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XML::LibXML::InputCallback(3)        User Contributed Perl Documentation       XML::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 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 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.. Perllevel..
           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( $parser )
           Initializes the callback system for the provided parser before starting 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
       1.88

COPYRIGHT
       2001-2007, AxKit.com Ltd.

       2002-2006, Christian Glahn.

       2006-2009, Petr Pajas.



perl v5.12.5                                     2011-09-21                    XML::LibXML::InputCallback(3)