Moose::Cookbook::Basics::Person_BUILDARGSAndBUILD - Using BUILDARGS and BUILD to hook into object construction |
Moose::Cookbook::Basics::Person_BUILDARGSAndBUILD - Using BUILDARGS and BUILD to hook into object construction
version 2.1605
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; } }
This recipe demonstrates the use of BUILDARGS
and BUILD
. By
defining these methods, we can hook into the object construction
process without overriding new
.
The BUILDARGS
method is called before an object has been
created. It is called as a class method, and receives all of the
parameters passed to the new
method. It is expected to do something
with these arguments and return a hash reference. The keys of the hash
must be attribute init_arg
s.
The primary purpose of BUILDARGS
is to allow a class to accept
something other than named arguments. In the case of our 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 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 $_[0]
is not a reference
before assuming it is a social security number.
We call the original BUILDARGS
method to handle all the other
cases. You should always do this in your own BUILDARGS
methods,
since the Moose::Object manpage provides its own BUILDARGS
method that
handles hash references and a list of key-value pairs.
The BUILD
method is called after the object is constructed, but
before it is returned to the caller. The 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 Person
class, we need to check the relationship between two
attributes, ssn
and country_of_residence
. We throw an exception
if the object is not logically consistent.
This recipe is made significantly simpler because all of the
attributes are read-only. If the country_of_residence
attribute
were settable, we would need to check that a Person had an ssn
if
the new country was usa
. This could be done with a before
modifier.
We have repeatedly discouraged overriding new
in Moose
classes. This recipe shows how you can use BUILDARGS
and BUILD
to hook into object construction without overriding new
.
The BUILDARGS
method lets us expand on Moose's built-in parameter
handling for constructors. The BUILD
method lets us implement
logical constraints across the whole object after it is created.
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.
Moose::Cookbook::Basics::Person_BUILDARGSAndBUILD - Using BUILDARGS and BUILD to hook into object construction |