
=pod

=head1 NAME

Moose::Cookbook::Basics::Recipe10 - Using BUILDARGS and BUILD to hook into object construction

=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 AUTHOR

Dave Rolsky E<lt>autarch@urth.orgE<gt>

=head1 COPYRIGHT AND LICENSE

Copyright 2006-2009 by Infinity Interactive, Inc.

L<http://www.iinteractive.com>

This library is free software; you can redistribute it and/or modify
it under the same terms as Perl itself.

=cut
