An introduction to Wobzilla

  1. The Mozilla plattform
  2. Views in Wobzilla
  3. Similarities and differences to XForms
  4. Drawbacks of XForms and the model presented so far
  5. The bookmark example (doing it the XSLT way)
  6. Getting into wobzilla

The Mozilla plattform

Wobzilla integrates the user-interface into the browser, as a vast number content maintenance tools do. It currently runs only with Mozilla-browser Version 1.3 or later (and derived browsers like fireball and netscape). Decisive for the Mozilla was the large number of supported plattforms, keeping browser independence wasn't a priority for the first protoype. Apart from that, little Mozilla-specific extensions such as XUL or XPCOM are used. Wobzilla is largely based on the Web-Standards XHTML, DOM, XML, XPath and JavaScript. So maybe there will be an Internet Explorer- Version of Wobzilla someday, with the help of cross-browser toolkits like Dynapi.

Views in Wobzilla

Different from hardware-components, in software-technology the term component refers to the construction plan of which the computer constructs one or more instances of the component at runtime. This distinction is analog to the class/instance concept in OOP. Therefore we distingish in the following between view-components and view-component-instances, latter will be refered as views to simplify matters.

Views in Wobzilla are responsible for a given part of the output tree, thats the DHMTL-Tree rendered in the browser. Views can contain sub-views themselves (as of the compositum-pattern), the resulting data-structure will further be refered as view-tree.

The presentation-model in Wobzilla is a hierarchical configuration of views. The class-Attribut of a view-configuration contains the name of the view-component. The following configuration describes a view-tree that consists of two text-fields (both 20 columns wide). Each text-fields is embedded in a html div element:

<wbz:view class="WbzContainer">
    <wbz:view class="WbzTextField" cols="20"/>
    <wbz:view class="WbzTextField" cols="20"/>

The graphic shows the view-tree that is generated out of the configuration, and the output-tree that is generated by the specific views

HTML as output tree

Using HTML as the output-language has some benefits over the traditional gui-drawing.

Mapping of model and presentation

The mapping between data- and presentation-model is mediated with xpath-Expressions (as in XForms). The expressions are passed in the select attribute of a view-configuration. That way every view is assigned to a node in the data-model. Relative xpath-Expressions are evaluated on the data-model-node of the parent-view. The following example shows a configuration of two TextFields that are mapped to the first- and last-name of an author-entry.

<wbz:view class="WbzContainer" select="author">
    <wbz:view class="WbzTextField" cols="20" select="firstname"/>
    <wbz:view class="WbzTextField" cols="20" select="lastname"/>
simple mapping between model and presentation

Similarities and differences to XForms

The approach as introduced so far has many similarities to XForms:

Two important differences to XForms:

Drawbacks of XForms and the model presented so far

The last limition can easily be shown in the following example. The XForms specification uses a bookmarks example to demonstrate the nesting of two repeat elements. But an arbitrary nesting (as it is quite common in most bookmark-managers) is not possible in xforms. The next section shows an example of a recursive bookmark example and how it can be done wobzilla.

The bookmark example (doing it the XSLT way)

Consider the following xml-file:

<folder name="Christophers
 <folder name="plans">
   <bookmark name="Mensaplan" href=""/>
   <bookmark name="Fahrplan DB" href=""/>
 <folder name="projects">
   <folder name="diploma thesis">
     <bookmark name="XForms" href=""/>
     <bookmark name="Mozilla" href=""/>

Transfering the bookmark example to HTML can be done with a handfull of XSLT expressions:

<xsl:stylesheet xmlns:xsl="">

 <xsl:template match="folder">
   Foldername: <xsl:value-of select="@name"/>
   <xsl:for-each select="folder|bookmark">
     <xsl:apply-templates select="."/>

 <xsl:template match="bookmark"> 
   Link: <xsl:value-of select="@name"/> 
   Url: <xsl:value-of select="@href"/>


But XSLT is a pure transformation-language. The data-document gets transfered to an output document in one step. Editing on the other hand involves constantly changing the data-source and synchronising the changes with the representative in the output. We call the necessary mechanism a mapping in contrast to a transformation. A central idea of Wobzilla is to use a mapping-mechanism that combines the expressiveness of XSLT with the component-concept introduced above.

The following configuration describes an editor for bookmark-documents in Wobzilla:

<wbz:stylesheet xmlns:wbz="">

 <wbz:template match="folder">
   Foldername: <wbz:view class="WbzTextField" select="@name"/>
   <wbz:view class="WbzRepeat" select="folder|bookmark">
     <wbz:apply-templates select="."/>

 <wbz:template match="bookmark">
    Link: <wbz:view class="WbzTextField" select="@name"/>
    URL: <wbz:view class="WbzTextField" select="@href"/>


In comparison to the XSLT-version the following differences show:

  1. The namespace of the elemente is different. Wobzilla uses the wbz-Präfixes instead of xsl. Even though Wobzilla supports the XSLT-Syntax, the underlying concept is an extension of XSLT and not the original XSLT. This is accomodated by the use of a differnt namespace.
  2. xsl:value-of has been replaced with a TextField-view. The values are now editable. Wobzilla also provides a wbz:value-of-component, but only for read-only views.
  3. xsl:for-each has been replaces with repeat; repeat allows the selection, removal and creation of elements in the data-model. As above, there is wbz:for-each component for read only views.
  4. wbz:create declares the prototypes for the new folder or bookmark-instances, that can be created with the help ofrepeat.

Apart from this differences, writing Wobzilla-GUIs is quite similar to writing XSLT-Stylesheets. You won't have any difficulties in getting familiar with Wobzilla, if you had to deal with XSLT before.

Getting into wobzilla

To conclude the introduction we present a graphical overview of the described parts of Wobzilla. You will find a detailed explanation of the components and the mapping in the Wobzilla API also see How the framework works for an overview of the mechansims in Wobzilla.

Creating a GUI
Christopher Kohlhaas Email: kohlhaas at
Last modified: Wed Oct 15 14:13:41 CEST 2003