How to declare 2 dependant attributes in Mooseish way?

Question

In my object constructor i had statement to initialize two attributes same time:

($self->{token}, $self->{token_start}) = $self->_get_authorized_token();

So i got token and it's starting time together in one statement.

Now i try to port my module to use Moo(se) and here i don't know how should i set those two bound attributes at same time?. Some pseudo code would be like that:

has qw/token token_start/ => (
    is  => 'rw',
    default => shift->_get_authorized_token(); 
);

But how to declare 2 bound attributes in Moo(se)ish way?

---

EDIT. I show the code of the method _get_authorized_token, maybe it will brings some ideas:

sub _get_authorized_token {
    my $self = shift;
    my $postData = { 'apikey' => $self->{key} };
    my $url = $self->{base_url} . '/seller';
    my $xml = $self->_post(url => $url,
                            postdata => $postData,
                        );
    my $ref = XMLin($xml, SuppressEmpty => '' );
    my $time = $ref->{Notification_Datetime};
    my $token = $ref->{Notification_Data}{body}{token};
    return ($token, $time); 
}

Answer 1

Once you end up with two attributes that are basically linked to the point where you always set them simultaneously ... the answer is usually to create a value object with two attributes for the purpose and then delegate the relevant methods to it. So, something like -

package MyApp::TokenInfo;

use Moo;

has token => (is => 'ro', required => 1);
has token_start => (is => 'ro', required => 1);

...

package MyApp::ThingWithAToken;

use Module::Runtime qw(use_module);
use Moo;

...

has token_info => (is => 'lazy', handles => [ qw(token token_start) ]);

sub _build_token_info {
  my ($self) = @_;
  my ($token, $token_start) = $self->_get_authorized_token;

  # this is equivalent to:
  #
  #   require MyApp::TokenInfo;
  #   return MyApp::TokenInfo->new(...);
  #
  # but more concise

  return use_module('MyApp::TokenInfo')->new(
    token => $token,
    token_start => $token_start
  );
}

...

my $thing = MyApp::ThingWithAToken->new(...);

$thing->token; # calls $thing->token_info->token;
$thing->token_start; # calls $thing->token_info->token_start

so the presence of the value object isn't required knowlede from externally but internally you've still got the two attributes tied together in a way that lets your implementation handle them as a single "thing".

-- mst