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|
# PODNAME: Moose::Cookbook::Basics::BinaryTree_BuilderAndLazyBuild
# ABSTRACT: Builder methods and lazy_build
__END__
=pod
=encoding UTF-8
=head1 NAME
Moose::Cookbook::Basics::BinaryTree_BuilderAndLazyBuild - Builder methods and lazy_build
=head1 VERSION
version 2.4000
=head1 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 );
}
=head1 DESCRIPTION
If you've already read
L<Moose::Cookbook::Basics::BinaryTree_AttributeFeatures>, then this example
should look very familiar. In fact, all we've done here is replace the
attribute's C<default> parameter with a C<builder>.
In this particular case, the C<default> and C<builder> options act in
exactly the same way. When the C<left> or C<right> attribute is read,
Moose calls the builder method to initialize the attribute.
Note that Moose calls the builder method I<on the object which has the
attribute>. Here's an example:
my $tree = BinaryTree->new();
my $left = $tree->left();
When C<< $tree->left() >> is called, Moose calls C<<
$tree->_build_child_tree() >> in order to populate the C<left>
attribute. If we had passed C<left> to the original constructor, the
builder would not be called.
There are some differences between C<default> and C<builder>. Notably,
a builder is subclassable, and can be composed from a role. See
L<Moose::Manual::Attributes> for more details.
=head2 The lazy_build shortcut
The C<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 C<predicate> and C<clearer>, making them
both start with an underscore. The C<builder> method I<always> starts
with an underscore.
You can read more about C<lazy_build> in L<Moose::Meta::Attribute>
=head1 CONCLUSION
The C<builder> option is a more OO-friendly version of the C<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.
=head1 AUTHORS
=over 4
=item *
Stevan Little <stevan@cpan.org>
=item *
Dave Rolsky <autarch@urth.org>
=item *
Jesse Luehrs <doy@cpan.org>
=item *
Shawn M Moore <sartak@cpan.org>
=item *
יובל קוג'מן (Yuval Kogman) <nothingmuch@woobling.org>
=item *
Karen Etheridge <ether@cpan.org>
=item *
Florian Ragwitz <rafl@debian.org>
=item *
Hans Dieter Pearcey <hdp@cpan.org>
=item *
Chris Prather <chris@prather.org>
=item *
Matt S Trout <mstrout@cpan.org>
=back
=head1 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.
=cut
|
