: <code>application/vnd.mozilla.xul+xml</code>
| paradigm = Declarative ([[markup language]])
| scope = 
| released = 
| designer = 
| developer = [[Mozilla Foundation]]
| latest release version = 
| latest release date = 
| latest preview version = 
| latest preview date = 
| typing = 
| implementations = [[Mozilla]]
| dialects = 
| influenced by = [[HTML]], [[XML]]
| influenced = 
| programming language = [[C++]]
| platform = [[Gecko (software)|Gecko]]/[[Goanna_(software)|Goanna]]
| operating system = [[Cross-platform]]
| license = [[Mozilla Public License|MPL]]
| website = https://wiki.mozilla.org/XUL:Home_Page
| wikibooks = 
}}
{{Use mdy dates|date=September 2022}}

'''XUL''' ({{IPAc-en|ˈ|z|uː|l}} {{respell|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 interface]]s, enabling developers to write user interface elements in a manner similar to [[web page]]s.

XUL [[application software|applications]] rely on the Mozilla [[codebase]] or a [[Fork (software development)|fork]] of it. The most prominent example is the [[Firefox]] web browser. However, Mozilla has significantly reduced the usage of XUL in Firefox after their rewrite of the browser engine in version 57.<ref name="life after xul"/><ref name = "problems with xul">{{Cite web|url=https://mozilla.github.io/firefox-browser-architecture/text/0003-problems-with-xul.html|title=Problems with XUL|website=mozilla.github.io|access-date=2019-06-07}}</ref> This resulted in the removal of the legacy [[Add-on (Mozilla)|add-on]] system, which relied on XUL and [[XPCOM]], another cross-platform technology Mozilla decided to deprecate shortly before the release of Firefox 57. Firefox originally permitted add-ons to extensively alter its user interface, but this capability was removed in 2017 and replaced with the less-permissive [[Browser extension#API conformity|WebExtensions API]].<ref name="57rel">{{Cite web|url=https://www.mozilla.org/en-US/firefox/57.0/releasenotes/|title=Firefox 57.0, See All New Features, Updates and Fixes|website=Mozilla |date=14 November 2017 |access-date=27 September 2022}}</ref><ref name="2015blog">{{cite web|title=The Future of Developing Firefox Add-ons|url=https://blog.mozilla.org/addons/2015/08/21/the-future-of-developing-firefox-add-ons/|date=2015-08-21|author=Kev Needham|access-date=2018-04-02|website=blog.mozilla.org}}</ref> Several [[Fork (software development)|fork]]s of Firefox retain support for XUL and XPCOM-based add-ons. [[Waterfox]] maintains a fork of the legacy Mozilla codebase for the ''Waterfox Classic'' browser.<ref>{{cite web|title=Waterfox, Its Legacy and Looking to the Future|url=https://blog.waterfoxproject.org/waterfox-its-legacy-and-looking-to-the-future|website=Waterfox blog|date=2018-04-28|access-date=2018-06-20|last=Kontos|first=Alex|archive-date=April 12, 2019|archive-url=https://web.archive.org/web/20190412134139/https://blog.waterfoxproject.org/waterfox-its-legacy-and-looking-to-the-future|url-status=dead}}</ref> The [[Pale Moon]] browser,<ref>{{cite web|url=https://www.palemoon.org/roadmap.shtml |title=Pale Moon future roadmap | access-date=2018-04-02 |publisher=Moonchild Productions |date=23 March 2022 |website=palemoon.org}}</ref> [[Basilisk (web browser)|Basilisk]] browser,<ref>{{cite news|title=Pale Moon team releases first version of Basilisk browser|url=https://www.ghacks.net/2017/11/17/pale-moon-team-releases-first-version-of-basilisk-browser/|newspaper=Ghacks Technology News|date=2017-11-17 |last=Brinkmann |first=Martin |access-date=2018-04-02}}</ref> and numerous smaller projects collectively use and maintain the Unified XUL Platform (UXP) implementation. 

== History ==

[[File:PeriodicTableofXUL Windows10 PaleMoon31.png|thumb|The Periodic Table of XUL Elements is a reference extension designed to demonstrate XUL's capabilities.]]

