NAME

       Moose::Cookbook::Basics::BinaryTree_BuilderAndLazyBuild - Builder methods and lazy_build

VERSION

       version 2.1005

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::BinaryTree_AttributeFeatures, 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

       Moose is maintained by the Moose Cabal, along with the help of many contributors. See
       "CABAL" in Moose and "CONTRIBUTORS" in Moose for details.

COPYRIGHT AND LICENSE

       This software is copyright (c) 2013 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 v5.18.1                         Moose::Cookbook::Basics::BinaryTree_BuilderAndLazyBuild(3pm)