Default attribute namespaces.

When an attribute is prefixed with "xml" the default namespace should be used
automatically. This namespace is not registered on element level by default as
this namespace isn't registered manually, instead it's a "magic" namespace. This
also ensures we match the behaviour of libxml more closely, hopefully reducing
confusion.

lib/oga.rb

@@ -25,8 +25,8 @@ require_relative 'oga/xml/cdata'
 require_relative 'oga/xml/xml_declaration'
 require_relative 'oga/xml/processing_instruction'
 require_relative 'oga/xml/doctype'
-require_relative 'oga/xml/attribute'
 require_relative 'oga/xml/namespace'
+require_relative 'oga/xml/attribute'
 require_relative 'oga/xml/element'
 require_relative 'oga/xml/node_set'
 

lib/oga/xml/attribute.rb

@@ -21,6 +21,17 @@ module Oga
     class Attribute
       attr_accessor :name, :namespace_name, :element, :value
 
+      ##
+      # The default namespace available to all attributes. This namespace can
+      # not be modified.
+      #
+      # @return [Oga::XML::Namespace]
+      #
+      DEFAULT_NAMESPACE = Namespace.new(
+        :name => 'xml',
+        :uri  => 'http://www.w3.org/XML/1998/namespace'
+      ).freeze
+
       ##
       # @param [Hash] options
       #
@@ -44,7 +55,15 @@ module Oga
       # @return [Oga::XML::Namespace]
       #
       def namespace
-        return @namespace ||= element.available_namespaces[namespace_name]
+        unless @namespace
+          if namespace_name == DEFAULT_NAMESPACE.name
+            @namespace = DEFAULT_NAMESPACE
+          else
+            @namespace = element.available_namespaces[namespace_name]
+          end
+        end
+
+        return @namespace
       end
 
       ##

spec/oga/xml/attribute_spec.rb

@@ -24,11 +24,17 @@ describe Oga::XML::Attribute do
         :name           => 'a',
         :element        => element
       )
+
+      @default = described_class.new(:namespace_name => 'xml', :name => 'x')
     end
 
     example 'return a Namespace instance' do
       @attribute.namespace.should == @namespace
     end
+
+    example 'return the default XML namespace when the "xml" prefix is used' do
+      @default.namespace.should == Oga::XML::Attribute::DEFAULT_NAMESPACE
+    end
   end
 
   context '#to_s' do