/usr/lib/perl5/Moose/Cookbook/Basics/Recipe8.pod is in libmoose-perl 2.0401-1.
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 | package Moose::Cookbook::Basics::Recipe8;
# ABSTRACT: Builder methods and lazy_build
=pod
=head1 NAME
Moose::Cookbook::Basics::Recipe8 - Builder methods and lazy_build
=head1 VERSION
version 2.0401
=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::Recipe3>, 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 AUTHOR
Moose is maintained by the Moose Cabal, along with the help of many contributors. See L<Moose/CABAL> and L<Moose/CONTRIBUTORS> for details.
=head1 COPYRIGHT AND LICENSE
This software is copyright (c) 2011 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
__END__
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