I'm going to take away jnapiorkowski's tab key!

[gitmo/Moose.git] / lib / Moose / Util / MetaRole.pm

package Moose::Util::MetaRole;

use strict;
use warnings;

use List::MoreUtils qw( all );

sub apply_metaclass_roles {
    my %options = @_;

    my $for = $options{for_class};

    my $meta = _make_new_metaclass( $for, \%options );

    for my $tor_class ( grep { $options{ $_ . '_roles' } }
        qw( constructor_class destructor_class ) ) {

        my $class = _make_new_class(
            $meta->$tor_class(),
            $options{ $tor_class . '_roles' }
        );

        $meta->$tor_class($class);
    }

    return $meta;
}

sub _make_new_metaclass {
    my $for     = shift;
    my $options = shift;

    return $for->meta()
        unless grep { exists $options->{ $_ . '_roles' } }
            qw(
            metaclass
            attribute_metaclass
            method_metaclass
            instance_metaclass
    );

    my $new_metaclass
        = _make_new_class( ref $for->meta(), $options->{metaclass_roles} );

    my $old_meta = $for->meta();

    # This could get called for a Moose::Meta::Role as well as a Moose::Meta::Class
    my %classes = map {
        $_ => _make_new_class( $old_meta->$_(), $options->{ $_ . '_roles' } )
        }
        grep { $old_meta->can($_) }
        qw(
        attribute_metaclass
        method_metaclass
        instance_metaclass
    );

    return $new_metaclass->reinitialize( $for, %classes );
}

sub apply_base_class_roles {
    my %options = @_;

    my $for = $options{for_class};

    my $meta = $for->meta();

    my $new_base = _make_new_class(
        $for,
        $options{roles},
        [ $meta->superclasses() ],
    );

    $meta->superclasses($new_base)
        if $new_base ne $meta->name();
}

sub _make_new_class {
    my $existing_class = shift;
    my $roles          = shift;
    my $superclasses   = shift || [$existing_class];

    return $existing_class unless $roles;

    my $meta = $existing_class->meta();

    return $existing_class
        if $meta->can('does_role') && all { $meta->does_role($_) } @{$roles};

    return Moose::Meta::Class->create_anon_class(
        superclasses => $superclasses,
        roles        => $roles,
        cache        => 1,
    )->name();
}

1;

__END__

=head1 NAME

Moose::Util::MetaRole - Apply roles to any metaclass, as well as the object base class

=head1 SYNOPSIS

  package MyApp::Moose;

  use strict;
  use warnings;

  use Moose ();
  use Moose::Exporter;
  use Moose::Util::Meta::Role;

  use MyApp::Role::Meta::Class;
  use MyApp::Role::Meta::Method::Constructor;
  use MyApp::Role::Object;

  Moose::Exporter->setup_import_methods( also => 'Moose' );

  sub init_meta {
      shift;
      my %options = @_;

      Moose->init_meta(%options);

      Moose::Util::MetaRole::apply_metaclass_roles(
          for_class               => $options{for_class},
          metaclass_roles         => ['MyApp::Role::Meta::Class'],
          constructor_class_roles => ['MyApp::Role::Meta::Method::Constructor'],
      );

      Moose::Util::MetaRole::apply_base_class_roles(
          for_class => $options{for_class},
          roles     => ['MyApp::Role::Object'],
      );

      return $options{for_class}->meta();
  }

=head1 DESCRIPTION

B<The whole concept behind this module is still considered
experimental, and it could go away in the future!>

This utility module is designed to help authors of Moose extensions
write extensions that are able to cooperate with other Moose
extensions. To do this, you must write your extensions as roles, which
can then be dynamically applyied to the caller's metaclasses.

This module makes sure to preserve any existing superclasses and roles
already set for the meta objects, which means that any number of
extensions can apply roles in any order.

=head1 USAGE

B<It is very important that you only call this module's functions when
your module is imported by the caller>. The process of applying roles
to the metaclass reinitializes the metaclass object, which wipes out
any existing attributes already defined. However, as long as you do
this when your module is imported, the caller should not have any
attributes defined yet.

The easiest way to ensure that this happens is to use
L<Moose::Exporter> and provide an C<init_meta> method that will be
called when imported.

=head1 FUNCTIONS

This module provides two functions.

=head2 apply_metaclass_roles( ... )

This function will apply roles to one or more metaclasses for the
specified class. It accepts the following parameters:

=over 4

=item * for_class => $name

This specifies the class for which to alter the meta classes.

=item * metaclass_roles => \@roles

=item * attribute_metaclass_roles => \@roles

=item * method_metaclass_roles => \@roles

=item * instance_metaclass_roles => \@roles

=item * constructor_class_roles => \@roles

=item * destructor_class_roles => \@roles

These parameter all specify one or more roles to be applied to the
specified metaclass. You can pass any or all of these parameters at
once.

=back

=head2 apply_base_class_roles( for_class => $class, roles => \@roles )

This function will apply the specified roles to the object's base class.

=head1 PROBLEMS WITH METACLASS ROLES AND SUBCLASS

Because of the way this module works, there is an ordering problem
which occurs in certain situations. This sequence of events causes an
error:

=over 4

=item 1.

There is a class (ClassA) which uses some extension(s) that apply
roles to the metaclass.

=item 2.

You have another class (ClassB) which wants to subclass ClassA and
apply some more extensions.

=back

Normally, the call to C<extends> will happen at run time, I<after> the
additional extensions are applied. This causes an error when we try to
make the metaclass for ClassB compatible with the metaclass for
ClassA.

We hope to be able to fix this in the future.

For now the workaround is for ClassB to make sure it extends ClassA
I<before> it loads extensions:

  package ClassB;

  use Moose;

  BEGIN { extends 'ClassA' }

  use MooseX::SomeExtension;

=head1 AUTHOR

Dave Rolsky E<lt>autarch@urth.orgE<gt>

=head1 COPYRIGHT AND LICENSE

Copyright 2008 by Infinity Interactive, Inc.

L<http://www.iinteractive.com>

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

=cut