Question

Something confuses me here:

The NSXMLParser method has a namespaceURI attribute:

- (void)parser:(NSXMLParser *)parser 
 didEndElement:(NSString *)elementName 
  namespaceURI:(NSString *)namespaceURI 
 qualifiedName:(NSString *)qName

From the documentation I couldn’t figure out what they mean by “namespace”. Can someone explain with an example what a namespace is in a XML and why I would want that?

Edit: Yep, I noticed wikipedia. But it’s confusing, again. What sense does it make to put one single namespace declaration at the top of the XML file like

xmlns:xhtml="http://www.w3.org/1999/xhtml"

?? Again, that makes just zero sense. Wikipedia has no useful example either to get it, why I would really want namespaces and -more importantly- how this looks in an XML file. They say it’s for resolving ambiguity of multiple same-named elements like ID, but there’s no example how multiple namespaces would resolve that.

Answer 1

XML namespaces work like namespaces everywhere else.

They provide a means to uniquely distinguish equally named elements or attributes. This is done by declaring a namespace URI, and optionally attaching a prefix to the node name. This prefix is (optionally) defined along with the namespace declaration.

<!-- node without any namespace (it's in the default namespace) -->
<node>
  <child /><!-- descendants are in the parent namespace by default -->
</node>

<!-- node with explicit default namespace -->
<node xmlns="http://some/namespace/uri/">
  <child /><!-- descendants are in the parent namespace by default -->
</node>

<!-- NS declaration with prefix (node is still in the default namespace!) -->
<node xmlns:prefix="http://some/namespace/uri/">
  <child /><!-- descendants are in the parent's namespace -->
  <prefix:child><!-- explicit namespace assignment by prefix -->
    <grandchild /><!-- prefixes don't propagate, this is in the default namespace! -->
  </prefix:child>
</node>

Namespaces are strictly scoped. They are available to the node and its descendants only. To have a namespace available in the entire XML document, it must be declared at the top level element (document element).

In your case, on the example or <prefix:child />:

didEndElement = "child"
 namespaceURI = "http://some/namespace/uri/"
qualifiedName = "prefix:child"