XML::LibXML::InputCallback
XML::LibXML::InputCalUserkContributed Perl DocumXML::LibXML::InputCallback(3)
NAME
XML::LibXML::InputCallback - XML::LibXML Class for Input Callbacks
SYNOPSIS
use XML::LibXML;
DESCRIPTION
You may get unexpected results if you are trying to load external doc-
uments during libxml2 parsing if the location of the resource is not a
HTTP, FTP or relative location but a absolute path for example. To get
around this limitation, you may add your own input handler to open,
read and close particular types of locations or URI classes. Using
this input callback handlers, you can handle your own custom URI
schemes for example.
The input callbacks are used whenever LibXML has to get something
other than externally parsed entities from somewhere. They are imple-
mented using a callback stack on the Perl layer in analogy to
libxml2’s native callback stack.
The XML::LibXML::InputCallback class transparently registers the input
callbacks for the libxml2’s parser processes.
How does XML::LibXML::InputCallback work?
The libxml2 library offers a callback implementation as global func-
tions only. To work-around the troubles resulting in having only
global callbacks - for example, if the same global callback stack is
manipulated by different applications running together in a single
Apache Web-server environment -, XML::LibXML::InputCallback comes with
a object-oriented and a function-oriented part.
Using the function-oriented part the global callback stack of libxml2
can be manipulated. Those functions can be used as interface to the
callbacks on the C- and XS Layer. At the object-oriented part, opera-
tions for working with the "pseudo-localized" callback stack are
implemented. Currently, you can register and de-register callbacks on
the Perl layer and initialize them on a per parser basis.
Callback Groups
The libxml2 input callbacks come in groups. One group contains a URI
matcher (match), a data stream constructor (open), a data stream
reader (read), and a data stream destructor (close). The callbacks can
be manipulated on a per group basis only.
The Parser Process
The parser process work on a XML data stream, along which, links to
other resources can be embedded. This can be links to external DTDs or
XIncludes for example. Those resources are identified by URIs. The
callback implementation of libxml2 assumes that one callback group can
handle a certain amount of URIs and a certain URI scheme. Per default,
callback handlers for file://*, file:://*.gz, http://* and ftp://* are
registered.
Callback groups in the callback stack are processed from top to bot-
tom, meaning that callback groups registered later will be processed
before the earlier registered ones.
While parsing the data stream, the libxml2 parser checks if a regis-
tered callback group will handle a URI - if they will not, the URI
will be interpreted as file://URI. To handle a URI, the match callback
will have to return ’1’. If that happens, the handling of the URI will
be passed to that callback group. Next, the URI will be passed to the
open callback, which should return a reference to the data stream if
it successfully opened the file, ’0’ otherwise. If opening the stream
was successful, the read callback will be called repeatedly until it
returns an empty string. After the read callback, the close callback
will be called to close the stream.
Organisation of callback groups in XML::LibXML::InputCallback
Callback groups are implemented as a stack (Array), each entry holds a
reference to an array of the callbacks. For the libxml2 library, the
XML::LibXML::InputCallback callback implementation appears as one sin-
gle callback group. The Perl implementation however allows to manage
different callback stacks on a per libxml2-parser basis.
Using XML::LibXML::InputCallback
After object instantiation using the parameter-less constructor, you
can register callback groups.
my $input_callbacks = XML::LibXML::InputCallback->new();
$input_callbacks->register_callbacks([ $match_cb1, $open_cb1,
$read_cb1, $close_cb1 ] );
$input_callbacks->register_callbacks([ $match_cb2, $open_cb2,
$read_cb2, $close_cb2 ] );
$input_callbacks->register_callbacks( [ $match_cb3, $open_cb3,
$read_cb3, $close_cb3 ] );
$parser->input_callbacks( $input_callbacks );
$parser->parse_file( $some_xml_file );
What about the old callback system prior to XML::LibXML::InputCall-
back?
