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1 | |
2 | =pod |
3 | |
4 | =head1 NAME |
5 | |
6 | Moose::Cookbook::Recipe9 - Builder methods and lazy_build |
7 | |
8 | =head1 SYNOPSIS |
9 | |
10 | package BinaryTree; |
11 | use Moose; |
12 | |
13 | has 'node' => (is => 'rw', isa => 'Any'); |
14 | |
15 | has 'parent' => ( |
16 | is => 'rw', |
17 | isa => 'BinaryTree', |
18 | predicate => 'has_parent', |
19 | weak_ref => 1, |
20 | ); |
21 | |
22 | has 'left' => ( |
23 | is => 'rw', |
24 | isa => 'BinaryTree', |
25 | predicate => 'has_left', |
26 | lazy => 1, |
27 | builder => '_build_child_tree', |
28 | ); |
29 | |
30 | has 'right' => ( |
31 | is => 'rw', |
32 | isa => 'BinaryTree', |
33 | predicate => 'has_right', |
34 | lazy => 1, |
35 | builder => '_build_child_tree', |
36 | ); |
37 | |
38 | before 'right', 'left' => sub { |
39 | my ($self, $tree) = @_; |
40 | $tree->parent($self) if defined $tree; |
41 | }; |
42 | |
43 | sub _build_child_tree { |
44 | my $self = shift; |
45 | |
46 | return BinaryTree->new( parent => $self ); |
47 | } |
48 | |
49 | =head1 DESCRIPTION |
50 | |
51 | If you've already read L<Moose::Cookbook::Recipe3>, then this example |
52 | should look awfully familiar. In fact, all we've done here is replace |
53 | the attribute C<default> with a C<builder> method. |
54 | |
55 | In this particular case, the C<default> and C<builder> act exactly the |
56 | same. When the C<left> or C<right> attribute is first accessed before |
57 | it has been set, Moose will call the specified C<builder> method to |
58 | populate the attribute. |
59 | |
60 | =head2 Subclassable |
61 | |
62 | There are some differences between C<default> and C<builder>. Because |
63 | C<builder> is called I<by name>, it goes through Perl's normal |
64 | inheritance system. This means that builder methods are both |
65 | inheritable and overrideable. |
66 | |
67 | For example, we might make a C<BinaryTree> subclass: |
68 | |
69 | package TrinaryTree; |
70 | use Moose; |
71 | |
72 | extends 'BinaryTree'; |
73 | |
74 | has 'middle' => ( |
75 | is => 'rw', |
76 | isa => 'BinaryTree', |
77 | predicate => 'has_middle', |
78 | lazy => 1, |
79 | builder => '_build_child_tree', |
80 | ); |
81 | |
82 | This doesn't quite work though. If you look closely at the |
83 | C<_build_child_tree> method defined in C<BinaryTree>, you'll notice |
84 | that it hard-codes a class name. Naughty us! |
85 | |
86 | Also, as a bonus, we'll pass C<@_> through, so subclasses can override |
87 | the method to pass additional options to the constructor. |
88 | |
89 | Good object-oriented code should allow itself to be subclassed |
90 | gracefully. Let's tweak C<_build_child_tree>: |
91 | |
92 | sub _build_child_tree |
93 | my $self = shift; |
94 | |
95 | return (ref $self)->new( parent => $self, @_ ); |
96 | } |
97 | |
98 | Now C<_build_child_tree> can be gracefully inherited and overridden. |
99 | |
100 | =head2 Composable |
101 | |
102 | There's more to builders than just subclassing, though. The fact that |
103 | builders are called by name also makes them suitable for use in a |
104 | role. |
105 | |
106 | package HasAnimal; |
107 | use Moose::Role; |
108 | |
109 | requires '_build_animal'; |
110 | |
111 | has 'animal' => ( |
112 | is => 'ro', |
113 | isa => 'Animal', |
114 | lazy => 1, |
115 | builder => '_build_animal', |
116 | ); |
117 | |
118 | This role provides an animal attribute, but requires that the consumer |
119 | of the role provide a builder method it. |
120 | |
121 | package CatLover; |
122 | use Moose; |
123 | |
124 | with 'HasAnimal'; |
125 | |
126 | sub _build_animal { |
127 | return Cat->new(); |
128 | } |
129 | |
130 | This simply could not be done using a C<default>. |
131 | |
132 | =head2 The lazy_build shortcut |
133 | |
134 | The C<lazy_build> attribute parameter can be used as sugar to specify |
135 | a whole bunch of options at once. |
136 | |
137 | has 'animal' => ( |
138 | is => 'ro', |
139 | isa => 'Animal', |
140 | lazy_build => 1, |
141 | ); |
142 | |
143 | This is a shorthand for this: |
144 | |
145 | has 'animal' => ( |
146 | is => 'ro', |
147 | isa => 'Animal', |
148 | required => 1, |
149 | lazy => 1, |
150 | builder => '_build_animal', |
151 | predicate => 'has_animal', |
152 | clearer => 'clear_animal', |
153 | ); |
154 | |
155 | If your attribute starts with an underscore, Moose is smart and will |
156 | do the right thing with the C<predicate> and C<clearer>, making them |
157 | both start with an underscore. The C<builder> method I<always> starts |
158 | with an underscore, since you will want this to be private the vast |
159 | majority of the time. |
160 | |
161 | =head1 CONCLUSION |
162 | |
163 | The C<builder> option is a more OO-friendly version of the C<default> |
164 | functionality. It also has the property of separating out the code |
165 | into a separate well-defined method. This alone makes it valuable. It |
166 | is quite ugly to jam a long default code reference into your attribute |
167 | definition. |
168 | |
169 | Here are some good rules for determining when to use C<builder> vs |
170 | C<default>. |
171 | |
172 | If the default value is a simple scalar that only needs to be |
173 | calculated once (or a constant), use C<default>. |
174 | |
175 | If the default value is an empty reference that needs to be wrapped in |
176 | a coderef like C<sub { [] }>, use C<default>. |
177 | |
178 | Otherwise, use C<builder>. |
179 | |
180 | =head1 AUTHOR |
181 | |
182 | Dave Rolsky E<lt>autarch@urth.orgE<gt> |
183 | |
184 | =head1 COPYRIGHT AND LICENSE |
185 | |
186 | Copyright 2006-2008 by Infinity Interactive, Inc. |
187 | |
188 | L<http://www.iinteractive.com> |
189 | |
190 | This library is free software; you can redistribute it and/or modify |
191 | it under the same terms as Perl itself. |
192 | |
193 | =cut |