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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>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.'
   23: 
   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.
   27: 
   28: <p><b>Understanding XPFE</b>
   29: 
   30: <p>The technologies that XPFE uses are all existing Web standards, such as Cascading Style Sheets, JavaScript and XML (the XML component is a new
   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 
   32: standards.
   33: 
   34: <center>
   35: <p><img src="http://books.mozdev.org/screenshots/moz_0101.gif"><br>
   36: <font size="-1"><i>Figure 1: XPFE Framework</i></font>
   37: </center>
   38: 
   39: <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
   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.
   42: 
   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. 
   46: 
   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 
   48: page...
   49: 
   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.
   52: 
   53: <hr>
   54: 
   55: 
   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.
   58: 
   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.
   62: 
   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
   67: technologies.
   68: 
   69: <p><b>Comparing XPFE and DHTML</b>
   70: 
   71: <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
   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
   74: programs.
   75: 
   76: <p>Figure 2 below illustrates the similarities between XPFE and DHTML.  Both use JavaScript to create functionality, both use CSS to format the design
   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.
   79: 
   80: <center>
   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>
   83: </center>
   84: 
   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.
   89: 
   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.
   94: 
   95: <p><b>Oversimplifying in the Metaphor</b>
   96: 
   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.
  100: 
  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:
  103: 
  104: <FONT COLOR="#800000">
  105: <UL>
  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>
  108: <LI>JavaScript - Used to create the functionality 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>
  112: <LI>XBL (eXtensible Binding Language) - Used to create reusable widgets using a combination of XUL and JavaScript.<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>
  114: <LI>XPCOM/XPConnect - Used to allow JavaScript, or potentially any other scripting language, to access and utilize C and C++ libraries.
  115: </UL>
  116: </FONT>
  117: 
  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.
  121: 
  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.
  126: 
  127: <p><b>Judge For Yourself</b>
  128: 
  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.
  132: 
  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
  137: Initiative</a>.
  138: 
  139: <br><br>
  140: 
  141: <p><i>Thanks to Julia Kleyman for creating the illustrations used in this article.</i>
  142: 
  143: </body>
  144: </html>
  145: 
  146: 

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