NAME¶
Moose::Cookbook::Basics::BinaryTree_BuilderAndLazyBuild - Builder methods and
lazy_build
VERSION¶
version 2.1213
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.
AUTHORS¶
- •
- Stevan Little <stevan.little@iinteractive.com>
- •
- Dave Rolsky <autarch@urth.org>
- •
- Jesse Luehrs <doy@tozt.net>
- •
- Shawn M Moore <code@sartak.org>
- •
- XXXX XXX'XX (Yuval Kogman) <nothingmuch@woobling.org>
- •
- Karen Etheridge <ether@cpan.org>
- •
- Florian Ragwitz <rafl@debian.org>
- •
- Hans Dieter Pearcey <hdp@weftsoar.net>
- •
- Chris Prather <chris@prather.org>
- •
- Matt S Trout <mst@shadowcat.co.uk>
COPYRIGHT AND LICENSE¶
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.