Moose::Cookbook::Meta::GlobRef_InstanceMetaclass(3)

NAME
Moose::Cookbook::Meta::GlobRef_InstanceMetaclass - Creating a glob reference meta-instance class

VERSION
version 2.2012

SYNOPSIS
  package My::Meta::Instance;

  use Scalar::Util qw( weaken );
  use Symbol qw( gensym );

  use Moose::Role;

  sub create_instance {
      my $self = shift;
      my $sym = gensym();
      bless $sym, $self->_class_name;
  }

  sub clone_instance {
      my ( $self, $instance ) = @_;

      my $new_sym = gensym();
      %{*$new_sym} = %{*$instance};

      bless $new_sym, $self->_class_name;
  }

  sub get_slot_value {
      my ( $self, $instance, $slot_name ) = @_;
      return *$instance->{$slot_name};
  }

  sub set_slot_value {
      my ( $self, $instance, $slot_name, $value ) = @_;
      *$instance->{$slot_name} = $value;
  }

  sub deinitialize_slot {
      my ( $self, $instance, $slot_name ) = @_;
      delete *$instance->{$slot_name};
  }

  sub is_slot_initialized {
      my ( $self, $instance, $slot_name ) = @_;
      exists *$instance->{$slot_name};
  }

  sub weaken_slot_value {
      my ( $self, $instance, $slot_name ) = @_;
      weaken *$instance->{$slot_name};
  }

  sub inline_create_instance {
      my ( $self, $class_variable ) = @_;
      return 'do { my $sym = Symbol::gensym(); bless $sym, ' . $class_variable . ' }';
  }

  sub inline_slot_access {
      my ( $self, $instance, $slot_name ) = @_;
      return '*{' . $instance . '}->{' . $slot_name . '}';
  }

  package MyApp::User;

  use Moose;
  Moose::Util::MetaRole::apply_metaroles(
      for => __PACKAGE__,
      class_metaroles => {
          instance => ['My::Meta::Instance'],
      },
  );

  has 'name' => (
      is  => 'rw',
      isa => 'Str',
  );

  has 'email' => (
      is  => 'rw',
      isa => 'Str',
  );

DESCRIPTION
This recipe shows how to build your own meta-instance. The meta instance is the metaclass that creates object instances and helps manages access to attribute slots.

In this example, we're creating a meta-instance that is based on a glob reference rather than a hash reference. This example is largely based on the Piotr Roszatycki's MooseX::GlobRef module.

Our extension is a role which will be applied to Moose::Meta::Instance, which creates hash reference based objects. We need to override all the methods which make assumptions about the object's data structure.

The first method we override is "create_instance":

  sub create_instance {
      my $self = shift;
      my $sym = gensym();
      bless $sym, $self->_class_name;
  }

This returns an glob reference which has been blessed into our meta-instance's associated class.

We also override "clone_instance" to create a new array reference:

  sub clone_instance {
      my ( $self, $instance ) = @_;

      my $new_sym = gensym();
      %{*$new_sym} = %{*$instance};

      bless $new_sym, $self->_class_name;
  }

After that, we have a series of methods which mediate access to the object's slots (attributes are stored in "slots"). In the default instance class, these expect the object to be a hash reference, but we need to change this to expect a glob reference instead.

  sub get_slot_value {
      my ( $self, $instance, $slot_name ) = @_;
      *$instance->{$slot_name};
  }

This level of indirection probably makes our instance class slower than the default. However, when attribute access is inlined, this lookup will be cached:

  sub inline_slot_access {
      my ( $self, $instance, $slot_name ) = @_;
      return '*{' . $instance . '}->{' . $slot_name . '}';
  }

The code snippet that the "inline_slot_access" method returns will get "eval"'d once per attribute.

Finally, we use this meta-instance in our "MyApp::User" class:

  Moose::Util::MetaRole::apply_metaroles(
      for => __PACKAGE__,
      class_metaroles => {
          instance => ['My::Meta::Instance'],
      },
  );

We actually don't recommend the use of Moose::Util::MetaRole directly in your class in most cases. Typically, this would be provided by a Moose::Exporter-based module which handles applying the role for you.

CONCLUSION
This recipe shows how to create your own meta-instance class. It's unlikely that you'll need to do this yourself, but it's interesting to take a peek at how Moose works under the hood.

SEE ALSO
There are a few meta-instance class extensions on CPAN:

•   MooseX::Singleton

    This module extends the instance class in order to ensure that the object is a singleton. The instance it uses is still a blessed hash reference.

•   MooseX::GlobRef

    This module makes the instance a blessed glob reference. This lets you use a handle as an object instance.

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.