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Moose::Cookbook::Extending::Recipe2.3pm

NAME

Moose::Cookbook::Extending::Recipe2 - Providing a role for the base object class

SYNOPSIS

   package MooseX::Debugging;
 
   use strict;
   use warnings;
 
   use Moose ();
   use Moose::Exporter;
   use Moose::Util::MetaRole;
 
   Moose::Exporter->setup_import_methods;
 
   sub init_meta {
       shift;
       my %options = @_;
 
       Moose->init_meta(%options);
 
       Moose::Util::MetaRole::apply_base_class_roles(
           for_class => $options{for_class},
           roles     => ['MooseX::Debugging::Role::Object'],
       );
   }
 
   package MooseX::Debugging::Role::Object;
 
   use Moose::Role;
 
   after 'BUILD' => sub {
       my $self = shift;
 
       warn "Made a new " . ref $self . " object\n";
   };
 
 

DESCRIPTION

In this example, we provide a role for the base object class that adds some simple debugging output. Every time an object is created, it spits out a warning saying what type of object it was.

Obviously, a real debugging role would do something more interesting, but this recipe is all about how we apply that role.

In this case, with the combination of Moose::Exporter and Moose::Util::MetaRole, we ensure that when a module does "use MooseX::Debugging", it automatically gets the debugging role applied to its base object class.

There are a few pieces of code worth looking at more closely.

   Moose::Exporter->setup_import_methods;
 
 

This creates an "import" method in the "MooseX::Debugging" package. Since we are not actually exporting anything, we do not pass "setup_import_methods" any parameters. However, we need to have an "import" method to ensure that our "init_meta" method is called.

Then in our "init_meta" method we have this line:

       Moose->init_meta(%options);
 
 

This is a bit of boilerplate that almost every extension will use. This ensures that the caller has a normal Moose metaclass before we go and add traits to it.

The "Moose->init_meta" method does ensures that the caller has a sane metaclass, and we don't want to replicate that logic in our extension. If the "Moose->init_meta" was already called (because the caller did "use Moose" before using our extension), then calling "Moose->init_meta" again is effectively a no-op.

AUTHOR

Dave Rolsky <autarch@urth.org>

COPYRIGHT AND LICENSE

Copyright 2009 by Infinity Interactive, Inc.

<http://www.iinteractive.com>

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