Overview

About this document

This document proposes a new property tag. This tag behaves just like the set tag, but follows the model of the input tag in HTML and the property tag in Apache Ant.

This is a proposal for a change in the XVCL specification. It was written by Gilbert Herschberger (30 September 2005).

Note: Modifying the DTD. Add the property tag to the dtd/xvcl.dtd document.

Details

 <property name="n" value="v"/>

is equivalent to

 <set var="n" value="v"/>

Goal

Explore the mechanism to extend the XVCL processor with custom tags.

Apache Ant provides a mechanism to extend a script with custom tags. The taskdef tag defines a custom tag. The mechanism is fully supported and encouraged and documented.

The XVCL processor has a mechanism to define tags. Tags are not defined in Java source code, but an external resource. It might be possible to add a tag to the XVCL processor by modifying this resource.

Contents

This document contains the following sections:

• Section A - Defining the set Tag
• Section B - Defining the property Tag

Section A - Defining the set Tag

Introduction

This section describes how the var tag is defined.

Mechanism

The following mechanism defines a tag, using the set tag as an example.

1. A set tag corresponds to a Java class.
2. A list of tags is maintained in an external resource.
3. The XVCL processor parses a frame.
4. The XVCL processor handles the parsed frame.

External resource

A list of tags is maintained in an external resource.

 package-name.config.properties

Defining

Here is how a tag is associated with a Java class.

 ####set Command
 set=package-name.SetVar
 SetVar_var=var
 SetVar_value=value
 SetVar_defer-evaluation=defer-evaluation

Java class

The set tag is associated with the SetVar class. A tag corresponds to a Java class. The class must be compiled and must be available on the Java classpath.

Parsing

A tag is part of an element. When parsing an XML-based frame with JDOM, an element becomes an instance of org.jdom.Element.

When parsing with MyBuilder, an element becomes an instance of XVCLNode. Therefore, a custom tag must extend XVCLNode.

For the set tag,

• The SetVar class extends XVCLNode.

1. XVCLNode extends MyElement.
2. MyElement extends Element.

Attributes

The default attributes of a set tag are

• var
• value
• defer-evaluation

An attribute name is mapped. The getAttributeExternalName() method implements the map from a constant internal name to a configure-able external name.

Using the default configuration, getAttributeExternalName( "SetVar_var" ) returns "var".

Section B - Defining the property Tag

Introduction

This section describes how to implement the proposed change in the XVCL specification.

Defining

Here is how a property tag is associated with a Java class.

 ####property Command
 set=package-name.Property
 Property_var=name
 Property_value=value
 Property_defer-evaluation=defer-evaluation

Alternative 1

The SetVar and Property classes share common code. Let's say that Property extends SetVar.

The parseAttributes() method should be the same except:

• SetVar_ becomes Property_
• var becomes name

Case study

This is a case study in the intent and purpose of the XVCL processor.

Source code shared by SetVar and Property is written into a frame called setvar-parseattributes.xml and re-used by setvar.xml and property.xml. The classname variable is defined, giving

• classname_var,
• classname_value, and
• classname_defer-evaluation.

Inside setvar-parseattributes.xml, the <value-of> tag emits the "?@classname?" expression.