In XML::LibXML versions prior to 1.59 - i.e. without the
XML::LibXML::InputCallback module - you could define your callbacks
either using globally or locally. You still can do that using
XML::LibXML::InputCallback, and in addition to that you can define the
callbacks on a per parser basis!
If you use the old callback interface through global callbacks,
XML::LibXML::InputCallback will treat them with a lower priority as
the ones registered using the new interface. The global callbacks will
not override the callback groups registered using the new interface.
Local callbacks are attached to a specific parser instance, therefore
they are treated with highest priority. If the match callback of the
callback group registered as local variable is identical to one of the
callback groups registered using the new interface, that callback
group will be replaced.
Users of the old callback implementation whose open callback returned
a plain string, will have to adapt their code to return a reference to
that string after upgrading to version >= 1.59. The new callback sys-
tem can only deal with the open callback returning a reference!
INTERFACE DESCRIPTION
Global Variables
$_CUR_CB
Stores the current callback and can be used as shortcut to access
the callback stack.
@_GLOBAL_CALLBACKS
Stores all callback groups for the current parser process.
@_CB_STACK
Stores the currently used callback group. Used to prevent parser
errors when dealing with nested XML data.
Global Callbacks
_callback_match
Implements the interface for the match callback at C-level and for
the selection of the callback group from the callbacks defined at
the Perl-level.
_callback_open
Forwards the open callback from libxml2 to the corresponding call-
back function at the Perl-level.
_callback_read
Forwards the read request to the corresponding callback function
at the Perl-level and returns the result to libxml2.
_callback_close
Forwards the close callback from libxml2 to the corresponding
callback function at the Perl-level..
Class methods
new()
A simple constructor.
register_callbacks( [ $match_cb, $open_cb, $read_cb, $close_cb ])
The four callbacks have to be given as array reference in the
above order match, open, read, close!
unregister_callbacks( [ $match_cb, $open_cb, $read_cb, $close_cb ])
With no arguments given, "unregister_callbacks()" will delete the
last registered callback group from the stack. If four callbacks
are passed as array reference, the callback group to unregister
will be identified by the match callback and deleted from the
callback stack. Note that if several identical match callbacks are
defined in different callback groups, ALL of them will be deleted
from the stack.
init_callbacks()
Initializes the callback system before a parsing process.
cleanup_callbacks()
Resets global variables and the libxml2 callback stack.
lib_init_callbacks()
Used internally for callback registration at C-level.
lib_cleanup_callbacks()
Used internally for callback resetting at the C-level.
EXAMPLE CALLBACKS
The following example is a purely fictitious example that uses a
MyScheme::Handler object that responds to methods similar to an
IO::Handle.
# Define the four callback functions
sub match_uri {
my $uri = shift;
return $uri =~ /^myscheme:/; # trigger our callback group at a ’myscheme’ URIs
}
sub open_uri {
my $uri = shift;
my $handler = MyScheme::Handler->new($uri);
return $handler;
}
# The returned $buffer will be parsed by the libxml2 parser
sub read_uri {
my $handler = shift;
my $length = shift;
my $buffer;
read($handler, $buffer, $length);
return $buffer; # $buffer will be an empty string ’’ if read() is done
}
# Close the handle associated with the resource.
sub close_uri {
my $handler = shift;
close($handler);
}
# Register them with a instance of XML::LibXML::InputCallback
my $input_callbacks = XML::LibXML::InputCallback->new();
$input_callbacks->register_callbacks([ \&match_uri, \&open_uri,
\&read_uri, \&close_uri ] );
# Register the callback group at a parser instance
$parser->input_callbacks( $input_callbacks );
# $some_xml_file will be parsed using our callbacks
$parser->parse_file( $some_xml_file );
AUTHORS
Matt Sergeant, Christian Glahn, Petr Pajas
VERSION
1.66
COPYRIGHT
2001-2007, AxKit.com Ltd; 2002-2006 Christian Glahn; 2006-2008 Petr
Pajas, All rights reserved.
perl v5.8.8 2008-11-11 XML::LibXML::InputCallback(3)