XUL was devised at [[Netscape]] in 1997 as part of the development effort that eventually became the [[Mozilla]] [[codebase]].<ref>
{{cite web |url = https://arstechnica.com/information-technology/2004/06/collins-interview/2/ |title = Ars Technica sits down with Scott Collins from Mozilla.org |author = Jorge O. Castro |publisher = Ars Technica |date = 2004-06-15 |access-date=2018-11-28}}
</ref> XUL is similar to web technologies implemented by the [[Gecko (software)|Gecko]] rendering engine. When XUL was introduced, it added features beyond the then standard [[HTML|HTML 4]]. It was designed as an extension of HTML to be used with applications rather than documents.<ref name=essay>{{cite web | url=https://yoric.github.io/post/why-did-mozilla-remove-xul-addons/ | title=Why Did Mozilla Remove XUL Add-ons? |last=Teller |first=David |date=Aug 20, 2020}}</ref>

XUL was used by various Mozilla projects, [[Fork (software development)|forks]] of Mozilla projects, and projects closely related to Mozilla. Some software projects such as [[Songbird (software)|Songbird]], [[Komodo IDE]], and [[Zotero]] started as desktop applications that utilized [[XULRunner]] to render and compose their UI. In the early 2000s, there was some interest in using XUL by other parties, including [[Amazon (company)|Amazon]],<ref>{{cite web|url=http://www.oreillynet.com/pub/a/mozilla/2003/05/02/casestudy2.html |archive-url=https://web.archive.org/web/20120218221243/http://www.oreillynet.com/pub/a/mozilla/2003/05/02/casestudy2.html |archive-date=18 February 2012 |last=King |first=Brian |title=Remote Application Development with Mozilla, Part 2: A Case Study of the Mozilla Amazon Browser (MAB)|work=[[Oreillynet]] |date=2003-02-05 |quote=The Mozilla Amazon Browser is an interesting case study for a number of reasons. First, it's a working prototype application, not an example constructed solely for the purpose of demonstrating the technology. Second, it exercises many areas of Mozilla's support for remote development and thus shows off the capabilities, potential, and limitations of remote XUL applications. Finally, it demonstrates Mozilla's support for web services, an important emerging technology for web application development. }}
</ref> but those have switched over to [[HTML5]].<ref name="life after xul" /><ref name="problems with xul" /> Many of the features of modern web technologies such as HTML5, CSS3, ECMAScript 5+, WebGL, and WebAssembly that made rich and modern web applications possible, without the need of utilizing legacy [[NPAPI]]/[[PPAPI]]/[[ActiveX]] browser plug-in technologies like [[Adobe Flash Player|Adobe Flash]], [[Microsoft Silverlight|Silverlight]], [[Java Web Start|Java]], or [[Adobe Shockwave Player|Shockwave]], also made much of XUL redundant.<ref name=essay/>

Mozilla released Firefox 57, also known as Firefox Quantum, in November 2017. As part of this, Firefox finished transitioning to a multiprocess architecture. Although XUL add-ons could work in this architecture, they often created significant performance issues. Because of this, Mozilla removed support for legacy [[Add-on (Mozilla)|add-ons]], including the use of custom XUL code.<ref name="57rel"/><ref name="2015blog"/> Mozilla switched their browser extensions system to the cross-browser WebExtensions API, which is largely based on the Chrome extension API. In late 2017, Firefox still included 289 XUL bindings for native browser components. By the end of 2019, Mozilla had removed all XUL files from their <code>mozilla-central</code> repository.<ref>{{cite web | url=https://wiki.mozilla.org/Firefox/XUL_and_XBL_Replacement | title=Firefox/XUL and XBL Replacement |access-date=27 September 2022 |last=Grinstead |first=Brian |date=16 December 2019}}</ref><ref name="life after xul">{{cite web|last=Hirsch|first=Jared|date=2 September 2018|title=Life After XUL|url=https://developer.mozilla.org/en-US/docs/Mozilla/Tech/Life_After_XUL:_Building_Firefox_Interfaces_with_HTML|url-status=dead|archive-url=https://web.archive.org/web/20181129013045/https://developer.mozilla.org/en-US/docs/Mozilla/Tech/Life_After_XUL%3A_Building_Firefox_Interfaces_with_HTML|archive-date=29 November 2018|access-date=28 November 2018|website=Mozilla Developer Network (MDN) |publisher=Mozilla}}</ref><ref name = "problems with xul"/>

