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 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  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
 any operating system.    any operating system.  
   
<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<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
published on LinuxToday, summarizes the history of XPFE well when he says: 'The application platform capabilities of Mozilla came about through a happyapplication.  For example, Netscape 6 does use XPFE to allow for the creation of <a href="http://www.netscape.com/themes/">different themes</a> for
coincidence of Open Source development, good design, and far-sighted developers who were paying attention.'their browser suite, but the browser suite itself is created using XPFE as well.
 
 <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
 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
 well when he says: 'The application platform capabilities of Mozilla came about through a happy coincidence of Open Source development, good design, and
 far-sighted developers who were paying attention.'
   
 <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  <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
unintended consequence of lowering the barriers to entry for application developers.  To better understand this happy coincidence and why it can be sounintended consequence of lowering the barriers to entry to application development.  To better understand this happy coincidence and why it can be so
 useful for developers it is necessary to take a closer look at what XPFE is made of.  useful for developers it is necessary to take a closer look at what XPFE is made of.
   
 <p><b>Understanding XPFE</b>  <p><b>Understanding XPFE</b>
   
<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<p>XPFE uses a number of existing Web standards, such as <a href="http://www.w3.org/Style/CSS/">Cascading Style Sheets</a>, <a
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 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
standards.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.
 Viewed together these three standards can be seen forming XPFE in Figure 1 below.
   
 <center>  <center>
 <p><img src="http://books.mozdev.org/screenshots/moz_0101.gif"><br>  <p><img src="http://books.mozdev.org/screenshots/moz_0101.gif"><br>
 <font size="-1"><i>Figure 1: XPFE Framework</i></font>  <font size="-1"><i>Figure 1: XPFE Framework</i></font>
 </center>  </center>
   
<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<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
Mozilla-based application, Cascading Style Sheets are used for formatting the look and feel, and XUL is used for creating the application's structure.  Mozilla-based application, Cascading Style Sheets are used for formatting the look and feel, and XUL is used for creating the application's
Viewed together these three standards can be seen forming XPFE in Figure 1 above.structure.
<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<p>Instead of using platform-specific C code to create an application, XPFE uses these well understood Web standards that are by design inherently
page, XPFE creates an application in the same way a Web page is created.  Or to put it another way, the application is a Web page.  Mozilla itselfplatform independent.  Since the framework of XPFE is inherently platform independent, so are the applications that are created with it.  Since the
certainly doesn't distinguish between a Web page and an XPFE application.  Gecko, the rendering engine that Mozilla uses to draw a Web page in theframework 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
browser, also draws the Mozilla application on the desktop.create a cross-platform application.
<p>Instead of using platform-specific C code to create an application, XPFE uses well understood Web standards that are by design inherently platform<p>Although the actual creation of Mozilla-based applications can be much more complicated than building a Web page, XPFE allows developers to create
independent.  Since the framework of XPFE is inherently platform independent, so are the applications that are created with it.  Since the framework is applications in the same way they would create a Web page.  Or to put it another way, the application is now a Web page.  
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. 
<p>In some ways Mozilla doesn't even make a distinction between Web pages and XPFE applications.  For instance, Gecko, the layout engine that Mozilla
<hr>uses to render Web pages in the browser, also renders Mozilla-based applications on the desktop.
 
 
<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 
ability to create their own applications using Mozilla. 
 
<p>Web developers and designers who use Mozilla are naturally attracted to the fact that they can create applications using the same skills and 
techniques that they used to create Web pages in the past.  The specifics involved with creating Mozilla applications are different but should seem very 
familiar to anyone who has created for the Web before. 
 
<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 
Mozilla-based application, but includes the functionality and structure of that application as well.  More simply put, XPFE allows users to do more than 
just create a skin for an application.  For example, Netscape 6 does use this functionality to allow for the creation of <a 
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 
technologies. 
   
 <p><b>Comparing XPFE and DHTML</b>  <p><b>Comparing XPFE and DHTML</b>
   
Line 72  and CSS that allows a developer to creat Line 64  and CSS that allows a developer to creat
 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  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
 programs.  programs.
   
<p>Figure 2 below illustrates the similarities between XPFE and DHTML.  Both use JavaScript to create functionality, both use CSS to format the design<p>Figure 2 below illustrates the similarities between XPFE and DHTML.  Both use JavaScript to create functionality, both use CSS to format design and
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 islayout, 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
HTML and the other is XUL.and the other is XUL.
   
 <center>  <center>
 <p><img src="http://books.mozdev.org/screenshots/moz_0102.gif"><br>  <p><img src="http://books.mozdev.org/screenshots/moz_0102.gif"><br>
Line 86  link together separate text documents on Line 78  link together separate text documents on
 can't make it an appropriate language to use for developing applications.  XUL is a language specifically designed for creating user interfaces, so it  can't make it an appropriate language to use for developing applications.  XUL is a language specifically designed for creating user interfaces, so it
 makes sense that XPFE is more suited for application development than DHTML.  makes sense that XPFE is more suited for application development than DHTML.
   
