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.IX Title "Moose::Cookbook::Extending::Debugging_BaseClassRole 3"
.TH Moose::Cookbook::Extending::Debugging_BaseClassRole 3 "2021-11-07" "perl v5.26.3" "User Contributed Perl Documentation"
.\" For nroff, turn off justification. Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
Moose::Cookbook::Extending::Debugging_BaseClassRole \- Providing a role for the base object class
.SH "VERSION"
.IX Header "VERSION"
version 2.2201
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& package MooseX::Debugging;
\&
\& use Moose::Exporter;
\&
\& Moose::Exporter\->setup_import_methods(
\& base_class_roles => [\*(AqMooseX::Debugging::Role::Object\*(Aq],
\& );
\&
\& package MooseX::Debugging::Role::Object;
\&
\& use Moose::Role;
\&
\& sub BUILD {}
\& after BUILD => sub {
\& my $self = shift;
\&
\& warn "Made a new " . ( ref $self ) . " object\en";
\& };
.Ve
.SH "DESCRIPTION"
.IX Header "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.
.PP
Obviously, a real debugging role would do something more interesting,
but this recipe is all about how we apply that role.
.PP
In this case, with the combination of Moose::Exporter and
Moose::Util::MetaRole, we ensure that when a module does \f(CW\*(C`use MooseX::Debugging\*(C'\fR, it automatically gets the debugging role applied
to its base object class.
.PP
There are a few pieces of code worth looking at more closely.
.PP
.Vb 3
\& Moose::Exporter\->setup_import_methods(
\& base_class_roles => [\*(AqMooseX::Debugging::Role::Object\*(Aq],
\& );
.Ve
.PP
This creates an \f(CW\*(C`import\*(C'\fR method in the \f(CW\*(C`MooseX::Debugging\*(C'\fR package. Since we
are not actually exporting anything, we do not pass \f(CW\*(C`setup_import_methods\*(C'\fR
any parameters related to exports, but we need to have an \f(CW\*(C`import\*(C'\fR method to
ensure that our \f(CW\*(C`init_meta\*(C'\fR method is called. The \f(CW\*(C`init_meta\*(C'\fR is created by
\&\f(CW\*(C`setup_import_methods\*(C'\fR for us, since we passed the \f(CW\*(C`base_class_roles\*(C'\fR
parameter. The generated \f(CW\*(C`init_meta\*(C'\fR will in turn call
Moose::Util::MetaRole::apply_base_class_roles.
.PP
.Vb 4
\& sub BUILD {}
\& after BUILD => sub {
\& ...
\& };
.Ve
.PP
Due to the way role composition currently works, if the class that a role is
composed into contains a \f(CW\*(C`BUILD\*(C'\fR method, then that will override the \f(CW\*(C`BUILD\*(C'\fR
method in any roles it composes, which is typically not what you want. Using a
method modifier on \f(CW\*(C`BUILD\*(C'\fR 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 \f(CW\*(C`BUILD\*(C'\fR method exists in the class.
.SH "AUTHORS"
.IX Header "AUTHORS"
.IP "\(bu" 4
Stevan Little <stevan@cpan.org>
.IP "\(bu" 4
Dave Rolsky <autarch@urth.org>
.IP "\(bu" 4
Jesse Luehrs <doy@cpan.org>
.IP "\(bu" 4
Shawn M Moore <sartak@cpan.org>
.IP "\(bu" 4
יובל קוג'מן (Yuval Kogman) <nothingmuch@woobling.org>
.IP "\(bu" 4
Karen Etheridge <ether@cpan.org>
.IP "\(bu" 4
Florian Ragwitz <rafl@debian.org>
.IP "\(bu" 4
Hans Dieter Pearcey <hdp@cpan.org>
.IP "\(bu" 4
Chris Prather <chris@prather.org>
.IP "\(bu" 4
Matt S Trout <mstrout@cpan.org>
.SH "COPYRIGHT AND LICENSE"
.IX Header "COPYRIGHT AND LICENSE"
This software is copyright (c) 2006 by Infinity Interactive, Inc.
.PP
This is free software; you can redistribute it and/or modify it under
the same terms as the Perl 5 programming language system itself.