When Mozilla removed support from Firefox, there were still several applications using XUL. Mozilla began gradually removing XUL support from their email client, [[Mozilla Thunderbird|Thunderbird]]. They released Thunderbird 78 without support for XUL-based extensions in 2020.<ref>{{cite web|url=https://developer.thunderbird.net/add-ons/updating/tb78/changes |title=Adapt to Changes in Thunderbird 69-78 |website=developer.thunderbird.net |access-date=27 September 2022 |date=2022}}</ref>
The [[SeaMonkey]] internet suite chose to re-implement XUL for the Firefox Quantum codebase, and the suite's development has subsequently slowed. The [[Pale Moon]] developers forked Mozilla's entire Firefox 52 ESR codebase to create the Unified XUL Platform [[Pale Moon#Unified XUL Platform (UXP)|(UXP)]] for their [[Goanna (software)|Goanna]] rendering engine. UXP maintains the traditional XUL capabilities.<ref>{{cite web |url=https://forum.palemoon.org/viewtopic.php?f=62&t=17213 |title=UXP vs goanna |last=Straver |first=Markus |access-date=27 September 2022 |date=2 November 2017 |quote=The plan is to switch Pale Moon over from our current platform to UXP (long-term plans) because a developed and maintained XUL-based platform is the only way a XUL application (like Pale Moon) has any chance of surviving without falling into obsolescence, with Mozilla abandoning this technology. That has been the main reason why I decided to start on this platform to begin with! Regardless, the platform will not be solely developed for Pale Moon's potential future use, it is developed for any future XUL application that will otherwise be dead in the water. Basically we're taking over the torch from Mozilla in developing and maintaining a platform for XUL applications of any kind; Mozilla should not be seen as "upstream" because it isn't. |archive-date=November 11, 2020 |archive-url=https://web.archive.org/web/20201111192226/https://forum.palemoon.org/viewtopic.php?f=62&t=17213 |url-status=dead }}</ref> As a result, Pale Moon and other UXP applications are more extensible compared to modern versions of Firefox and Thunderbird.<ref>{{cite web |title=There is only XUL |url=http://thereisonlyxul.org/ |archive-url=https://web.archive.org/web/20180917071644/http://thereisonlyxul.org/ |archive-date=17 September 2018 |last=Tobin |first=Matt |access-date=18 September 2018}}</ref>

== Usage ==
XUL can only be used with the [[Mozilla]] [[codebase]] (or a [[Fork (software development)|fork]] of it) because the [[Gecko (software)|Gecko]] (or [[Goanna (software)|Goanna]]) [[browser engine|engine]] does the XUL rendering.<ref>{{Cite web|title=Gecko FAQ {{!}} MDN|url=https://developer.mozilla.org/en-US/docs/Gecko/FAQ|access-date=2021-01-05|website=developer.mozilla.org|archive-date=2019-10-08|archive-url=https://web.archive.org/web/20191008071852/https://developer.mozilla.org/en-US/docs/Gecko/FAQ|url-status=dead}}</ref>

Application programmers need to 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
# 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:
* Top-level elements: [[window (computing)|window]], page, [[dialog box|dialog]], [[wizard (software)|wizard]], etc.
* [[widget (computing)|Widgets]]: label, [[button (computing)|button]], [[text box]], list box, [[combo box]], [[radio button]], [[check box]], [[Tree view|tree]], [[menu (computing)|menu]], [[toolbar]], group box, [[tab (GUI)|tab box]], color-picker, spacer, splitter, etc.
* Box model: box, grid, stack, deck, etc.
* Events and scripts: script, command, key, broadcaster, observer, etc.
* Data source: template, rule, etc.
* Others: overlay, iframe, browser, editor, etc.

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

===XBL===
{{main|XBL}}
XML Binding Language ([[XBL]]) is an [[XML]]-based [[markup language]] for altering the behavior of a XUL [[Graphical widget|widget]], such as a [[scroll bar]].<ref>{{cite web|author=Castro|first=Jorge O.|date=2004-06-15|title=Ars Technica sits down with Scott Collins from Mozilla.org|url=https://arstechnica.com/information-technology/2004/06/collins-interview/2/|url-status=live|archive-url=https://web.archive.org/web/20180222104837/https://arstechnica.com/information-technology/2004/06/collins-interview/2/|archive-date=2018-02-22|access-date=2018-11-28|website=[[Ars Technica]]|page=2}}</ref><ref>{{cite web|title=XBL 2.0 Acknowledgments|url=http://www.w3.org/TR/xbl/#acknowledgments|website=www.w3.org}}</ref> An XBL file contains [[data binding|bindings]], each of which describes the behavior of a XUL widget. The root element of an XBL file is the <code><nowiki><bindings></nowiki></code> element, which contains one or more <code><nowiki><binding></nowiki></code> elements. Each <code><nowiki><binding></nowiki></code> element declares one binding, which can be attached to any XUL element. It may also possess an <code><nowiki>id</nowiki></code> attribute. A binding is assigned to an element by setting the CSS property <code><nowiki>-moz-binding</nowiki></code> to the [[URL]] of the binding's file.<ref>{{cite web|title=XBL 2.0|url=http://www.w3.org/TR/xbl/|website=www.w3.org}}</ref> For example:
<syntaxhighlight lang="css">
scrollbar {
  -moz-binding: url('somefile.xml#binding1');
}
</syntaxhighlight>

