new_instances

[gitmo/Class-MOP.git] / examples / LazyClass.pod

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

use strict;
use warnings;

use Carp 'confess';

our $VERSION = '0.03';

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

sub initialize_instance_slot {
    my ($self, $class, $meta_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();
    # try to fetch the init arg from the %params ...
    my $val;        
    $val = $params->{$init_arg} if exists $params->{$init_arg};
    # now add this to the instance structure
    # only if we have found a value at all
    $meta_instance->add_slot($self->name, $val) if defined $val;    
}


sub generate_accessor_method {
    my ($self, $attr_name) = @_;
    sub {
        if (scalar(@_) == 2) {
            $_[0]->{$attr_name} = $_[1];
        }
        else {
            if (!exists $_[0]->{$attr_name}) {
                my $attr = $self->associated_class->get_attribute($attr_name);
                $_[0]->{$attr_name} = $attr->has_default ? $attr->default($_[0]) : undef;           
            }            
            $_[0]->{$attr_name};            
        }
    };
}

sub generate_reader_method {
    my ($self, $attr_name) = @_; 
    sub {
        confess "Cannot assign a value to a read-only accessor" if @_ > 1;        
        if (!exists $_[0]->{$attr_name}) {
            my $attr = $self->associated_class->get_attribute($attr_name);
            $_[0]->{$attr_name} = $attr->has_default ? $attr->default($_[0]) : undef;           
        }
        $_[0]->{$attr_name};
    };   
}

1;

__END__

=pod

=head1 NAME

LazyClass - An example metaclass with lazy initialization

=head1 SYNOPSIS

  package BinaryTree;
  
  use metaclass 'Class::MOP::Class' => (
      ':attribute_metaclass' => 'LazyClass::Attribute'
  );
  
  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

=head1 DESCRIPTION

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. 

=head1 AUTHOR

Stevan Little E<lt>stevan@iinteractive.comE<gt>

=head1 COPYRIGHT AND LICENSE

Copyright 2006 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