3: <title>XPFE vs. DHTML</title>
5: <body bgcolor="#FFFFFF">
7: <font size="+1"><b>XPFE vs. DHTML</b></font>
9: <p>'In the beginning, there were 3 front ends: Mac, Windows and Unix. Each took a suite of developers to maintain. Adding a new feature (even just a
10: button) required 3 engineers to waste at least a day (more often a week) slaving away until the feature was complete. This had to change.'
12: <p>This <a href="http://mozilla.org/xpfe/ElevatorTouchyFeely.html">quote</a> is posted on mozilla.org and describes how the Netscape 4.x browsers
13: required a different set of engineers to create and maintain the code for the user interface, even though the browser looked nearly identical on each
14: different supported platform.
16: <p>For a company committed to creating an application that runs on a wide range of different systems, using platform specific code was a big waste of
17: time. XPFE, Mozilla's cross-platform front end, was designed to solve this problem by enabling engineers to create one interface that would then work on
18: any operating system.
20: <p>This new technology started out as a time-saving technique and turned into one of Mozilla's most powerful innovations. Mike Cornall, in an article
21: published on LinuxToday, summarizes the history of XPFE well when he says: 'The application platform capabilities of Mozilla came about through a happy
22: coincidence of Open Source development, good design, and far-sighted developers who were paying attention.'
24: <p>Mozilla engineers were trying to create a more efficient process that would save them time and effort, but this technology ended up having the
25: unintended consequence of lowering the barriers to entry for application developers. To better understand this happy coincidence and why it can be so
26: useful for developers it is necessary to take a closer look at what XPFE is made of.
28: <p><b>Understanding XPFE</b>
31: language called XUL, the XML-based User Interface Language). In it's most simple form, XPFE can be thought of a simply the union of each of these
35: <p><img src="http://books.mozdev.org/screenshots/moz_0101.gif"><br>
36: <font size="-1"><i>Figure 1: XPFE Framework</i></font>
40: Mozilla-based application, Cascading Style Sheets are used for formatting the look and feel, and XUL is used for creating the application's structure.
41: Viewed together these three standards can be seen forming XPFE in Figure 1 above.
43: <p>Instead of using platform-specific C code to create an application, XPFE uses well understood Web standards that are by design inherently platform
44: independent. Since the framework of XPFE is inherently platform independent, so are the applications that are created with it. Since the framework is
45: also made up of tools that are used to create Web page, anyone familiar with creating a Web page can use XPFE to create a cross-platform application.
47: <p>The basic idea behind XPFE is simple, although the actual creation of Mozilla-based applications can be much more complicated than building a Web
50: <p>Mozilla itself doesn't distinguish between a Web page and an XPFE application. Gecko, the rendering engine that Mozilla uses to draw a Web page in
51: the browser, also draws the Mozilla application on the desktop.
56: <p>Since well understood Web standards are being used to create applications instead of platform-specific C code, a whole new group of people now have the
57: ability to create their own applications using Mozilla.
59: <p>Web developers and designers who use Mozilla are naturally attracted to the fact that they can create applications using the same skills and
60: techniques that they used to create Web pages in the past. The specifics involved with creating Mozilla applications are different but should seem very
61: familiar to anyone who has created for the Web before.
63: <p>When talking about front ends it is important to clarify what this means. In this context a front end is more than the look and feel of a
64: Mozilla-based application, but includes the functionality and structure of that application as well. More simply put, XPFE allows users to do more than
65: just create a skin for an application. For example, Netscape 6 does use this functionality to allow for the creation of <a
66: href="http://www.netscape.com/themes/">different themes</a> for their browser suite, but the browser suite itself is also created out of these same
69: <p><b>Comparing XPFE and DHTML</b>
72: and CSS that allows a developer to create a Web application that is contained within the content area of a browser. XPFE provides a logical evolution to
73: this idea by allowing the creation of applications that are more powerful, more flexible and that can live outside of the browser window as stand-alone
77: and layout, and both use a fairly simple mark-up language to describe content. The difference between the two is that one of these mark-up languages is
78: HTML and the other is XUL.
