[xsd-users] Resolving xlinks in a GML file

[Laurence Davies]

Hi Boris,

Thanks for your reply.

You've noted the xlink that points to raw data, so perhaps I provided a bad example.

In GML there is the concept of the property type (if you're interested, it is discussed here: https://www.seegrid.csiro.au/wiki/AppSchemas/GmlImplementation#Implementation_of_properties ) which is my main concern at present. The below XML should illustrate the URLs that I need to resolve and parse if the resolution succeeds:

<geo:PositionTimeSeries gml:id="ID_PTS_1" srsName="http://www.opengis.net/gml/srs/epsg.xml#4938">
    <geo:atNode xlink:href="http://wfs.geodesyml.org/nodes?identifer" />
    ...
</geo:PositionTimeSeries>

HTTP result from the hypothetical web service http://wfs.geodesyml.org/nodes?hashidentifer would look like:

<wfs:FeatureCollection numberMatched="1" numberReturned="1" timeStamp="2015-06-29T00:06:01.221Z" xsi:schemaLocation="urn:xml-gov-au:icsm:egeodesy:0.2 http://icsm.govspace.gov.au/files/2015/05/GeodesyML.xsd http://www.opengis.net/gml/3.2 http://schemas.opengis.net/gml/3.2.1/gml.xsd http://www.opengis.net/wfs/2.0 http://schemas.opengis.net/wfs/2.0/wfs.xsd">
    <wfs:member>
        <geo:Node gml:id="node_11328">
            <gml:name codeSpace="urn:vic-gov-au:icsm:egeodesy:geodesyml_node-name">node_11328</gml:name>
            ...
        </geo:Node>
    </wfs:member>
</wfs:FeatureCollection>

Would the added layer of parsing a WFS response pose a problem? If the URL just returns a <geo:Node>...</geo:Node> perhaps I could use the following template:

/* second argument takes the form:
 * std::[auto|unique]_ptr<type> name (const std::basic_string<C>& uri, xml_schema::flags = 0, const xml_schema::properties& = xml_schema::properties ());
 */
template<class P, class T>
std::auto_ptr<T> PropertyResolver(P &p, 
	std::auto_ptr<T> (*create_global_element) (const ::std::string& uri, xml_schema::flags, const xml_schema::properties&)
	)
{
	typedef ::xlink::hrefType href_type;
	typedef ::xsd::cxx::tree::optional< href_type > href_optional;

	href_optional ho = p.href();
	if (ho.present()) {
		href_type h = ho.get();
		if (h.at(0) == '#') 
		{
			xml_schema::idref ref(h.substr(1), 0, &p);
			return ref.get();
		}
		else
		{
			/* Resolve the URI externally */
			xercesc::XMLUri xuri(xercesc::XMLString::transcode(h));
			if (xuri.isValidURI())
			{
				/* @todo Is this an XPointer? Split at the # and resolve the base, 
				 *       then the XPointer expression */
				/* Try parsing using the xsd type */
				return create_global_element(h,0,xml_schema::properties ());
			}
			else
			{
				throw runtime_error("URI for xlink:href is not valid.");
			}
		}
	}
	return std::auto_ptr<T>(null);
}

This leads to another issue that I'm having: The generation of parsers for global elements. I have global elements that substitute for a global abstract element, and the parsers (documented here: http://www.codesynthesis.com/projects/xsd/documentation/cxx/tree/manual/#3) don't seem to be generated for these elements. Am I missing a crucial option to enable the generation of these parsers? For example, consider the following extract from my schema:

<element name="AbstractPosition" substitutionGroup="gml:AbstractFeature"/>

<element name="Position" type="geo:PositionType" substitutionGroup="geo:AbstractPosition"/>

<complexType name="PositionType">
	<complexContent>
		<extension base="gml:AbstractFeatureType">
 ....
		</extension>
	</complexContent>
</complexType>

Thank you again and kind regards,

Laurence Davies
____________________
Research Assistant in eGeodesy
CRC-SI, UNSW
Desk phone: (03) 8636 2373
Mobile: 0427 519 289

________________________________________
From: Boris Kolpackov [boris at codesynthesis.com]
Sent: Saturday, 27 June 2015 3:46 AM
To: Laurence Davies
Cc: xsd-users at codesynthesis.com
Subject: Re: [xsd-users] Resolving xlinks in a GML file

Hi Laurence,

Laurence Davies <laurence.davies at unsw.edu.au> writes:

> I found a relevant solution you provided in 2008 on this subject in
> the email below that covers the gml:id case but not the latter. Would
> you know what call I would need to make to resolve a URL and, if
> possible, a URN?

If href is to a gml:id, it means it is to a node in the same object
model (or to some element in the same document, in XML terms). If
href is a URL/URN like below, what does it point to?

xlink:href="ftp://ftp.ga.gov.au/geodesy-outgoing/gnss/solutions/apref/apr18287.snx"

When you say "what call I would need to make to resolve a URL" what
do you want it to be resolved to? A byte stream? A file (that got
downloaded) on disk? Maybe, if it is always an image, you want this
call to display the image on the user's computer?

Boris