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
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
|
# PODNAME: Moose::Cookbook::Basics::Person_BUILDARGSAndBUILD
# ABSTRACT: Using BUILDARGS and BUILD to hook into object construction
__END__
=pod
=encoding UTF-8
=head1 NAME
Moose::Cookbook::Basics::Person_BUILDARGSAndBUILD - Using BUILDARGS and BUILD to hook into object construction
=head1 VERSION
version 2.1405
=head1 SYNOPSIS
package Person;
has 'ssn' => (
is => 'ro',
isa => 'Str',
predicate => 'has_ssn',
);
has 'country_of_residence' => (
is => 'ro',
isa => 'Str',
default => 'usa'
);
has 'first_name' => (
is => 'ro',
isa => 'Str',
);
has 'last_name' => (
is => 'ro',
isa => 'Str',
);
around BUILDARGS => sub {
my $orig = shift;
my $class = shift;
if ( @_ == 1 && ! ref $_[0] ) {
return $class->$orig(ssn => $_[0]);
}
else {
return $class->$orig(@_);
}
};
sub BUILD {
my $self = shift;
if ( $self->country_of_residence eq 'usa' ) {
die 'Cannot create a Person who lives in the USA without an ssn.'
unless $self->has_ssn;
}
}
=head1 DESCRIPTION
This recipe demonstrates the use of C<BUILDARGS> and C<BUILD>. By
defining these methods, we can hook into the object construction
process without overriding C<new>.
The C<BUILDARGS> method is called I<before> an object has been
created. It is called as a class method, and receives all of the
parameters passed to the C<new> method. It is expected to do something
with these arguments and return a hash reference. The keys of the hash
must be attribute C<init_arg>s.
The primary purpose of C<BUILDARGS> is to allow a class to accept
something other than named arguments. In the case of our C<Person>
class, we are allowing it to be called with a single argument, a
social security number:
my $person = Person->new('123-45-6789');
The key part of our C<BUILDARGS> is this conditional:
if ( @_ == 1 && ! ref $_[0] ) {
return $class->$orig(ssn => $_[0]);
}
By default, Moose constructors accept a list of key-value pairs, or a
hash reference. We need to make sure that C<$_[0]> is not a reference
before assuming it is a social security number.
We call the original C<BUILDARGS> method to handle all the other
cases. You should always do this in your own C<BUILDARGS> methods,
since L<Moose::Object> provides its own C<BUILDARGS> method that
handles hash references and a list of key-value pairs.
The C<BUILD> method is called I<after> the object is constructed, but
before it is returned to the caller. The C<BUILD> method provides an
opportunity to check the object state as a whole. This is a good place
to put logic that cannot be expressed as a type constraint on a single
attribute.
In the C<Person> class, we need to check the relationship between two
attributes, C<ssn> and C<country_of_residence>. We throw an exception
if the object is not logically consistent.
=head1 MORE CONSIDERATIONS
This recipe is made significantly simpler because all of the
attributes are read-only. If the C<country_of_residence> attribute
were settable, we would need to check that a Person had an C<ssn> if
the new country was C<usa>. This could be done with a C<before>
modifier.
=head1 CONCLUSION
We have repeatedly discouraged overriding C<new> in Moose
classes. This recipe shows how you can use C<BUILDARGS> and C<BUILD>
to hook into object construction without overriding C<new>.
The C<BUILDARGS> method lets us expand on Moose's built-in parameter
handling for constructors. The C<BUILD> method lets us implement
logical constraints across the whole object after it is created.
=head1 AUTHORS
=over 4
=item *
Stevan Little <stevan.little@iinteractive.com>
=item *
Dave Rolsky <autarch@urth.org>
=item *
Jesse Luehrs <doy@tozt.net>
=item *
Shawn M Moore <code@sartak.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@weftsoar.net>
=item *
Chris Prather <chris@prather.org>
=item *
Matt S Trout <mst@shadowcat.co.uk>
=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
|
