fix docs for MMA->new (RT78202)

[gitmo/Moose.git] / lib / Moose / Cookbook / Meta / PrivateOrPublic_MethodMetaclass.pod

package Moose::Cookbook::Meta::PrivateOrPublic_MethodMetaclass;

# ABSTRACT: A method metaclass for marking methods public or private

__END__


=pod

=head1 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',
      )
  );

=head1 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 C<policy> attribute, and
C<_add_policy_wrapper> method.

You'll note that we have to explicitly set the C<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 C<wrap>
method. We call that to build our object, but it does not include
subclass-specific attributes.

The C<_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 C<goto> so that the wrapper does not show up in the
call stack.

Finally, we replace the value of C<< $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 C<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 C<is_public> and C<is_private> methods on them to get
more information about the method.

=head1 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 L<Devel::Declare> to create full-blown new keywords in Perl.

=begin testing

package main;

use Test::Fatal;

my $user = MyApp::User->new( password => 'foo!' );

like( exception { $user->_reset_password },
qr/The MyApp::User::_reset_password method is private/,
    '_reset_password method dies if called outside MyApp::User class');

{
    package MyApp::User;

    sub run_reset { $_[0]->_reset_password }
}

$user->run_reset;

is( $user->password, 'reset', 'password has been reset' );

=end testing

=cut