Moose::Cookbook::BasMoose::Cookbook::Basics::BinaryTree_BuilderAndLazyBuild(3)NAMEMoose::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 v5.18.1Moose::Cookbook::Basics::BinaryTree_BuilderAndLazyBuild(3)
