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Posted by Jonathan Allen on Apr 09, 2007
Last week we ran a short piece on the future of rich client frameworks. At the time we over-looked XUL as a proprietary language for Mozilla add-ons. It seems that was a mistake.
XML User Interface Language (XUL) shares many of the features with the other frameworks we covered last week, including an XML based presentation and JavaScript on the back end. But it also has a few key features that set it apart.
The first thing most Windows developers ask about a new framework is, “Will it look right?” Or in other words, does the framework make a reasonable attempt to look and feel like a native application. The answer for XUL appears to be yes. A test drive of the Mozilla Amazon Browser (MAP) shows that it can be used to make good looking rich clients . Though running in Firefox, it feels just like a native application. A demo site called the XUL Periodic Timetable (Mozilla Only) shows that it already supports most of the controls that rich client developers expect.
Ten years ago it was considered acceptable to have isolated platforms and programming languages that didn’t interact with other platforms. These days, that is no longer tolerated. To wit, XPCOM and XPConnect are offered.
XPCOM, which stands for Cross Platform Component Object Model, is a framework for writing cross-platform, modular software. XPCOM components can be written in C, C++, and JavaScript, and they can be used from C, C++, JavaScript, Python, Java, and Perl.
XPConnect is a technology which enables simple interoperation between XPCOM and JavaScript. XPConnect allows JavaScript objects to transparently access and manipulate XPCOM objects. It also enables JavaScript objects to present XPCOM compliant interfaces to be called by XPCOM objects.
Another hot topic these days is internationalization. XUL has that covered by storing human readable text in DTD and property files. In what will surely make translation easier, these files can be created and packaged separately from the XUL code itself.
Unlike many open source projects, tool support is not lacking either. The XUL homepage lists several tools and utilities including XUL Explorer, a lightweight IDE.
At first glance it may appear that XUL has the same problems as Gran Paradiso, namely being tied to the Mozilla family of browsers. However, that isn’t the case. Another Mozilla product, XULRunner, allows XUL based applications to run outside the browser. Currently it is available for Windows, OS X, and Linux.
XUL isn’t a hundred percent solution though. While testing in Firefox 2.0.0.3, it had a problem with accelerator keys such as
With a bit of publicity and polish, XUL could very well give WPF/E and Adobe Flex a run for their money.
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My guess would be that XUL remains a niche experimental technology that only hard core open source folks try to stick with. The feverish pace of advancement in the Flex & Apollo arena provide a very compelling platform. WPF/E has earned the reputation of vaporware, I'm sure once they kick it into high gear all .NET folks will become loyal to it.
XUL doesn't provide any kind of distinct advantage. So, the biggest challenge I see with XUL is who exactly is going to be loyal to it.
On a side note the ongoing plan for XUL is, I believe, to move towards a common runtime engine in the form of XulRunner a standalone XUL engine. This allows you to run XUL apps without having to sit on top of Firefox.
I agree with that: XUL has been created long before Adobe Flex and WPF/E, but has never really emerged. It has absolutely no competing advantage and not enough money to produce any good marketing buzz or tool integration (Microsoft can tie WPF/E to .Net and Adobe can tie Flex with its Flash design platform).
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