Moose::Cookbook::Basics::BinaryTree_BuilderAndLazyBuild - Builder methods and lazy_build |
Moose::Cookbook::Basics::BinaryTree_BuilderAndLazyBuild - Builder methods and lazy_build
version 2.1605
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 ); }
If you've already read
the Moose::Cookbook::Basics::BinaryTree_AttributeFeatures manpage, 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
the Moose::Manual::Attributes manpage for more details.
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 the Moose::Meta::Attribute manpage
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.
This software is copyright (c) 2006 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.
Moose::Cookbook::Basics::BinaryTree_BuilderAndLazyBuild - Builder methods and lazy_build |