LazyClass - An example metaclass with lazy initialization

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NAME

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LazyClass - An example metaclass with lazy initialization

SYNOPSIS

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  package BinaryTree;

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

  BinaryTree->meta->add_attribute('node' => (
      accessor => 'node',
      init_arg => ':node'
  ));

  BinaryTree->meta->add_attribute('left' => (
      reader  => 'left',
      default => sub { BinaryTree->new() }
  ));

  BinaryTree->meta->add_attribute('right' => (
      reader  => 'right',
      default => sub { BinaryTree->new() }    
  ));    

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

  # ... later in code

  my $btree = BinaryTree->new();
  # ... $btree is an empty hash, no keys are initialized yet

DESCRIPTION

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This is an example metclass in which all attributes are created lazily. This means that no entries are made in the instance HASH until the last possible moment.

The example above of a binary tree is a good use for such a metaclass because it allows the class to be space efficient without complicating the programing of it. This would also be ideal for a class which has a large amount of attributes, several of which are optional.

AUTHORS

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Stevan Little <stevan@iinteractive.com>

Yuval Kogman <nothingmuch@woobling.com>

COPYRIGHT AND LICENSE

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Copyright 2006-2008 by Infinity Interactive, Inc.

http://www.iinteractive.com

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

Moose documentation Contained in the Moose distribution. 
package # hide the package from PAUSE
    LazyClass::Attribute;

use strict;
use warnings;

use Carp 'confess';

our $VERSION = '0.05';

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

sub initialize_instance_slot {
    my ($self, $meta_instance, $instance, $params) = @_;

    # if the attr has an init_arg, use that, otherwise,
    # use the attributes name itself as the init_arg
    my $init_arg = $self->init_arg();

	if ( exists $params->{$init_arg} ) {
		my $val = $params->{$init_arg};
		$meta_instance->set_slot_value($instance, $self->name, $val);
	}
}

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

package # hide the package from PAUSE
    LazyClass::Method::Accessor;

use strict;
use warnings;

use Carp 'confess';

our $VERSION = '0.01';

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

sub _generate_accessor_method {
    my $attr = (shift)->associated_attribute;

	my $attr_name = $attr->name;
	my $meta_instance = $attr->associated_class->get_meta_instance;

    sub {
        if (scalar(@_) == 2) {
			$meta_instance->set_slot_value($_[0], $attr_name, $_[1]);
        }
        else {
			unless ( $meta_instance->is_slot_initialized($_[0], $attr_name) ) {
				my $value = $attr->has_default ? $attr->default($_[0]) : undef;
				$meta_instance->set_slot_value($_[0], $attr_name, $value);
            }

            $meta_instance->get_slot_value($_[0], $attr_name);
        }
    };
}

sub _generate_reader_method {
    my $attr = (shift)->associated_attribute;

	my $attr_name = $attr->name;
	my $meta_instance = $attr->associated_class->get_meta_instance;

    sub {
        confess "Cannot assign a value to a read-only accessor" if @_ > 1;        

		unless ( $meta_instance->is_slot_initialized($_[0], $attr_name) ) {
			my $value = $attr->has_default ? $attr->default($_[0]) : undef;
			$meta_instance->set_slot_value($_[0], $attr_name, $value);
		}

		$meta_instance->get_slot_value($_[0], $attr_name);
    };   
}

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

use strict;
use warnings;

our $VERSION = '0.01';

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

sub initialize_all_slots {}

1;

__END__