Rename Extending::Recipe2 to Extending::Debugging_BaseClassRole

[gitmo/Moose.git] / lib / Moose / Cookbook / Extending / Debugging_BaseClassRole.pod

package Moose::Cookbook::Extending::Debugging_BaseClassRole;

# ABSTRACT: Providing a role for the base object class

__END__


=pod

=begin testing-SETUP

use Test::Requires {
    'Test::Output' => '0',
};

=end testing-SETUP

=head1 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";
  };

=head1 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 L<Moose::Exporter> and
L<Moose::Util::MetaRole>, we ensure that when a module does C<S<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 C<import> method in the C<MooseX::Debugging> package. Since we
are not actually exporting anything, we do not pass C<setup_import_methods>
any parameters related to exports, but we need to have an C<import> method to
ensure that our C<init_meta> method is called. The C<init_meta> is created by
C<setup_import_methods> for us, since we passed the C<base_class_roles>
parameter. The generated C<init_meta> will in turn call
L<Moose::Util::MetaRole::apply_base_class_roles|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 C<BUILD> method, then that will override the C<BUILD>
method in any roles it composes, which is typically not what you want. Using a
method modifier on C<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 C<BUILD> method exists in the class.

=begin testing

{
    package Debugged;

    use Moose;
    MooseX::Debugging->import;
}

stderr_is(
    sub { Debugged->new },
    "Made a new Debugged object\n",
    'got expected output from debugging role'
);

=end testing

=cut