Moose::Cookbook::Meta::PrivateOrPublic_MethodMetaclass(3)

NAME
Moose::Cookbook::Meta::PrivateOrPublic_MethodMetaclass - A method metaclass for marking methods public or private

VERSION
version 2.2012

SYNOPSIS
  package MyApp::Meta::Method::PrivateOrPublic;

  use Moose;
  use Moose::Util::TypeConstraints;

  extends 'Moose::Meta::Method';

  has '_policy' => (
      is       => 'ro',
      isa      => enum( [ qw( public private ) ] ),
      default  => 'public',
      init_arg => 'policy',
  );

  sub new {
      my $class   = shift;
      my %options = @_;

      my $self = $class->SUPER::wrap(%options);

      $self->{_policy} = $options{policy};

      $self->_add_policy_wrapper;

      return $self;
  }

  sub _add_policy_wrapper {
      my $self = shift;

      return if $self->is_public;

      my $name      = $self->name;
      my $package   = $self->package_name;
      my $real_body = $self->body;

      my $body = sub {
          die "The $package\::$name method is private"
              unless ( scalar caller() ) eq $package;

          goto &{$real_body};
      };

      $self->{body} = $body;
  }

  sub is_public  { $_[0]->_policy eq 'public' }
  sub is_private { $_[0]->_policy eq 'private' }

  package MyApp::User;

  use Moose;

  has 'password' => ( is => 'rw' );

  __PACKAGE__->meta()->add_method(
      '_reset_password',
      MyApp::Meta::Method::PrivateOrPublic->new(
          name         => '_reset_password',
          package_name => __PACKAGE__,
          body         => sub { $_[0]->password('reset') },
          policy       => 'private',
      )
  );

DESCRIPTION
This example shows a custom method metaclass that models public versus private methods. If a method is defined as private, it adds a wrapper around the method which dies unless it is called from the class where it was defined.

The way the method is added to the class is rather ugly. If we wanted to make this a real feature, we'd probably want to add some sort of sugar to allow us to declare private methods, but that is beyond the scope of this recipe. See the Extending recipes for more on this topic.

The core of our custom class is the "policy" attribute, and "_add_policy_wrapper" method.

You'll note that we have to explicitly set the "policy" attribute in our constructor:

      $self->{_policy} = $options{policy};

That is necessary because Moose metaclasses do not use the meta API to create objects. Most Moose classes have a custom "inlined" constructor for speed.

In this particular case, our parent class's constructor is the "wrap" method. We call that to build our object, but it does not include subclass-specific attributes.

The "_add_policy_wrapper" method is where the real work is done. If the method is private, we construct a wrapper around the real subroutine which checks that the caller matches the package in which the subroutine was created.

If they don't match, it dies. If they do match, the real method is called. We use "goto" so that the wrapper does not show up in the call stack.

Finally, we replace the value of "$self->{body}". This is another case where we have to do something a bit gross because Moose does not use Moose for its own implementation.

When we pass this method object to the metaclass's "add_method" method, it will take the method body and make it available in the class.

Finally, when we retrieve these methods via the introspection API, we can call the "is_public" and "is_private" methods on them to get more information about the method.

SUMMARY
A custom method metaclass lets us add both behavior and meta-information to methods. Unfortunately, because the Perl interpreter does not provide easy hooks into method declaration, the API we have for adding these methods is not very pretty.

That can be improved with custom Moose-like sugar, or even by using a tool like Devel::Declare to create full-blown new keywords in Perl.

AUTHORS

•   Stevan Little <stevan.little@iinteractive.com>

•   Dave Rolsky <autarch@urth.org>

•   Jesse Luehrs <doy@tozt.net>

•   Shawn M Moore <code@sartak.org>

•   יובל קוג'מן (Yuval Kogman) <nothingmuch@woobling.org>

•   Karen Etheridge <ether@cpan.org>

•   Florian Ragwitz <rafl@debian.org>

•   Hans Dieter Pearcey <hdp@weftsoar.net>

•   Chris Prather <chris@prather.org>

•   Matt S Trout <mst@shadowcat.co.uk>

COPYRIGHT AND LICENSE
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.