Re: [xml] XPath with default namespace

[Daniel Veillard <veillard redhat com>]

On Tue, Aug 07, 2001 at 08:53:55PM +0100, Stephen Lord wrote:
> How can I ensure that I have the correct namespaces set up in the context
>  before using xPath to query for an element that defines it's namespace
>  within that element?

  Are you within an XSLT document, if yes then the namespaces in scope
are used.
  If you are within an extension function, build a list of namespaces
in scope at the current extension function, assuming the current node
is inst, then:

    int nsNr = 0;
    xmlNsPtr *nsList = xmlGetNsList(inst->doc, inst);

    if (nsList != NULL) {
        while (cur->nsList[i] != NULL)
            i++;
    }

  Then pass it to the XPath context
    xmlXPathContextPtr xpathCtxt;


    xpathCtxt->namespaces = nsList;
    xpathCtxt->nsNr = nsNr;

  Don't forget to xmlFree(nsList) once done.

> If I try :
>  //definitions/types/xsd:schema/*
>
> XPath returns:
>  Error xpath.c:7976: Undefined namespace prefix
>  //ofwsdl:definitions/ofwsdl:types/xsd:schema/*
>                                                ^
>  Error xpath.c:7970: Invalid type
>  //ofwsdl:definitions/ofwsdl:types/xsd:schema/*
>                                                ^
>  Which is obviously because the namespace is not defined yet.

  Right
  
> I can hack it by predefining an alternative prefix e.g ofxsd2000 to the URI
>  in question but that implies that I have to evaluate a path for each
>  possible value of xsd: i.e.
>
>          xmlXPathRegisterNs(ctx, (xmlChar *)"ofxsd2001", (xmlChar
> *)"http://www.w3.org/2001/XMLSchema";);
>          xmlXPathRegisterNs(ctx, (xmlChar *)"ofxsd2000", (xmlChar
> *)"http://www.w3.org/2000/10/XMLSchema";);
>          xmlXPathRegisterNs(ctx, (xmlChar *)"ofxsd1999", (xmlChar
> *)"http://www.w3.org/1999/XMLSchema";);

  xmlXPathRegisterNs() is another, more global (and hence less reliable
considering that namespaces are not global) way of doing this. I don't
recommend it, it only makes sense for XPointers evaluation (since theu
don't inherit namespaces from their context).

> and then search with all three possible mappings of xsd to the above URIs.
>  Or do I have to walk the tree to the schema element and then load the
>  namespaces?

  Use xmlGetNsList() to be sure.

> Also, I am unable to figure out how to list out the current namespaces in a
>  given context. Any help would be greatly appreciated.

  If you have a node then you can compute what is the prefix associated
to a namespace in scope or what is the namespace name associated to a given
prefix:

xmlNsPtr        xmlSearchNs             (xmlDocPtr doc,
                                         xmlNodePtr node,
                                         const xmlChar *nameSpace);
xmlNsPtr        xmlSearchNsByHref       (xmlDocPtr doc,
                                         xmlNodePtr node,
                                         const xmlChar *href);
 
  the prefix (if any) is in the xmlNsPtr returned as ns->prefix, the
namespace name is ns->href.

  Namespaces are shared within the full subtree, that's a non-direct
mapping of the XPath data model but is equivalent (different memory/cpu
tradeoff).

Daniel

-- 
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