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File: Callback.pm
# Copyright 2001-2004 Six Apart. This code cannot be redistributed without
# permission from www.movabletype.org.
#
# $Id: Callback.pm,v 1.5 2004/04/29 02:42:57 ezra Exp $

package MT::Callback;

use strict;

use MT::ErrorHandler;
@MT::Callback::ISA = qw( MT::ErrorHandler );

sub new {
    my $class = shift;
    my ($this) = ref$_[0] ? @_ : {@_};
    bless $this, $class;
}

sub invoke {
    my $this = shift;
    return $this->{code}->($this, @_);
}

1;
__END__

=head1 NAME

MT::Callback - Movable Type wrapper for executable code with error state

=head1 SYNOPSIS


  $cb = new MT::Callback(<name>, sub { <callback code> } );

E<lt>nameE<gt> is a human-readable string which identifies the
surrounding body of code, for example the name of a plugin--the name
will help identify errors in the activity log.

=head1 CALLBACK CALLING CONVENTIONS

The parameters passed to each callback routine depends on the operation
in questions, as follows:

=over 4

=item * load(), load_iter()

Before loading items from the database, load() and load_iter()
call the callback registered as <class>::pre_load, allowing a callback
writer to munge the arguments before the database is
called.

An example E<lt>classE<gt>::pre_load might be written as follows:

    sub pre_load {
	my ($eh, $terms, $args) = @_;
	....
    }

Each object I<returned> by load() or by an iterator will,
before it is returned, be processeed by all callbacks registered as
E<lt>classE<gt>::post_load. An example E<lt>classE<gt>::post_load
function 
    
    sub post_load {
	my ($eh, $terms, $args, $obj) = @_;
        ....
    }

=item * save()

Callbacks for the save method might be written as follows:

    sub pre_save {
	my ($eh, $obj) = @_;
	....
    }

    sub post_save {
	my ($eh, $obj) = @_;
        ....
    }

By altering the $obj in pre_save, you can affect what data gets stored
in the database.

By creating pre_save and post_load functions which have inverse
effects on the object, you might be able to store data in the database
in a special form, while keeping the usual in-memory representation.

=item * remove()

E<lt>classE<gt>::pre_remove and E<lt>classE<gt>::post_remove
are called at the very beginning and very end of the respective
operations. The callback routine is called as follows:

    sub pre_remove {
        my ($eh, $obj) = @_;
        ....
    }

The signature for the post_remove operation is the same.

E<lt>classE<gt>::pre_remove_all and
E<lt>classE<gt>::post_remove_all are called at the very beginning and
very end of the respective operations, with no arguments except the
MT::Callback object.

=back

=head1 ERROR HANDLING

The first argument to any callback routine is an MT::ErrorHandler
object. You can use this object to return errors to MT. The errors
will be displayed in the MT Activity Log (accessible from the main
weblog list).

To use the error handler object, just use it's error() method:

    sub my_callback {
	my ($eh, $arg2, $arg3) = @_;
	
	....

	if (some_condition) {
	    return $eh->error("The foofiddle was invalid.");
	} 
	...
    }

=head1 AUTHOR & COPYRIGHTS

Please see the I<MT> manpage for author, copyright, and license information.

=cut