=head1 NAME

XML::LibXML::DOM - XML::LibXML DOM Implementation


=head1 DESCRIPTION

XML::LibXML provides an lightwight interface to modify a node of the document
tree generated by the XML::LibXML parser. This interface follows as far as
possible the DOM Level 3 specification. Additionally to the specified functions
the XML::LibXML supports some functions that are more handy to use in the perl
environment.

One also has to remember, that XML::LibXML is an interface to libxml2 nodes
which actually reside on the C-Level of XML::LibXML. This means each node is a
reference to a structure different than a perl hash or array. The only way to
access these structure's values is through the DOM interface provided by
XML::LibXML. This also means, that one can't simply inherit a XML::LibXML node
and add new member variables as they were hash keys.

The DOM interface of XML::LibXML does not intend to implement a full DOM
interface as it is done by XML::GDOME and used for full featured application.
Moreover, it offers an simple way to build or modify documents that are created
by XML::LibXML's parser.

Another target of the XML::LibXML interface is to make the interfaces of
libxml2 available to the perl community. This includes also some workarounds to
some features where libxml2 assumes more control over the C-Level that most
perl users don't have.

One of the most important parts of the XML::LibXML DOM interface is, that the
interfaces try do follow the DOM Level 3 specification rather strictly. This
means the interface functions are named as the DOM specification says and not
what widespread Java interfaces claim to be standard. Although there are
several functions that have only a singular interface that conforms to the DOM
spec XML::LibXML provides an additional Java style alias interface.

Also there are some function interfaces left over from early stages of
XML::LibXML for compatibility reasons. These interfaces are for compatibility
reasons only. They might disappear in one of the future versions of
XML::LibXML, so a user is requested to switch over to the official functions.

More recent versions of perl (e.g. 5.6.1 or higher) support special flags to
disinguish between UTF-8 and so called binary data. XML::LibXML provides for
these versions functionality to make efficient use of these flags: If a
document has set an encoding other than UTF-8 all strings that are not already
in UTF-8 are implicitly encoded from the document encoding to UTF-8. On output
these strings are commonly returned as UTF-8 unless a user does request
explicitly the original (aka. document) encoding.

Older version of perl (such as 5.00503 or less) do not support these flags. If
XML::LibXML is build for these versions, all strings have to get encoded to
UTF-8 manualy before they are passed to any DOM functions.

NOTE: XML::LibXML's magic encoding may not work on all plattforms. Some
platforms are known to have a broken iconv(), which is partly used by libxml2.
To test if your platform works correctly with your language encoding, build a
simple document in the particular encoding and try to parse it with
XML::LibXML. If your document gets parsed with out causing any segmentation
faults, bus errors or whatever your OS throws. An example for such a test can
be found in test 19encoding.t of the distribution.

Namespaces and XML::LibXML's DOM implementation

XML::LibXML's DOM implementation is limitted by the DOM implementation of
libxml2 which treats namespaces slightly differently than required by the DOM
Layer 2 specification.

According to the DOM Layer 2 specification, namespaces of elements and
attributes should be persistent, and nodes should be permanently bound to
namespace URIs as they get created; it should be possible to manipulate the
special attributes used for declaring XML namespaces just as other attributes
without affecting the namespaces of other nodes. In DOM Layer 2, the
application is responsible for creating the special attributes consistently
and/or for correct serialization of the document.

This is both unconvenient, causes problems in serialization of DOM to XML, and
most importantly, seems almost impossible to implement over libxml2.

In libxml2, namespace URI and prefix of a node is provided by a pointer to a
namespace declaration (appearing as a special xmlns attribute in the XML
document). If the prefix or namespace URI of the declaration changes, the
prefix and namespace URI of all nodes that point to it changes as well.
Moreover, in contrast to DOM, a node (element or attribute) can only be bound
to a namespace URI if there is some namespace declaration in the document to
point to.

Therefore current DOM implementation in XML::LibXML tries to treat namespace
declarations in a compromise between reason, common sense, limitations of
libxml2, and the DOM Layer 2 specification.

In XML::LibXML, special attributes declaring XML namespaces are often created
automatically, usually when a namespaced node is attached to a document and no
existing declaration of the namespace and prefix is in the scope to be reused.
In this respect, XML::LibXML DOM implementation differs from the DOM Layer 2
specification according to which special attributes for declaring the
appropriate XML namespaces should not be added when a node with a namespace
prefix and namespace URI is created.

Namespace declarations are also created when XML::LibXML::Document's
createElementNS() or createAttributeNS() function are used. If the a namespace
is not declared on the documentElement, the namespace will be localy declared
for the newly created node. In case of Attributes this may look a bit
confusing, since these nodes cannot have namespace declarations itself. In this
case the namespace is internally applied to the attribute and later declared on
the node the attribute is appended to (if required).

The following example may explain this a bit:

   my $doc = XML::LibXML->createDocument;
   my $root = $doc->createElementNS( "", "foo" );
   $doc->setDocumentElement( $root );
  
   my $attr = $doc->createAttributeNS( "bar", "bar:foo", "test" );
   $root->setAttributeNodeNS( $attr );               

This piece of code will result in the following document:

   <?xml version="1.0"?>
   <foo xmlns:bar="bar" bar:foo="test"/>

The namespace is declared on the document element during the
setAttributeNodeNS() call.

Namespaces can be also declared explicitly by the use of XML::LibXML:Element's
setNamespace() function. Since 1.61, they can also be manipulated with
functions setNamespaceDeclPrefix() and setNamespaceDeclURI() (not available in
DOM). Changing an URI or prefix of an existing namespace declaration affects
the namespace URI and prefix of all nodes which point to it (that is the nodes
in its scope).

It is also important to repeat the specification: While working with namespaces
you should use the namespace aware functions instead of the simplified
versions. For example you should never use setAttribute() but setAttributeNS().

=head1 AUTHORS

Matt Sergeant, 
Christian Glahn, 
Petr Pajas, 

=head1 VERSION

1.61

=head1 COPYRIGHT

2001-2006, AxKit.com Ltd; 2002-2006 Christian Glahn; 2006 Petr Pajas, All rights reserved.

=cut
