Removed Meta recipe2 (an attribute metaclass)

[gitmo/Moose.git] / lib / Moose / Cookbook / Meta / Recipe5.pod

package Moose::Cookbook::Meta::Recipe5;

# ABSTRACT: The "table" attribute as a metaclass trait

__END__


=pod

=head1 SYNOPSIS

  package MyApp::Meta::Class::Trait::HasTable;
  use Moose::Role;

  has table => (
      is  => 'rw',
      isa => 'Str',
  );

  package Moose::Meta::Class::Custom::Trait::HasTable;
  sub register_implementation { 'MyApp::Meta::Class::Trait::HasTable' }

  package MyApp::User;
  use Moose -traits => 'HasTable';

  __PACKAGE__->meta->table('User');

=head1 DESCRIPTION

This recipe takes the metaclass table attribute from
L<Moose::Cookbook::Meta::Recipe4> and implements it as a metaclass
trait. Traits are just roles, as we saw in
L<Moose::Cookbook::Meta::Recipe3>.

The advantage of using traits is that it's easy to combine multiple
traits, whereas combining multiple metaclass subclasses requires
creating yet another subclass. With traits, Moose takes care of
applying them to your metaclass.

=head2 Using this Metaclass Trait in Practice

Once this trait has been applied to a metaclass, it looks exactly like
the example we saw in L<Moose::Cookbook::Meta::Recipe4>:

  my $table = MyApp::User->meta->table;

  # the safe version
  $table = MyApp::User->meta->table
      if MyApp::User->meta->meta->can('does')
         and MyApp::User->meta->meta->does('MyApp::Meta::Class');

The safe version is a little complicated. We have to check that the
metaclass object's metaclass has a C<does> method, in which case we
can ask if the the metaclass does a given role.

It's simpler to just write:

  $table = MyApp::User->meta->table
      if MyApp::User->meta->can('table');

In theory, this is a little less correct, since the metaclass might be
getting its C<table> method from a I<different> role. In practice, you
are unlikely to encounter this sort of problem.

=head1 SEE ALSO

L<Moose::Cookbook::Meta::Recipe3> - Labels implemented via attribute
traits

L<Moose::Cookbook::Meta::Recipe4> - Adding a "table" attribute to the
metaclass

=pod