<xsl:template>

We have just looked at <xsl:output>, which is known as a top-level element as it can only occur as a child of the <xsl:stylesheet> element. We'll now look at <xsl:template>, which is another top-level element.

 

The <xsl:template> element contains the information required to produce a node in the result tree. We have seen several examples in the stylesheets we have been producing.

 

In the examples we have used so far, we have employed a match attribute, for example:

 

 

The attribute value is an XPath expression that will tell us which nodes in the source tree will cause this template to execute.

 

When using XPath expressions, there is always the possibility that several templates will match a single element. For example, if you look back to the listing of HamletExtract.xml, you will see that both acts and scenes have titles. In our previous stylesheet Hamlet.xsl, we had the templates:

 

What if we had another template:

 

 

Which template would be executed when our <xsl:apply-templates> finds a scene's title?

 

One thing we can do is add a priority attribute to the <template> element:

 

 

This will allow the stylesheet to select the highest priority template, and only execute that. The value of the priority attribute can be any positive or negative integer or real number: the higher the number, the higher the priority. The default priority is between -0.5 and 0.5, so this new template will always be matched for any <TITLE> element, and the more specific ACT/TITLE and SCENE/TITLE templates will never be matched.

 

This is just one method of resolving conflicts when there are multiple templates matching a pattern. Later, in the section Template Match Conflicts, we will look at the others and the rules that determine in general which template is instantiated.

 

The <xsl:template> element has two further optional attributes. One is name, which is used with the <xsl:call-template> element, and the other is mode, which provides additional flexibility in the use of templates. We will meet both these attributes in Chapters 4 and 5.

<xsl:apply-templates>

Along with the <xsl:template> and <xsl:value-of> elements, this is the workhorse of the XSLT world. It is the element that controls which templates are used at any point while building the result tree.

 

It can have two attributes, of which we have already met the select attribute. This simply takes as its value an XPath expression that controls which elements in the source tree will be processed. This pattern can be simple, like most of those we have been using, or much more complicated, using any of the rules of XPath. Note that the select attribute is optional. If it is missing, all child nodes will be processed.

 

The second attribute is the mode attribute. Since this relates to the mode attribute in <xsl:template>, we will again be leaving this to Chapter 4.

 

As well as its attributes, <xsl:apply-templates> can have two sub-elements. The first is <xsl:sort>, which alters the order in which selected nodes are processed, and the second is <xsl:with-param>, which provides a method of passing parameters to templates. We will look at <xsl:sort> shortly, but leave <xsl:with-param> to Chapter 4.

<xsl:value-of>

This element is always contained within a template and simply writes the text node (or nodes) of the element pointed to by the XPath expression identified in its select attribute to the result tree.

 

The select attribute works in much the same way as it does in <xsl:apply-templates>, taking an XPath expression as its value. However, with <xsl:apply-templates>, if there are several matches, the relevant templates are instantiated multiple times. With <xsl:value-of>, only the first instance is used. Also, unlike with <xsl:apply-templates>, in this case the attribute is mandatory.

 

The value of this expression is always written to the result tree as a string, so some conversion might need to take place first. A number is converted to the string representation of that number. If a node contains sub-nodes, the value is the concatenation of the values of all the sub-nodes. So, for example, for our document HamletExtract.xml, we could include within the <SCENE> template a line:

 

 

This will return a value that is the concatenation of the values of the <SPEAKER> and <LINE> elements of the third <SPEECH>. This is the speech that will be selected:

 

 

So the text returned will be BERNARDOLong Live the king!.

 

Finally, if the result is a Boolean, it will be converted to one of the strings true or false.