Moose::Cookbook::Extending::Debugging_BaseClassRole - Providing a role for the base object class
version 2.2206
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"; };
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.
use MooseX::Debugging
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.
import
MooseX::Debugging
setup_import_methods
init_meta
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.
BUILD
Stevan Little <stevan@cpan.org>
Dave Rolsky <autarch@urth.org>
Jesse Luehrs <doy@cpan.org>
Shawn M Moore <sartak@cpan.org>
יובל קוג'מן (Yuval Kogman) <nothingmuch@woobling.org>
Karen Etheridge <ether@cpan.org>
Florian Ragwitz <rafl@debian.org>
Hans Dieter Pearcey <hdp@cpan.org>
Chris Prather <chris@prather.org>
Matt S Trout <mstrout@cpan.org>
This software is copyright (c) 2006 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.
To install Moose, copy and paste the appropriate command in to your terminal.
cpanm
cpanm Moose
CPAN shell
perl -MCPAN -e shell install Moose
For more information on module installation, please visit the detailed CPAN module installation guide.