/ 
Moose-2.0204
River stage four • 2950 direct dependents • 4061 total dependents
/ Moose::Cookbook::Extending::Recipe2

NAME

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

VERSION

version 2.0204

SYNOPSIS

package MooseX::Debugging;

use Moose::Exporter;

Moose::Exporter->setup_import_methods(
    base_class_roles => ['MooseX::Debugging::Role::Object'],
);

package MooseX::Debugging::Role::Object;

use Moose::Role;

sub BUILD {}
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(
    base_class_roles => ['MooseX::Debugging::Role::Object'],
);

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 related to exports, but we need to have an import method to ensure that our init_meta method is called. The init_meta is created by setup_import_methods for us, since we passed the base_class_roles parameter. The generated init_meta will in turn call Moose::Util::MetaRole::apply_base_class_roles.

sub BUILD {}
after BUILD => sub {
    ...
};

Due to the way role composition currently works, if the class that a role is composed into contains a BUILD method, then that will override the BUILD method in any roles it composes, which is typically not what you want. Using a method modifier on BUILD avoids this issue, since method modifiers compose together rather than being overridden. Method modifiers require that a method exists in order to wrap, however, so we also provide a stub method to wrap if no BUILD method exists in the class.

AUTHOR

Stevan Little <stevan@iinteractive.com>

COPYRIGHT AND LICENSE

This software is copyright (c) 2011 by Infinity Interactive, Inc..

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.