=== Example ===

This example shows three buttons stacked on top of each other in a vertical box container:<ref>{{Cite web|date=2017-12-09|title=The Box Model - Mozilla {{!}} MDN|url=https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XUL/Tutorial/The_Box_Model|access-date=2021-01-05|archive-url=https://web.archive.org/web/20171209141220/https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XUL/Tutorial/The_Box_Model|archive-date=2017-12-09}}</ref>
[[File:Boxes-ex1.png|right]]

<syntaxhighlight lang="xul+mozpreproc">
<?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>
</syntaxhighlight>

==''Ghostbusters'' reference ==
XUL contains a reference to the 1984 film ''[[Ghostbusters]]''.<ref name="Frederick-2010"/> The film's antagonist is the fictional deity Zuul, the Gatekeeper.<ref name="Garcia-Ventura-2020">{{cite book |last1=Garcia-Ventura |first1=Agnes |last2=Verderame |first2=Lorenzo |title=Receptions of the Ancient Near East in Popular Culture and Beyond |date=1 March 2020 |publisher=Lockwood Press |isbn=978-1-948488-25-9 |pages=109 |url=https://books.google.com/books?id=-E7VDwAAQBAJ&dq=%22XUL%22+%22zuul%22&pg=PA109 |language=en}}</ref> Zuul [[Spirit possession|possesses]] the character Dana Barrett and declares: "There is no Dana. There is only Zuul."<ref name="Quinlan-2014">{{cite web |last1=Quinlan |first1=Erin |title='Ghostbusters' turns 30: Revisit film's supernatural wisdom |url=https://www.today.com/popculture/ghostbusters-turns-30-revisit-films-supernatural-wisdom-2d79768136 |website=Today |access-date=25 May 2024 |language=en |date=6 June 2014}}</ref><ref name="Frederick-2010"/> The creators of XUL, pronounced the same as Zuul,<ref name="Holzner-2001">{{cite book |last1=Holzner |first1=Steven |title=Inside XML |date=2001 |publisher=New Riders |isbn=978-0-7357-1020-7 |page=47 |url=https://books.google.com/books?id=olpC8FqAQpEC&dq=%22XUL%22+%22zuul%22&pg=PA47 |language=en}}</ref> made the slogan, "There is no data. There is only XUL!".<ref>{{Cite web|url=https://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul|title=Mozilla XML Namespace|website=www.mozilla.org}}</ref> The slogan and Zuul as a gatekeeper are referenced in XUL's [[XML namespace]] [[Uniform Resource Identifier|URI]] and were previously present in all Firefox add-ons:<ref name="Frederick-2010">{{cite book |last1=Frederick |first1=Gail |last2=Lal |first2=Rajesh |title=Beginning Smartphone Web Development: Building JavaScript, CSS, HTML and Ajax-based Applications for iPhone, Android, Palm Pre, BlackBerry, Windows Mobile and Nokia S60 |date=15 April 2010 |publisher=Apress |isbn=978-1-4302-2621-5 |page=23 |url=https://books.google.com/books?id=rLqJNQ3_hpsC&dq=%22There+is+no+data.+There+is+only+XUL!%22&pg=PA23 |language=en}}</ref>

{{blockquote
|text=<code><nowiki>http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul</nowiki></code>
|author=Mozilla<ref name="Frederick-2010"/>
}}

== See also ==
* [[Glade Interface Designer]] 

== References ==
{{Reflist|2}}

{{Mozilla projects}}
{{Widget toolkits}}

{{Authority control}}

{{DEFAULTSORT:Xul}}
[[Category:Mozilla]]
[[Category:RDF data access]]
[[Category:User interface markup languages]]
[[Category:XML-based standards]]
[[Category:1997 introductions]]
[[Category:Software that uses XUL]]