Rename Meta::Recipe7 to Meta::GlobRef_InstanceMetaclass

[gitmo/Moose.git] / lib / Moose / Cookbook / Roles / ApplicationToInstance.pod

package Moose::Cookbook::Roles::ApplicationToInstance;

# ABSTRACT: Applying a role to an object instance

__END__

=pod

=begin testing-SETUP

{
    # Not in the recipe, but needed for writing tests.
    package Employee;

    use Moose;

    has 'name' => (
        is       => 'ro',
        isa      => 'Str',
        required => 1,
    );

    has 'work' => (
        is        => 'rw',
        isa       => 'Str',
        predicate => 'has_work',
    );
}

=end testing-SETUP

=head1 SYNOPSIS

  package MyApp::Role::Job::Manager;

  use List::Util qw( first );

  use Moose::Role;

  has 'employees' => (
      is  => 'rw',
      isa => 'ArrayRef[Employee]',
  );

  sub assign_work {
      my $self = shift;
      my $work = shift;

      my $employee = first { !$_->has_work } @{ $self->employees };

      die 'All my employees have work to do!' unless $employee;

      $employee->work($work);
  }

  package main;

  my $lisa = Employee->new( name => 'Lisa' );
  MyApp::Role::Job::Manager->meta->apply($lisa);

  my $homer = Employee->new( name => 'Homer' );
  my $bart  = Employee->new( name => 'Bart' );
  my $marge = Employee->new( name => 'Marge' );

  $lisa->employees( [ $homer, $bart, $marge ] );
  $lisa->assign_work('mow the lawn');

=head1 DESCRIPTION

In this recipe, we show how a role can be applied to an object. In
this specific case, we are giving an employee managerial
responsibilities.

Applying a role to an object is simple. The L<Moose::Meta::Role>
object provides an C<apply> method. This method will do the right
thing when given an object instance.

  MyApp::Role::Job::Manager->meta->apply($lisa);

We could also use the C<apply_all_roles> function from L<Moose::Util>.

  apply_all_roles( $person, MyApp::Role::Job::Manager->meta );

The main advantage of using C<apply_all_roles> is that it can be used
to apply more than one role at a time.

We could also pass parameters to the role we're applying:

  MyApp::Role::Job::Manager->meta->apply(
      $lisa,
      -alias => { assign_work => 'get_off_your_lazy_behind' },
  );

We saw examples of how method exclusion and alias working in
L<Moose::Cookbook::Roles::Restartable_AdvancedComposition>.

=head1 CONCLUSION

Applying a role to an object instance is a useful tool for adding
behavior to existing objects. In our example, it is effective used to
model a promotion.

It can also be useful as a sort of controlled monkey-patching for
existing code, particularly non-Moose code. For example, you could
create a debugging role and apply it to an object at runtime.

=begin testing

{
    my $lisa = Employee->new( name => 'Lisa' );
    MyApp::Role::Job::Manager->meta->apply($lisa);

    my $homer = Employee->new( name => 'Homer' );
    my $bart  = Employee->new( name => 'Bart' );
    my $marge = Employee->new( name => 'Marge' );

    $lisa->employees( [ $homer, $bart, $marge ] );
    $lisa->assign_work('mow the lawn');

    ok( $lisa->does('MyApp::Role::Job::Manager'),
        'lisa now does the manager role' );

    is( $homer->work, 'mow the lawn',
        'homer was assigned a task by lisa' );
}

=end testing

=cut