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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.  (In this context a front end is more than the look and feel of a Mozilla-based application, but can also include the functionality
   19: and structure of that application.  For example, Netscape 6 does use XPFE to allow for the creation of <a
   20: href="http://www.netscape.com/themes/">different themes</a> for their browser suite, but the browser suite itself is also created using XPFE.)
   21: 
   22: <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
   23: 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
   24: well when he says: 'The application platform capabilities of Mozilla came about through a happy coincidence of Open Source development, good design, and
   25: far-sighted developers who were paying attention.'
   26: 
   27: <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
   28: unintended consequence of lowering the barriers to entry to application development.  To better understand this happy coincidence and why it can be so
   29: useful for developers it is necessary to take a closer look at what XPFE is made of.
   30: 
   31: <p><b>Understanding XPFE</b>
   32: 
   33: <p>The technologies that XPFE uses are all existing Web standards, such as <a href="http://www.w3.org/Style/CSS/">Cascading Style Sheets</a>, 
   34: <a 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
   35: language 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 
   36: standards.
   37: 
   38: <center>
   39: <p><img src="http://books.mozdev.org/screenshots/moz_0101.gif"><br>
   40: <font size="-1"><i>Figure 1: XPFE Framework</i></font>
   41: </center>
   42: 
   43: <p>To understand how XPFE works, we can look at how the different components of it fit together.  JavaScript is used to create the functionality for a
   44: Mozilla-based application, Cascading Style Sheets are used for formatting the look and feel, and XUL is used for creating the application's structure.  
   45: Viewed together these three standards can be seen forming XPFE in Figure 1 above.
   46: 
   47: <p>Instead of using platform-specific C code to create an application, XPFE uses these well understood Web standards that are by design inherently
   48: platform independent.  Since the framework of XPFE is inherently platform independent, so are the applications that are created with it.  Since the
   49: framework is also made up of tools that are used to create Web pages, anyone familiar with creating a Web page can use XPFE to create a cross-platform
   50: application.
   51: 
   52: <p>Although the actual creation of Mozilla-based applications can be much more complicated than building a Web page, XPFE allows developers to create
   53: applications in the same way they would create a Web page.  Or to put it another way, the application is now a Web page.  Gecko, the rendering engine
   54: that Mozilla uses to draw a Web page in the browser, even draws the Mozilla application on the desktop.
   55: 
   56: <p><b>Comparing XPFE and DHTML</b>
   57: 
   58: <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 JavaScript
   59: 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
   60: 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
   61: programs.
   62: 
   63: <p>Figure 2 below illustrates the similarities between XPFE and DHTML.  Both use JavaScript to create functionality, both use CSS to format design and
   64: 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
   65: and the other is XUL.
   66: 
   67: <center>
   68: <p><img src="http://books.mozdev.org/screenshots/moz_0102.gif"><br>
   69: <font size="-1"><i>Figure 2: Comparison of DHTML and XPFE</i></font>
   70: </center>
   71: 
   72: <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
   73: link together separate text documents on the Internet.  Later additions to the HTML standard have extended its functionality, but even these enhancements
   74: can't make it an appropriate language to use for developing applications.  XUL is a language specifically designed for creating user interfaces, so it
   75: makes sense that XPFE is more suited for application development than DHTML.
   76: 
   77: <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
   78: Mozilla-based applications.  Even if you have never used HTML before, XUL uses a straight-forward <a
   79: 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
   80: accustomed to using XUL you will be ready to start using XPFE to create your own applications.
   81: 
   82: <p><b>Oversimplifying Things</b>
   83: 
   84: <p>Describing XPFE as a more sophisticated version of DHTML is an oversimplification and deliberately leaves out much important information.  These
   85: 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
   86: back and talk about the rest of the functionality available with Mozilla that makes it such a powerful framework for creating applications.
   87: 
   88: <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
   89: discussion group about Mozilla as Platform.  In this session he listed all of the following as components of a Mozilla-based application:
   90: 
   91: <UL>
   92: <LI>XUL (XML-based User Interface Language) - Used to create the structure and content of an application.<br><br>
   93: <LI>CSS (Cascading Style Sheets) - Used to create the look and feel of an application.<br><br>
   94: <LI>JavaScript - Used to create the functionality of an application. (Other scripting languages, such as Python, Perl or Ruby, can be used in 
   95: place of Javascript to create functionality for an application.)<br><br>
   96: <LI>XPInstall (Cross-Platform Install) - Used to package applications so that they can be installed on any platform.<br><br>
   97: <LI>RDF (Resource Description Framework) - Used to store data and transmit information.  Generally regarded to be one of the most complicated 
   98: aspects of XPFE.<br><br>
   99: <LI>DTD (Document Type Definition) - Used for localization and internationalization, more commonly referred to in short-hand as L12N and I18N 
  100: respectively.<br><br>
  101: <LI>XBL (eXtensible Binding Language) - Used to create reusable widgets using a combination of XUL and JavaScript.<br><br>
  102: <LI>XUL templates - Used to create a framework for importing data into an application with a combination of RDF and XUL.<br><br>
  103: <LI>XPCOM/XPConnect - Used to allow JavaScript, or potentially any other scripting language, to access and utilize C and C++ libraries.
  104: </UL>
  105: 
  106: <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
  107: 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
  108: powerful extra features that can be used in addition to the basic functionality.
  109: 
  110: <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
  111: 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
  112: 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
  113: XUL though.
  114: 
  115: <hr>
  116: 
  117: <p><b>Judge For Yourself</b>
  118: 
  119: <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
  120: 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
  121: Mozilla so you can judge for yourself.
  122: 
  123: <p>If you are interested in trying out some of these, there are currently over 40 different Mozilla-based applications being hosted on <a
  124: href="http://www.mozdev.org/projects.html">mozdev.org</a> that have been created using XPFE.  Other applications using the same technology include
  125: ActiveState's <a href="http://www.activestate.com/ASPN/Downloads/Komodo/More">Komodo</a> IDE, Rob Ginda's <a
  126: href="http://www.hacksrus.com/~ginda/chatzilla/">ChatZilla</a> IRC client, and Zope's <a href="http://www.zope.org/Resources/Mozilla/">Mozilla
  127: Initiative</a>.
  128: 
  129: <br><br>
  130: 
  131: <p><i>Thanks to Julia Kleyman for creating the illustrations used in this article.</i>
  132: 
  133: </body>
  134: </html>
  135: 
  136: 

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