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✏️ Editing: InputCallback.pod

=head1 NAME

XML::LibXML::InputCallback - XML::LibXML Class for Input Callbacks

=head1 SYNOPSIS

use XML::LibXML;

=head1 DESCRIPTION

You may get unexpected results if you are trying to load external documents
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 XML::LibXML has to get something other
than externally parsed entities from somewhere. They are implemented 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.

=head2 How does XML::LibXML::InputCallback work?

The libxml2 library offers a callback implementation as global functions 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, operations 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.

=head3 Callback Groups

The libxml2 input callbacks come in groups. One group contains a URI matcher (I<<<<<< match >>>>>>), a data stream constructor (I<<<<<< open >>>>>>), a data stream reader (I<<<<<< read >>>>>>), and a data stream destructor (I<<<<<< close >>>>>>). The callbacks can be manipulated on a per group basis only.

=head3 The Parser Process

The parser process works on an 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 I<<<<<< file://* >>>>>>, I<<<<<< file:://*.gz >>>>>>, I<<<<<< http://* >>>>>> and I<<<<<< ftp://* >>>>>> are registered.

Callback groups in the callback stack are processed from top to bottom, 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 registered
callback group will handle a URI - if they will not, the URI will be
interpreted as I<<<<<< file://URI >>>>>>. To handle a URI, the I<<<<<< 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 I<<<<<< open >>>>>> callback, which should return a I<<<<<< reference >>>>>> to the data stream if it successfully opened the file, '0' otherwise. If
opening the stream was successful, the I<<<<<< read >>>>>> callback will be called repeatedly until it returns an empty string. After the
read callback, the I<<<<<< close >>>>>> callback will be called to close the stream.

=head3 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 single
callback group. The Perl implementation however allows one to manage different
callback stacks on a per libxml2-parser basis.

=head2 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 );

=head2 What about the old callback system prior to XML::LibXML::InputCallback?

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 I<<<<<< 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 I<<<<<< 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
system can only deal with the I<<<<<< open >>>>>> callback returning a reference!

=head1 INTERFACE DESCRIPTION

=head2 Global Variables

=over 4

=item $_CUR_CB

Stores the current callback and can be used as shortcut to access the callback
stack.

=item @_GLOBAL_CALLBACKS

Stores all callback groups for the current parser process.

=item @_CB_STACK

Stores the currently used callback group. Used to prevent parser errors when
dealing with nested XML data.

=back

=head2 Global Callbacks

=over 4

=item _callback_match

Implements the interface for the I<<<<<< match >>>>>> callback at C-level and for the selection of the callback group from the
callbacks defined at the Perl-level.

=item _callback_open

Forwards the I<<<<<< open >>>>>> callback from libxml2 to the corresponding callback function at the Perl-level.

=item _callback_read

Forwards the read request to the corresponding callback function at the
Perl-level and returns the result to libxml2.

=item _callback_close

Forwards the I<<<<<< close >>>>>> callback from libxml2 to the corresponding callback function at the
Perl-level..

=back

=head2 Class methods

=over 4

=item new()

A simple constructor.

=item register_callbacks( [ $match_cb, $open_cb, $read_cb, $close_cb ])

The four callbacks I<<<<<< have >>>>>> to be given as array reference in the above order I<<<<<< match >>>>>>, I<<<<<< open >>>>>>, I<<<<<< read >>>>>>, I<<<<<< close >>>>>>!

=item unregister_callbacks( [ $match_cb, $open_cb, $read_cb, $close_cb ])

With no arguments given, C<<<<<< 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 I<<<<<< match >>>>>> callback and deleted from the callback stack. Note that if several identical I<<<<<< match >>>>>> callbacks are defined in different callback groups, ALL of them will be deleted
from the stack.

=item init_callbacks( $parser )

Initializes the callback system for the provided parser before starting a
parsing process.

=item cleanup_callbacks()

Resets global variables and the libxml2 callback stack.

=item lib_init_callbacks()

Used internally for callback registration at C-level.

=item lib_cleanup_callbacks()

Used internally for callback resetting at the C-level.

=back

=head1 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 );

=head1 AUTHORS

Matt Sergeant,
Christian Glahn,
Petr Pajas

=head1 VERSION

2.0210

=head1 COPYRIGHT

2001-2007, AxKit.com Ltd.

2002-2006, Christian Glahn.

2006-2009, Petr Pajas.

=cut

=head1 LICENSE

This program is free software; you can redistribute it and/or modify it under
the same terms as Perl itself.

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