81: <p><img src="http://books.mozdev.org/screenshots/moz_0102.gif"><br>
82: <font size="-1"><i>Figure 2: Comparison of DHTML and XPFE</i></font>
85: <p>Although HTML has been put to many different uses, it was <a href="http://www.w3.org/MarkUp/#historical">originally designed</a> as a simple system to
86: link together separate text documents on the Internet. Later additions to the HTML standard have extended its functionality, but even these enhancements
87: can't make it an appropriate language to use for developing applications. XUL is a language specifically designed for creating user interfaces, so it
88: makes sense that XPFE is more suited for application development than DHTML.
90: <p>Fortunately since XUL as a language is structurally similar to HTML it is simple enough to learn if you are already familiar with the basic language
91: of the Web. Even if you have never used HTML before, XUL uses a straight-forward <a href="http://www.mozilla.org/xpfe/xulref/">collection of tags</a>
92: that makes it easy to get comfortable with it in a short time. Once you become accustomed to using XUL you will be ready to start using XPFE to create
93: your own applications.
95: <p><b>Oversimplifying in the Metaphor</b>
97: <p>This overview of XPFE as a simple evolution of DHTML is an oversimplification of the story and deliberately leaves out much important information.
98: These details were ignored at first to give a conceptual framework for understanding the new ideas that XPFE represent. Now that we've gotten past the
99: basics, we can go back and talk about the rest of the functionality available with using Mozilla to create applications.
101: <p>At the <a href="http://meetzilla.mozdev.org/second_meeting.html">Second Mozilla Developer Meeting</a>, Rob Ginda, the creator of ChatZilla, lead a
102: discussion group about Mozilla as Platform. In this session he listed all of the following as components of a Mozilla-based application:
104: <FONT COLOR="#800000">
106: <LI>XUL (XML-based User Interface Language) - Used to create the structure and content of an application.<br><br>
107: <LI>CSS (Cascading Style Sheets) - Used to create the look and feel of an application.<br><br>
109: <LI>XPInstall (Cross-Platform Install) - Used to package applications so that they can be installed on any platform.<br><br>
110: <LI>RDF (Resource Description Framework) - Used to store data and transmit information. Generally regarded to be one of the most complicated aspects of XPFE.<br><br>
111: <LI>DTD (Document Type Definition) - Used for localization and internationalization, more commonly referred to in short-hand as L12N and I18N respectively.<br><br>
113: <LI>XUL templates - Used to create a framework for importing data into an application with a combination of RDF and XUL.<br><br>
118: <p>Each of these technologies is important and several of these deserve to have whole books devoted to them. Although each of these technologies is
119: important there is a distinction to be made among them. Some of these are essential to the creation of a Mozilla application and some of them provide
120: powerful extra features that can be used in addition to the basic functionality.
122: <p>For example, <a href="http://www.mozilla.org/rdf/doc/">RDF</a> is an extremely powerful technology for using data in Mozilla but it is possible to
123: create an application without using it. <a href="http://www.mozilla.org/projects/l10n/mlp.html">Localization</a> also provides Mozilla with a great
124: amount of flexibility and usability but there are many existing applications that don't take advantage of this feature. It wouldn't be possible to
125: create an application without XUL though.
127: <p><b>Judge For Yourself</b>
129: <p>XPFE is a new technology that has yet to prove itself to the Web community and many people are skeptical about the need for an application framework
130: such as this. Before you make up your mind about XPFE, you should take a look at the many different applications that have already been created using
131: Mozilla so you can judge for yourself.
133: <p>If you are interested in trying out some of these, there are currently over 40 different Mozilla-based applications being hosted on <a
134: href="http://www.mozdev.org/projects.html">mozdev.org</a> that have been created using XPFE. Other applications using the same technology include
135: ActiveState's <a href="http://www.activestate.com/ASPN/Downloads/Komodo/More">Komodo</a> IDE, Rob Ginda's <a
136: href="http://www.hacksrus.com/~ginda/chatzilla/">ChatZilla</a> IRC client, and Zope's <a href="http://www.zope.org/Resources/Mozilla/">Mozilla
141: <p><i>Thanks to Julia Kleyman for creating the illustrations used in this article.</i>