Moose - modify default attribute position in object hash - perl

Question

I'm dealing with some non-Moose legacy code and I want to extend it with a Moose class. This is a simplification of the legacy code:
package My::Legacy;
sub create {
my ($class, $args) = #_;
my $fields = { _fields => {}};
foreach my $key ( keys %$args ) {
$fields->{_fields}->{$key} = $args->{$key}
}
bless $fields, $class;
}
1;
The My::Legacy class handles all the CRUD operations, caching and other stuff. All the operations are performed on the values contained in the internal _field hash, so, for example, if you want to update a value it has to be in the _field hash. The My::Legacy class provides setter/getter for this.
The My::Legacy is subclassed by several classes that need the "sugar" provided by it: My::Legacy::ObjectA, My::Legacy::ObjectB, etc.
I need to add a further one and I want to extend it using Moose. The problem is that every time I will set an attribute, I will have to keep its value in sync in the internal _fields hash, so for example if I have...
package My::Legacy::MyMooseObj;
use Moose;
use MooseX::NonMoose;
use namespace::autoclean;
has _fields => (
isa => HashRef,
is => 'rw',
default => sub { {} },
);
has attr_a => (
isa => 'Int',
is => 'ro',
);
has attr_b => (
isa => 'Str',
is => 'ro',
);
__PACKAGE__->meta->make_immutable;
...and I do:
my $MyMooseObj = My::Legacy::MyMooseObj->new();
$MyMooseObj->attr_a(15);
...I want attr_a to be set in _fields as well, so if I dump out the object it will look like:
bless( {
'_fields' => {
'attr_a' => 15,
},
'attr_a' => 15,
}, 'My::Legacy::MyMooseObj' );
The way I come up to achieve this is add a trigger to each attribute in order to write its value in the _fields hash every time is set:
has attr_b => (
isa => 'Str',
is => 'ro',
trigger => sub { # Write in the _fields attribute attr_b value! },
);
This is a bit annoying because every time I add a new attribute I have to make sure it has the trigger set :/
Can you think of a better way of doing it ? Is there any way of telling Moose to read/write the attribute not in the "root" of the object hash by default (so in my case to read/write attributes from _fields) ?

Answer 1

This more or less does what you want...
use strict;
use warnings;
{
package My::Legacy::MyMooseObj;
use Moose;
use MooseX::FunkyAttributes;
use namespace::autoclean;
has _fields => (
isa => 'HashRef',
is => 'rw',
default => sub { {} },
lazy => 1, # you want this, for the rest to work
);
has attr_a => (
isa => 'Int',
is => 'ro',
traits => [ FunkyAttribute ],
custom_get => sub { $_->_fields->{attr_a} },
custom_set => sub { $_->_fields->{attr_a} = $_[-1] },
custom_has => sub { exists($_->_fields->{attr_a}) },
);
has attr_b => (
isa => 'Str',
is => 'rw',
traits => [ FunkyAttribute ],
custom_get => sub { $_->_fields->{attr_b} },
custom_set => sub { $_->_fields->{attr_b} = $_[-1] },
custom_has => sub { exists($_->_fields->{attr_b}) },
);
}
my $obj = My::Legacy::MyMooseObj->new( attr_a => 42 );
$obj->attr_b(666);
print $obj->dump;
With the current version of MooseX::FunkyAttributes, the constructor will not work correctly if you do the whole __PACKAGE__->meta->make_immutable though. :-(
Delving slightly deeper into metaprogramming...
use strict;
use warnings;
{
package My::Legacy::MyMooseObj;
use Moose;
use MooseX::FunkyAttributes;
use namespace::autoclean;
has _fields => (
isa => 'HashRef',
is => 'rw',
default => sub { {} },
lazy => 1, # you want this, for the rest to work
);
sub funky_has {
my ($attr, %opts) = #_;
has $attr => (
is => 'ro',
traits => [ FunkyAttribute ],
custom_get => sub { $_->_fields->{$attr} },
custom_set => sub { $_->_fields->{$attr} = $_[-1] },
custom_has => sub { exists($_->_fields->{$attr}) },
%opts,
);
}
funky_has attr_a => (isa => 'Int');
funky_has attr_b => (isa => 'Str', is => 'rw');
}
my $obj = My::Legacy::MyMooseObj->new( attr_a => 42 );
$obj->attr_b(666);
print $obj->dump;