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XML::LibXML::InputCallUser(Contributed 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
       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 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.

   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.

       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 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 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
       single 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::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 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 system
       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
           callback 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
       1.70

COPYRIGHT
       2001-2007, AxKit.com Ltd.

       2002-2006, Christian Glahn.

       2006-2009, Petr Pajas.

perl v5.12.1                      2009-10-07     XML::LibXML::InputCallback(3)
 

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