InsideOutClass - A set of example metaclasses which implement the Inside-Out technique

  package Foo;

  use metaclass (
    ':attribute_metaclass' => 'InsideOutClass::Attribute',
    ':instance_metaclass'  => 'InsideOutClass::Instance'
  );

  __PACKAGE__->meta->add_attribute('foo' => (
      reader => 'get_foo',
      writer => 'set_foo'
  ));    

  sub new  {
      my $class = shift;
      $class->meta->new_object(@_);
  } 

  # now you can just use the class as normal

package # hide the package from PAUSE
    InsideOutClass::Attribute;

use strict;
use warnings;

our $VERSION = '0.02';

use Carp         'confess';
use Scalar::Util 'refaddr';

use base 'Class::MOP::Attribute';

sub initialize_instance_slot {
    my ($self, $meta_instance, $instance, $params) = @_;
    my $init_arg = $self->init_arg;
    # try to fetch the init arg from the %params ...
    my $val;        
    $val = $params->{$init_arg} if exists $params->{$init_arg};
    # if nothing was in the %params, we can use the 
    # attribute's default value (if it has one)
    if (!defined $val && defined $self->default) {
        $val = $self->default($instance);
    }
    my $_meta_instance = $self->associated_class->get_meta_instance;
    $_meta_instance->initialize_slot($instance, $self->name);
    $_meta_instance->set_slot_value($instance, $self->name, $val);
}

sub accessor_metaclass { 'InsideOutClass::Method::Accessor' }

package # hide the package from PAUSE
    InsideOutClass::Method::Accessor;
    
use strict;
use warnings;

our $VERSION = '0.01';

use Carp         'confess';
use Scalar::Util 'refaddr';

use base 'Class::MOP::Method::Accessor';

## Method generation helpers

sub _generate_accessor_method {
    my $attr       = (shift)->associated_attribute;
    my $meta_class = $attr->associated_class;  
    my $attr_name  = $attr->name;
    return sub {
        my $meta_instance = $meta_class->get_meta_instance;
        $meta_instance->set_slot_value($_[0], $attr_name, $_[1]) if scalar(@_) == 2;
        $meta_instance->get_slot_value($_[0], $attr_name);
    };
}

sub _generate_reader_method {
    my $attr       = (shift)->associated_attribute;
    my $meta_class = $attr->associated_class;  
    my $attr_name  = $attr->name;
    return sub { 
        confess "Cannot assign a value to a read-only accessor" if @_ > 1;
        $meta_class->get_meta_instance
                   ->get_slot_value($_[0], $attr_name); 
    }; 
}

sub _generate_writer_method {
    my $attr       = (shift)->associated_attribute;
    my $meta_class = $attr->associated_class;  
    my $attr_name  = $attr->name;
    return sub { 
        $meta_class->get_meta_instance
                   ->set_slot_value($_[0], $attr_name, $_[1]);
    };
}

sub _generate_predicate_method {
    my $attr       = (shift)->associated_attribute;
    my $meta_class = $attr->associated_class;  
    my $attr_name  = $attr->name;
    return sub { 
        defined $meta_class->get_meta_instance
                           ->get_slot_value($_[0], $attr_name) ? 1 : 0;
    };   
}

package # hide the package from PAUSE
    InsideOutClass::Instance;

use strict;
use warnings;

our $VERSION = '0.01';

use Carp         'confess';
use Scalar::Util 'refaddr';

use base 'Class::MOP::Instance';

sub create_instance {
	my ($self, $class) = @_;
        bless \(my $instance), $self->_class_name;
}

sub get_slot_value {
	my ($self, $instance, $slot_name) = @_;
	$self->associated_metaclass->get_package_symbol('%' . $slot_name)->{refaddr $instance};
}

sub set_slot_value {
	my ($self, $instance, $slot_name, $value) = @_;
	$self->associated_metaclass->get_package_symbol('%' . $slot_name)->{refaddr $instance} = $value;
}

sub initialize_slot {
    my ($self, $instance, $slot_name) = @_;
    $self->associated_metaclass->add_package_symbol(('%' . $slot_name) => {})
        unless $self->associated_metaclass->has_package_symbol('%' . $slot_name); 
    $self->associated_metaclass->get_package_symbol('%' . $slot_name)->{refaddr $instance} = undef;
}

sub is_slot_initialized {
	my ($self, $instance, $slot_name) = @_;
	return 0 unless $self->associated_metaclass->has_package_symbol('%' . $slot_name);
	return exists $self->associated_metaclass->get_package_symbol('%' . $slot_name)->{refaddr $instance} ? 1 : 0;
}

1;

__END__