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    1: <html>
    2: <head>
    3: <title>XPFE vs. DHTML</title>
    4: </head>
    5: <body bgcolor="#FFFFFF">
    6: 
    7: <font size="+1"><b>XPFE vs. DHTML</b></font>
    8: 
    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.' 
   11: 
   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.
   15: 
   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.  
   19: 
   20: <p>In this context a front end is more than the look and feel of the application, but can also include the functionality and structure of that
   21: application.  For example, Netscape 6 does use XPFE to allow for the creation of <a href="http://www.netscape.com/themes/">different themes</a> for
   22: their browser suite, but the browser suite itself is created using XPFE as well.
   23: 
   24: <p>This new technology started out as a time-saving technique and turned into one of Mozilla's most powerful innovations.  Mike Cornall, in <a
   25: href="http://linuxtoday.com/news_story.php3?ltsn=2000-07-25-001-07-OP-SM-0036">an article</a> published on LinuxToday, summarizes the history of XPFE
   26: well when he says: 'The application platform capabilities of Mozilla came about through a happy coincidence of Open Source development, good design, and
   27: far-sighted developers who were paying attention.'
   28: 
   29: <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
   30: unintended consequence of lowering the barriers to entry to application development.  To better understand this happy coincidence and why it can be so
   31: useful for developers it is necessary to take a closer look at what XPFE is made of.
   32: 
   33: <p><b>Understanding XPFE</b>
   34: 
   35: <p>XPFE uses a number of existing Web standards, such as <a href="http://www.w3.org/Style/CSS/">Cascading Style Sheets</a>, <a
   36: href="http://www.ecma.ch/ecma1/STAND/ECMA-262.HTM">JavaScript</a> and <a href="http://www.w3.org/XML/">XML</a> (the XML component is a new language
   37: called XUL, the XML-based User Interface Language).  In it's most simple form, XPFE can be thought of as simply the union of each of these standards.
   38: Viewed together these can be seen forming XPFE in Figure 1 below.
   39: 
   40: <center>
   41: <p><img src="http://books.mozdev.org/screenshots/moz_0101.gif"><br>
   42: <font size="-1"><i>Figure 1: XPFE Framework</i></font>
   43: </center>
   44: 
   45: <p>To understand how XPFE works, we can look at how these different components fit together.  JavaScript is used to create the functionality for a
   46: Mozilla-based application, Cascading Style Sheets are used for formatting the look and feel, and XUL is used for creating the application's
   47: structure.
   48: 
   49: <p>Instead of using platform-specific C code to create an application, XPFE uses these well understood Web standards that are by design inherently
   50: platform independent.  Since the framework of XPFE is inherently platform independent, so are the applications that are created with it.  Since the
   51: framework is also made up of tools that are used to create Web pages, people familiar with creating a Web page can quickly learn how to use XPFE to
   52: create a cross-platform application.
   53: 
   54: <p>Although the actual creation of Mozilla-based applications can be much more complicated than building a Web page, XPFE allows developers to create
   55: applications in the same way they would create a Web page.  Or to put it another way, the application is now a Web page.  
   56: 
   57: <p><b>Comparing XPFE and DHTML</b>
   58: 
   59: <p>In many ways XPFE is very similar to <a href="http://www.webreference.com/dhtml/">DHTML</a>.  Dynamic HTML is a combination of HTML with
   60: JavaScript and CSS that allows a developer to create an application that is contained within the content area of a browser.  XPFE provides a logical
   61: evolution to this idea by allowing the creation of applications that are more powerful, more flexible and that can live outside of the browser window
   62: as stand-alone programs.
   63: 
   64: <p>Figure 2 below illustrates the similarities between XPFE and DHTML.  Both use JavaScript to create functionality, both use CSS to format design and
   65: 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 HTML
   66: and the other is XUL.
   67: 
   68: <center>
   69: <p><img src="http://books.mozdev.org/screenshots/moz_0102.gif"><br>
   70: <font size="-1"><i>Figure 2: Comparison of DHTML and XPFE</i></font>
   71: </center>
   72: 
   73: <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
   74: link together separate text documents on the Internet.  Later additions to the HTML standard have extended its functionality, but even these enhancements
   75: can't make it an appropriate language to use for developing applications.  XUL is a language specifically designed for creating user interfaces, so it
   76: makes sense that XPFE is more suited for application development than DHTML.
