package MooseX::Storage::Engine::Trait::OnlyWhenBuilt;
use Moose::Role;

# we should
# only serialize the attribute if it's already built. So, go ahead
# and check if the attribute has a predicate. If so, check if it's
# set  and then go ahead and look it up.
around 'collapse_attribute' => sub {
    my ($orig, $self, $attr, @args) = @_;

    my $pred = $attr->predicate if $attr->has_predicate;
    if ($pred) {
        return () unless $self->object->$pred();
    }

    return $self->$orig($attr, @args);
};

1;

__END__

=pod

=head1 NAME

MooseX::Storage::Engine::Trait::OnlyWhenBuilt - An engine trait to bypass serialization

=head1 SYNOPSIS

    {   package Point;
        use Moose;
        use MooseX::Storage;

        with Storage( traits => [qw|OnlyWhenBuilt|] );

        has 'x' => (is => 'rw', lazy_build => 1 );
        has 'y' => (is => 'rw', lazy_build => 1 );
        has 'z' => (is => 'rw', builder => '_build_z' );

        sub _build_x { 3 }
        sub _build_y { expensive_computation() }
        sub _build_z { 3 }
    }

    my $p = Point->new( 'x' => 4 );

    # the result of ->pack will contain:
    # { x => 4, z => 3 }
    $p->pack;

=head1 DESCRIPTION

Sometimes you don't want a particular attribute to be part of the
serialization if it has not been built yet. If you invoke C<Storage()>
as outlined in the C<Synopsis>, only attributes that have been built
(ie, where the predicate returns 'true') will be serialized.
This avoids any potentially expensive computations.

This trait is applied to an instance of L<MooseX::Storage::Engine>, for the
user-visible version shown in the SYNOPSIS, see L<MooseX::Storage::Traits::OnlyWhenBuilt>

=head1 METHODS

=head2 Introspection

=over 4

=item B<meta>

=back

=head1 BUGS

All complex software has bugs lurking in it, and this module is no
exception. If you find a bug please either email me, or add the bug
to cpan-RT.

=head1 AUTHOR

Stevan Little E<lt>stevan.little@iinteractive.comE<gt>

=head1 COPYRIGHT AND LICENSE

Copyright 2007-2008 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