<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<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
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>Mozilla-based applications.  Even if you have never used HTML before, XUL uses a straight-forward <a
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 createhref="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
your own applications.accustomed to using XUL you will be ready to start using XPFE to create your own applications.
<p><b>Oversimplifying in the Metaphor</b><p><b>Oversimplifying Things</b>
<p>This overview of XPFE as a simple evolution of DHTML is an oversimplification of the story and deliberately leaves out much important information.  <p>Describing XPFE as a more sophisticated version of DHTML is an oversimplification and deliberately leaves out much important information.  These
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 thedetails 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
basics, we can go back and talk about the rest of the functionality available with using Mozilla to create applications.back and talk about the rest of the functionality available with Mozilla that makes it such a powerful framework for creating applications.
   
<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<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
 discussion group about Mozilla as Platform.  In this session he listed all of the following as components of a Mozilla-based application:  discussion group about Mozilla as Platform.  In this session he listed all of the following as components of a Mozilla-based application:
   
 <FONT COLOR="#800000">  
 <UL>  <UL>
 <LI>XUL (XML-based User Interface Language) - Used to create the structure and content of an application.<br><br>  <LI>XUL (XML-based User Interface Language) - Used to create the structure and content of an application.<br><br>
 <LI>CSS (Cascading Style Sheets) - Used to create the look and feel of an application.<br><br>  <LI>CSS (Cascading Style Sheets) - Used to create the look and feel of an application.<br><br>
<LI>JavaScript - Used to create the functionality of an application.<br><br><LI>JavaScript - Used to create the functionality of an application, although other scripting languages, such as Python, Perl or Ruby, can be used in 
 place of Javascript.<br><br>
 <LI>XPInstall (Cross-Platform Install) - Used to package applications so that they can be installed on any platform.<br><br>  <LI>XPInstall (Cross-Platform Install) - Used to package applications so that they can be installed on any platform.<br><br>
<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><LI>RDF (Resource Description Framework) - Used to store data and transmit information.  Generally regarded to be one of the most complicated 
<LI>DTD (Document Type Definition) - Used for localization and internationalization, more commonly referred to in short-hand as L12N and I18N respectively.<br><br>aspects of XPFE.<br><br>
<LI>XBL (eXtensible Binding Language) - Used to create reusable widgets using a combination of XUL and JavaScript.<br><br><LI>DTD (Document Type Definition) - Used for localization and internationalization, more commonly referred to in short-hand as L12N and I18N 
 respectively.<br><br>
 <LI>XBL (eXtensible Binding Language) - Used to create reusable widgets with a combination of XUL and JavaScript.<br><br>
 <LI>XUL templates - Used to create a framework for importing data into an application with a combination of RDF and XUL.<br><br>  <LI>XUL templates - Used to create a framework for importing data into an application with a combination of RDF and XUL.<br><br>
 <LI>XPCOM/XPConnect - Used to allow JavaScript, or potentially any other scripting language, to access and utilize C and C++ libraries.  <LI>XPCOM/XPConnect - Used to allow JavaScript, or potentially any other scripting language, to access and utilize C and C++ libraries.
 </UL>  </UL>
 </FONT>  
   
 <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  <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
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 provideimportant there is a distinction to be made among them.  Some of these are essential to the creation of a Mozilla application and some provide
powerful extra features that can be used in addition to the basic functionality.powerful extra features.
   
 <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  <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
create an application without using it.  <a href="http://www.mozilla.org/projects/l10n/mlp.html">Localization</a> also provides Mozilla with a greatcreate an application without it.  <a href="http://www.mozilla.org/projects/l10n/mlp.html">Localization</a> also provides Mozilla with a great amount of
amount of flexibility and usability but there are many existing applications that don't take advantage of this feature.  It wouldn't be possible toflexibility but there are many existing applications that don't take advantage of this feature.  It wouldn't be possible to create an application without
create an application without XUL though.XUL though.
   
 <p><b>Judge For Yourself</b>  <p><b>Judge For Yourself</b>
   
<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<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
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 usingdevelopment framework such as this.  Before you make up your mind about XPFE though, you should take a look at the many different applications that
Mozilla so you can judge for yourself.have already been created using Mozilla so you can judge for yourself.
<p>If you are interested in trying out some of these, there are currently over 40 different Mozilla-based applications being hosted on <a<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
href="http://www.mozdev.org/projects.html">mozdev.org</a> that have been created using XPFE.  Other applications using the same technology includehref="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
ActiveState's <a href="http://www.activestate.com/ASPN/Downloads/Komodo/More">Komodo</a> IDE, Rob Ginda's <ause XPFE.  If you want to try out other projects using XPFE, there are currently over 40 different Mozilla-based applications being hosted on <a
 href="http://www.mozdev.org/projects.html">mozdev.org</a>.  Other applications using the same technology include ActiveState's <a
 href="http://www.activestate.com/ASPN/Downloads/Komodo/More">Komodo</a> IDE, Rob Ginda's <a
 href="http://www.hacksrus.com/~ginda/chatzilla/">ChatZilla</a> IRC client, and Zope's <a href="http://www.zope.org/Resources/Mozilla/">Mozilla  href="http://www.hacksrus.com/~ginda/chatzilla/">ChatZilla</a> IRC client, and Zope's <a href="http://www.zope.org/Resources/Mozilla/">Mozilla
 Initiative</a>.  Initiative</a>.
   

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