XSLT PI Parameters: Difference between revisions

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== Overview ==
#redirect [[mdc:XSLT:PI_Parameters]]
 
XSLT supports the concept of passing parameters to a stylesheet when executing it. This is currently possible when using the [http://www.mozilla.org/projects/xslt/js-interface.html XSLTProcessor in javascript]. However when using an <?xml-stylesheet?> processing instruction (PI) there is currently no way to provide parameters.
 
To solve this two new PIs are proposed, <?xslt-param?> and <?xslt-param-namespace?>. Both PIs can contain "pseudo attributes" the same way that the xml-stylesheet PI does.
 
The following document passes the two parameters "color" and "size" to the stylesheet "style.xsl".
 
  <?xslt-param name="color" value="blue"?>
  <?xslt-param name="size" select="2"?>
  <?xml-stylesheet type="text/xsl" href="style.xsl"?>
 
== Processing instructions ==
 
The attributes in the '''xslt-param''' and '''xslt-param-namespace''' PIs are parsed using the rules defined in [http://www.w3.org/TR/xml-stylesheet/ xml-stylesheet]. If any unrecognized attributes must be ignored. Parsing of any attribute must not fail due to the presence of an unrecognized attribute as long as that attribute follows the syntax in xml-stylesheet.
 
Both the '''xslt-param''' and the '''xslt-param-namespace''' PIs must appear in the prolog of the document, i.e. before the first element tag. All PIs in the prolog must be honored, both ones occuring before and onces occuring after any xml-stylesheet PIs.
 
If there are multiple xml-stylesheet PIs the parameters apply to all stylesheet as a consequence of that all stylesheets are imported into a single stylesheet per the XSLT spec.
 
=== xslt-param ===
 
The '''xslt-param''' PI supports 4 attributes:
 
* '''name'''<br>The local-name part of the parameter name. No syntax checking is done on the attribute, however if it is not a valid [http://www.w3.org/TR/REC-xml-names/#NT-NCName NCName] it will never match any parameter in the stylesheet.
* '''namespace'''<br>The namespace of the parameter name. No syntax checking is done on the attribute.
* '''value'''<br>Contains the string value for the parameter. The value of the attribute is used as value for the parameter. The datatype will always be ''string''.
* '''select'''<br>An XPath expression for the parameter. The value of the attribute is parsed as an XPath expression. The result of evaluating the expression is used as value for the parameter.
 
If the '''name''' attribute is missing or empty the PI is ignored.
 
If the '''namespace''' attribute is missing or empty the null namespace is used.
 
If both '''value''' and '''select''' is present the '''value''' attribute is ignored. If neither '''value''' nor '''select''' is present the PI is ignored.
 
It is not an error to specify a parameter name that does not exist in the stylesheet (or that is a variable in the stylesheet). The PI is simply ignored.
 
==== The select attribute context ====
The following context is used to parse and evaluate the expression in the '''select''' attribute.
 
* The context node is the node used as initial current node used when executing the stylesheet.
* The context position is the position of the context node in the initial current node list used when executing the stylesheet.
* The context size is the size of the  initial current node list used when executing the stylesheet.
* Variables: see TODO.
* The function library is the standard XPath function library.
* The namespace declarations are determined by the xslt-param-namespace PIs, see below.
 
If the '''select''' attribute fails to parse or execute the PI is ignored (in particular, it does not fallback to the value attribute).
 
=== xslt-param-namespace ===
 
The '''xslt-param-namespace''' uses two attributes:
 
* '''prefix'''<br>The prefix that is mapped.
* '''namespace'''<br>The namespace the prefix maps to.
 
An '''xslt-param-namespace''' PI affects the expression in the '''select''' attribute for all '''xslt-param'''s following the PI. This applies even if there are other nodes such as comments or other PIs between the '''xslt-param-namespace''' and '''xslt-param''' PIs.
 
It is not an error for multiple PIs to use the same prefix, every new PI just changes what namespace the prefix maps to.
 
If either '''prefix''' or '''namespace''' is missing or empty the PI is ignored. If '''prefix''' is not a valid NCName it is ignored.
 
== TODO ==
 
Should we allow variable references in the expression? The current patch for mozilla does allow any toplevel variable and parameter in the stylesheet, but it might be a tough requirement on other XSLT engines. Especially since that allows any parameter to reference any other parameter so evaluation order is tricky.
 
Should we allow any XSLT functions in the expression? document() seems useful, but it seems tricky to maintain the invariant that generate-id() should produce the same string for the same document.

Latest revision as of 13:39, 3 January 2007

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