class variables in Moose

Moose attribute didn't allow the reference rather than a CODE reference as a default value. That prevents the objects shared a attribute when initializing. Actually this is a good design. But How should I do when needing a shared attribute between objects? I may refer this as the concept of class variable.

The state variable in CODE reference may be competent with this role.

A Counter as an example here. We define a Counter class here.

	use Modern::Perl;
	use MooseX::Declare;
	class Counter {
	has value => (
	is => "rw",
	isa => "Num",
	traits => ['Counter'],
	default => 0,
	handles => {
	inc => "inc",
	},
	);
	};

view raw Counter.pm hosted with ❤ by GitHub

A counter has a attribute "value" and a method "inc" to increment it.

Then We define a Person class.

	use Modern::Perl;
	use MooseX::Declare;
	use Counter;
	class Person {
	has name => (
	is => "ro",
	isa => "Str",
	default => "nobody",
	required => 1,
	);
	has count => (
	is => "rw",
	isa => "Counter",
	default => sub {
	state $count = Counter->new;
	$count;
	},
	);
	sub BUILD {
	my ($self) = shift;
	$self->count->inc;
	}
	};

view raw Person.pm hosted with ❤ by GitHub

The Person class has a attribute "name" and a shared attribute "count". The Counter is a state variable return from a anonymous CODE reference. That's the trick.

The BUILD hook increment the counter everytime a new Person is constructed.

Then we examinate the shared variable.

	#!/usr/bin/env perl
	#-*- mode: cperl -*-
	use Modern::Perl;
	use Person;
	my $shelling = Person->new(name => "shelling");
	my $count = $shelling->count;
	say $count->value; #=> 1
	my $sherry = Person->new(name => "sherry");
	say $count->value; #=> 2
	my $appollo = Person->new(name => "appollo");
	say $count->value; #=> 3

view raw class-variables.pl hosted with ❤ by GitHub

A global variable in package name may still work. But this way uses 'has' syntax sugar. :p

If you use builder to replace the default option. the subroutine used by the builder is just the class variable accessor from Class name, returning value as $person->count().