   77: 
   78: <p>Since XUL is structurally similar to HTML, knowledge of building Web pages will give you a boost in learning how to create cross-platform
   79: Mozilla-based applications.  Even if you have never used HTML before, XUL uses a straight-forward <a
   80: href="http://www.mozilla.org/xpfe/xulref/">collection of tags</a> that makes it easy to get comfortable with it in a short time.  Once you become
   81: accustomed to using XUL you will be ready to start using XPFE to create your own applications.
   82: 
   83: <p><b>Oversimplifying Things</b>
   84: 
   85: <p>Describing XPFE as a more sophisticated version of DHTML is an oversimplification and deliberately leaves out much important information.  These
   86: details were ignored in the comparison to give a better understanding of the basic framework of XPFE.  Now that we've gotten past the basics, we can go
   87: back and talk about the rest of the functionality available with Mozilla that makes it such a powerful framework for creating applications.
   88: 
   89: <p>At the <a href="http://meetzilla.mozdev.org/second_meeting.html">Second Mozilla Developer Meeting</a>, Rob Ginda, the creator of ChatZilla, led a
   90: discussion group about Mozilla as Platform.  In this session he listed all of the following as components of a Mozilla-based application:
   91: 
   92: <UL>
   93: <LI>XUL (XML-based User Interface Language) - Used to create the structure and content of an application.<br><br>
   94: <LI>CSS (Cascading Style Sheets) - Used to create the look and feel of an application.<br><br>
   95: <LI>JavaScript - Used to create the functionality of an application, although other scripting languages, such as Python, Perl or Ruby, can be used in 
   96: place of Javascript.<br><br>
   97: <LI>XPInstall (Cross-Platform Install) - Used to package applications so that they can be installed on any platform.<br><br>
   98: <LI>RDF (Resource Description Framework) - Used to store data and transmit information.  Generally regarded to be one of the most complicated 
   99: aspects of XPFE.<br><br>
  100: <LI>DTD (Document Type Definition) - Used for localization and internationalization, more commonly referred to in short-hand as L12N and I18N 
  101: respectively.<br><br>
  102: <LI>XBL (eXtensible Binding Language) - Used to create reusable widgets with a combination of XUL and JavaScript.<br><br>
  103: <LI>XUL templates - Used to create a framework for importing data into an application with a combination of RDF and XUL.<br><br>
  104: <LI>XPCOM/XPConnect - Used to allow JavaScript, or potentially any other scripting language, to access and utilize C and C++ libraries.
  105: </UL>
  106: 
  107: <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
  108: important there is a distinction to be made among them.  Some of these are essential to the creation of a Mozilla application and some provide
  109: powerful extra features.
  110: 
  111: <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
  112: create an application without it.  <a href="http://www.mozilla.org/projects/l10n/mlp.html">Localization</a> also provides Mozilla with a great amount of
  113: flexibility but there are many existing applications that don't take advantage of this feature.  It wouldn't be possible to create an application without
  114: XUL though.
  115: 
  116: <p><b>Judge For Yourself</b>
  117: 
  118: <p>XPFE is a new technology and has yet to prove itself to the Web community.  Many people are also skeptical about the need for an application
  119: development framework such as this.  Before you make up your mind about XPFE though, you should take a look at the many different applications that
  120: have already been created using Mozilla so you can judge for yourself.
  121: 
  122: <p>The first place to start would be to try using the latest version of <a href="http://mozilla.org/releases/">Mozilla</a> or <a
  123: href="http://www.netscape.com/browsers/6/index.html">Netscape 6.1</a> if you haven't already, since these are the most well known applications that
  124: use XPFE.  If you want to try out other projects using XPFE, there are currently over 40 different Mozilla-based applications being hosted on <a
  125: href="http://www.mozdev.org/projects.html">mozdev.org</a>.  Other applications using the same technology include ActiveState's <a
  126: href="http://www.activestate.com/ASPN/Downloads/Komodo/More">Komodo</a> IDE, Rob Ginda's <a
  127: href="http://www.hacksrus.com/~ginda/chatzilla/">ChatZilla</a> IRC client, and Zope's <a href="http://www.zope.org/Resources/Mozilla/">Mozilla
  128: Initiative</a>.
  129: 
  130: <br><br>
  131: 
  132: <p><i>Thanks to Julia Kleyman for creating the illustrations used in this article.</i>
  133: 
  134: </body>
  135: </html>
  136: 
  137: 

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