NSXMLDocument, nodesForXPath with namespaces - cocoa

Question

I want to get a set of elements from a xml-file, but as soon the the elements involve namespaces, it fails.
This is a fragment of the xml file:
<gpx xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:xsd="http://www.w3.org/2001/XMLSchema"
version="1.0" creator="Groundspeak Pocket Query"
xsi:schemaLocation="http://www.topografix.com/GPX/1/0 http://www.topografix.com/GPX/1/0/gpx.xsd http://www.groundspeak.com/cache/1/0 http://www.groundspeak.com/cache/1/0/cache.xsd"
xmlns="http://www.topografix.com/GPX/1/0">
<name>My Finds Pocket Query</name>
<desc>Geocache file generated by Groundspeak</desc>
<author>Groundspeak</author>
<email>contact#groundspeak.com</email>
<time>2010-09-15T16:18:55.9846906Z</time>
<keywords>cache, geocache, groundspeak</keywords>
<bounds minlat="41.89687" minlon="5.561883" maxlat="70.669967" maxlon="25.74735" />
<wpt lat="62.244933" lon="25.74735">
<time>2010-01-11T08:00:00Z</time>
<name>GC22W1T</name>
<desc>Kadonneet ja karanneet by ooti, Traditional Cache (1.5/2)</desc>
<url>http://www.geocaching.com/seek/cache_details.aspx?guid=4af28fe9-401b-44df-b058-5fd5399fc083</url>
<urlname>Kadonneet ja karanneet</urlname>
<sym>Geocache Found</sym>
<type>Geocache|Traditional Cache</type>
<groundspeak:cache id="1521507" available="True" archived="False" xmlns:groundspeak="http://www.groundspeak.com/cache/1/0">
<groundspeak:name>Kadonneet ja karanneet</groundspeak:name>
<groundspeak:placed_by>ooti</groundspeak:placed_by>
<groundspeak:owner id="816431">ooti</groundspeak:owner>
<groundspeak:type>Traditional Cache</groundspeak:type>
<groundspeak:container>Small</groundspeak:container>
<groundspeak:difficulty>1.5</groundspeak:difficulty>
<groundspeak:terrain>2</groundspeak:terrain>
<groundspeak:country>Finland</groundspeak:country>
<groundspeak:state>
</groundspeak:state>
<groundspeak:short_description html="True">
</groundspeak:short_description>
<groundspeak:encoded_hints>
</groundspeak:encoded_hints>
<groundspeak:travelbugs />
</groundspeak:cache>
</wpt>
</gpx>
I want to get all the grounspeak:cache elements, but neither //groundspeak:cache nor //cache seems to return anything.
NSArray *caches = [self.xml nodesForXPath:#"//cache" error:&error];
Any clue?
Edit: Are there any cocoa-based software out there, where I can load my xml and test different xpaths? I'm quite new to objective-c and cocoa, so it would be nice to check that it is really my xpath that is wrong..

Answer 1

This //cache means: a descendant element under no namespace (or empty namespace)
Your groundspeak:cache element is under a namespace URI http://www.groundspeak.com/cache/1/0.
So, if you can't declare a namespace-prefix binding (I think you can't with cocoa...), you could use this XPath expression:
//*[namespace-uri()='http://www.groundspeak.com/cache/1/0' and
local-name()='cache']
If you don't want to be so strict about namespace...
//*[local-name()='cache']
But this last is a bad practice, because you could end up selecting wrong nodes, and because when dealing with XML, your tool should support namespaces.
As proof, this stylesheet:
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:template match="/">
<xsl:copy-of select="//*[namespace-uri() =
'http://www.groundspeak.com/cache/1/0' and
local-name() = 'cache']"/>
</xsl:template>
</xsl:stylesheet>
Output:
<groundspeak:cache id="1521507" available="True" archived="False"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:xsd="http://www.w3.org/2001/XMLSchema"
xmlns="http://www.topografix.com/GPX/1/0"
xmlns:groundspeak="http://www.groundspeak.com/cache/1/0">
<groundspeak:name>Kadonneet ja karanneet</groundspeak:name>
<groundspeak:placed_by>ooti</groundspeak:placed_by>
<groundspeak:owner id="816431">ooti</groundspeak:owner>
<groundspeak:type>Traditional Cache</groundspeak:type>
<groundspeak:container>Small</groundspeak:container>
<groundspeak:difficulty>1.5</groundspeak:difficulty>
<groundspeak:terrain>2</groundspeak:terrain>
<groundspeak:country>Finland</groundspeak:country>
<groundspeak:state></groundspeak:state>
<groundspeak:short_description html="True"></groundspeak:short_description>
<groundspeak:encoded_hints></groundspeak:encoded_hints>
<groundspeak:travelbugs />
</groundspeak:cache>

Answer 2

You need to add a new namespace attribute to the root node of your document, defining a prefix that you can use when querying the children:
NSXMLDocument *xmldoc = ...
NSXMLElement *namespace = [NSXMLElement namespaceWithName:#"mns" stringValue:#"http://mynamespaceurl.com/mynamespace"];
[xmldoc.rootElement addNamespace:namespace];
then when you query things later, you can use that prefix to refer to the namespace:
NSArray * caches = [xmldoc.rootElement nodesForXPath:#"//mns:caches" error:&error];

Answer 3

//groundspeak:cache should work. You might need a namespace-uri setting as well