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Moose::Cookbook::Basics::Recipe8(3)  User Contributed Perl Documentation Moose::Cookbook::Basics::Recipe8(3)



NAME
       Moose::Cookbook::Basics::Recipe8 - Builder methods and lazy_build

VERSION
       version 2.0205

SYNOPSIS
         package BinaryTree;
         use Moose;

         has 'node' => (is => 'rw', isa => 'Any');

         has 'parent' => (
             is        => 'rw',
             isa       => 'BinaryTree',
             predicate => 'has_parent',
             weak_ref  => 1,
         );

         has 'left' => (
             is        => 'rw',
             isa       => 'BinaryTree',
             predicate => 'has_left',
             lazy      => 1,
             builder   => '_build_child_tree',
         );

         has 'right' => (
             is        => 'rw',
             isa       => 'BinaryTree',
             predicate => 'has_right',
             lazy      => 1,
             builder   => '_build_child_tree',
         );

         before 'right', 'left' => sub {
             my ($self, $tree) = @_;
             $tree->parent($self) if defined $tree;
         };

         sub _build_child_tree {
             my $self = shift;

             return BinaryTree->new( parent => $self );
         }

DESCRIPTION
       If you've already read Moose::Cookbook::Basics::Recipe3, then this example should look very familiar.
       In fact, all we've done here is replace the attribute's "default" parameter with a "builder".

       In this particular case, the "default" and "builder" options act in exactly the same way. When the
       "left" or "right" attribute is read, Moose calls the builder method to initialize the attribute.

       Note that Moose calls the builder method on the object which has the attribute. Here's an example:

         my $tree = BinaryTree->new();

         my $left = $tree->left();

       When "$tree->left()" is called, Moose calls "$tree->_build_child_tree()" in order to populate the
       "left" attribute. If we had passed "left" to the original constructor, the builder would not be
       called.

       There are some differences between "default" and "builder". Notably, a builder is subclassable, and
       can be composed from a role. See Moose::Manual::Attributes for more details.

   The lazy_build shortcut
       The "lazy_build" attribute option can be used as sugar to specify a whole set of attribute options at
       once:

         has 'animal' => (
             is         => 'ro',
             isa        => 'Animal',
             lazy_build => 1,
         );

       This is a shorthand for:

         has 'animal' => (
             is        => 'ro',
             isa       => 'Animal',
             required  => 1,
             lazy      => 1,
             builder   => '_build_animal',
             predicate => 'has_animal',
             clearer   => 'clear_animal',
         );

       If your attribute starts with an underscore, Moose is smart and will do the right thing with the
       "predicate" and "clearer", making them both start with an underscore. The "builder" method always
       starts with an underscore.

       You can read more about "lazy_build" in Moose::Meta::Attribute

CONCLUSION
       The "builder" option is a more OO-friendly version of the "default" functionality. It also separates
       the default-generating code into a well-defined method. Sprinkling your attribute definitions with
       anonymous subroutines can be quite ugly and hard to follow.

AUTHOR
       Stevan Little <stevan@iinteractive.com>

COPYRIGHT AND LICENSE
       This software is copyright (c) 2011 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.



perl v5.12.5                                     2011-09-06              Moose::Cookbook::Basics::Recipe8(3)