目的是什么<xsl:call-template…>

Question

I have a XML file looks like below

- <select1>
   - <sel_details>
      <type>Primary Type</type> 
      <name>Some Name</name> 
      <id>ID Num</id>
     <sel_details>
     .....
  <select1>

And XSL looks like this

<xsl:for-each select="//select1">
     <xsl:call-template name="sel_details" />
     <xsl:with-param name="xmlSection" select="sel_details" />
     </xsl:call-template>
     <xsl:call-template name="....." />
     <xsl:with-param name="xmlSection" select="....." />
     </xsl:call-template>
     ..........
</xsl:for-each>

<xsl:template name="sel_details">
     <xsl:param name="xmlSection" />
        <xsl:for-each select="xmlSection">
            <xsl:value-of select="./type" />
        </xsl:for-each>
</xsl:template>

What is the purpose of <xsl:call-template...> then <xsl:template name...> ?

If I write only 3 lines like this, I could get the same value:

<xsl:for-each select="//select1/sel_details">
     <xsl:value-of select="./type" />
</xsl:for-each>

Answer 1

What is a named template?

In an XSLT stylesheet, xsl:call-template :

<xsl:call-template name="construct-header"/>

means invoking a so-called named template - an xsl:template element that has a name attribute:

<xsl:template name="construct-header">

As opposed to a template that matches input nodes and therefore has a match attribute:

<xsl:template match="book">

Since named templates are never matched to input nodes, calling them explicitly with xsl:call-template is the only way to execute the code inside them. Once a named template is invoked, the code is evaluated with no change to the context ( broadly speaking, at least ). So, a very legitimate question is

Why use a named template in general?

Named templates are used

• to implement recursive algorithms in XSLT. A well-known example is mimicking the tokenize() function in XSLT 1.0 which requires a recursive named template.
• to reuse code blocks in several places to avoid repetition
• to organize your stylesheet into meaningful parts. This can also be done by modularizing your code into several stylesheets that are included or imported by the main stylesheet.

Using a named template in your specific case

If the code is really as short as you show, I can see no reason to use a named template here. There is no recursive processing in the named template. But perhaps you have shortened it considerably before posting here or the named template is reused elsewhere in the stylesheet. Both would be sound reasons to keep the named template. (If you'd like clearer guidance on this, please show a full stylesheet.)

If you decide to reduce the code to an xsl:for-each , then also get rid of ./ because it is redundant, and, speaking from experience, I'd say there should not be a // in front of the path expression:

<xsl:for-each select="select1/sel_details">
     <xsl:value-of select="type" />
</xsl:for-each>

Answer 2

The code looks to me as if it was written by someone who had not yet mastered XSLT's apply-templates idiom. I imagine a more experienced XSLT programmer might have written something like this:

<xsl:template match="/">
  ...
  <xsl:apply-templates select="//select1"/>
  ...
</xsl:template>

<xsl:template match="select1">
     <xsl:apply-templates select="sel_details" />
     <xsl:apply-templates select="....." />
</xsl:template>

<xsl:template match="sel_details">
     <xsl:value-of select="type" />
</xsl:template>

Stylesheets in which xsl:for-each and xsl:call-template and xsl:choose are the dominant control structures generally suggest a programmer who is not yet comfortable with the XSLT idiom. Experienced XSLT coders use match templates and xsl:apply-templates instead.

Answer 3

Think of named function in an ordinary programming language. <xsl:call-template> is a direct analog of function invocation. Whether it's worth to move a certain piece of code to a separate function or not - it's the matter of coding style, number of code repetions and other other "fuzzy" considerations. In this particular case an author probably wished to assure that every section will be transfomed uniformly. Perhaps, in previous revisions of the file, the body of the function was less trivial.