XSLT: use parameters in xls:sort attributes (dynamic sorting)

Question

How do I apply a parameter to a select and order attribute in a xsl:sort element? I'ld like to do this dynamic with PHP with something like this:

$xsl = new XSLTProcessor();
$xslDoc = new DOMDocument(); 
$xslDoc->load( $this->_xslFilePath );
$xsl->importStyleSheet( $xslDoc );
$xsl->setParameter( '', 'sortBy', 'viewCount' );
$xsl->setParameter( '', 'order', 'descending' );

But I'ld first have to now how to get this to work. I tried the following, but it gives me a 'compilation error' : 'invalid value $order for order'. $sortBy doesn't seem to do anything either:

<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"&gt;
<xsl:output method="xml" indent="yes"/>
<xsl:param name="sortBy" select="viewCount"/>
<xsl:param name="order" select="descending"/>
<xsl:template match="/">
    <media>
    <xsl:for-each select="media/medium">
    <xsl:sort select="$sortBy" order="$order"/>
        // <someoutput>
    </xsl:for-each>
    </media>
</xsl:template>
</xsl:stylesheet>

Answer 1

You are close to the correct solution, but there are a few issues:

1. <xsl:param name="sortBy" select="viewCount"/> This defines the $sortBy parameter as the value of the viewCount child of the current node (the document node). Because the top element is not named viewCount, the $sortBy parameter so defined has no value at all.

2. <xsl:param name="order" select="descending"/> Ditto.

3. <xsl:sort select="$sortBy" order="$order"/> Even if issues 1. and 2. above are fixed, this xslt instruction is still problematic. It specifies the value of the order attribute as the literal string '$order' -- not as the value of the parameter $order. The way to do this in XSLT is by using AVT (Attribute Value Template). Whenever we want a to specify that within an attribute value we want a particular string to be evaluated as an XPath expression, then this string must be surrounded by curly braces.

So, the order attribute should be specified as: order = '{$order}'.

Unfortunately, AVTs cannot be used for the select attribute (another rule from the XSLT spec).

The way to specify the value of the select attribute is a little-bit more tricky:

select='*[name()=$sortBy]' This says: sort by the child element, whose name is the same as the value of the variable $sortBy.

To put all this together, here is the corrected transformation:

<xsl:stylesheet version="1.0"
 xmlns:xsl="http://www.w3.org/1999/XSL/Transform"&gt;
 <xsl:output omit-xml-declaration="yes" indent="yes"/>

 <xsl:strip-space elements="*"/>

 <xsl:param name="sortBy" select="'viewCount'"/>
 <xsl:param name="order" select="'descending'"/>

 <xsl:template match="/">
    <media>
      <xsl:for-each select="media/medium">
        <xsl:sort select="*[name()=$sortBy]" order="{$order}"/>

        <xsl:copy-of select="."/>
      </xsl:for-each>
    </media>
 </xsl:template>
</xsl:stylesheet>

When this transformation is applied on the following XML document:

<media>
 <medium>
   <viewCount>2</viewCount>
 </medium>
 <medium>
   <viewCount>1</viewCount>
 </medium>
 <medium>
   <viewCount>5</viewCount>
 </medium>
</media>

The correct result is produced:

<media>
   <medium>
      <viewCount>5</viewCount>
   </medium>
   <medium>
      <viewCount>2</viewCount>
   </medium>
   <medium>
      <viewCount>1</viewCount>
   </medium>
</media>