trAvis - MANAGER

Edit File: Restartable_AdvancedComposition.pod

package Moose::Cookbook::Roles::Restartable_AdvancedComposition

# ABSTRACT: Advanced Role Composition - method exclusion and aliasing

__END__

=pod

=head1 NAME

Moose::Cookbook::Roles::Restartable_AdvancedComposition - Advanced Role Composition - method exclusion and aliasing

=head1 VERSION

version 2.1005

=head1 SYNOPSIS

package Restartable;
  use Moose::Role;

has 'is_paused' => (
      is      => 'rw',
      isa     => 'Bool',
      default => 0,
  );

requires 'save_state', 'load_state';

sub stop { 1 }

sub start { 1 }

package Restartable::ButUnreliable;
  use Moose::Role;

with 'Restartable' => {
      -alias => {
          stop  => '_stop',
          start => '_start'
      },
      -excludes => [ 'stop', 'start' ],
  };

sub stop {
      my $self = shift;

$self->explode() if rand(1) > .5;

$self->_stop();
  }

sub start {
      my $self = shift;

$self->explode() if rand(1) > .5;

$self->_start();
  }

package Restartable::ButBroken;
  use Moose::Role;

with 'Restartable' => { -excludes => [ 'stop', 'start' ] };

sub stop {
      my $self = shift;

$self->explode();
  }

sub start {
      my $self = shift;

$self->explode();
  }

=head1 DESCRIPTION

In this example, we demonstrate how to exercise fine-grained control
over what methods we consume from a role. We have a C<Restartable>
role which provides an C<is_paused> attribute, and two methods,
C<stop> and C<start>.

Then we have two more roles which implement the same interface, each
putting their own spin on the C<stop> and C<start> methods.

In the C<Restartable::ButUnreliable> role, we want to provide a new
implementation of C<stop> and C<start>, but still have access to the
original implementation. To do this, we alias the methods from
C<Restartable> to private methods, and provide wrappers around the
originals (1).

Note that aliasing simply I<adds> a name, so we also need to exclude the
methods with their original names.

with 'Restartable' => {
      -alias => {
          stop  => '_stop',
          start => '_start'
      },
      -excludes => [ 'stop', 'start' ],
  };

In the C<Restartable::ButBroken> role, we want to provide an entirely
new behavior for C<stop> and C<start>. We exclude them entirely when
composing the C<Restartable> role into C<Restartable::ButBroken>.

It's worth noting that the C<-excludes> parameter also accepts a single
string as an argument if you just want to exclude one method.

with 'Restartable' => { -excludes => [ 'stop', 'start' ] };

=head1 CONCLUSION

Exclusion and renaming are a power tool that can be handy, especially
when building roles out of other roles. In this example, all of our
roles implement the C<Restartable> role. Each role provides same API,
but each has a different implementation under the hood.

You can also use the method aliasing and excluding features when
composing a role into a class.

=head1 FOOTNOTES

=over 4

=item (1)

The mention of wrapper should tell you that we could do the same thing
using method modifiers, but for the sake of this example, we don't.

=back

=begin testing

{
    my $unreliable = Moose::Meta::Class->create_anon_class(
        superclasses => [],
        roles        => [qw/Restartable::ButUnreliable/],
        methods      => {
            explode      => sub { },    # nop.
            'save_state' => sub { },
            'load_state' => sub { },
        },
    )->new_object();
    ok( $unreliable, 'made anon class with Restartable::ButUnreliable role' );
    can_ok( $unreliable, qw/start stop/ );
}

{
    my $cnt    = 0;
    my $broken = Moose::Meta::Class->create_anon_class(
        superclasses => [],
        roles        => [qw/Restartable::ButBroken/],
        methods      => {
            explode      => sub { $cnt++ },
            'save_state' => sub { },
            'load_state' => sub { },
        },
    )->new_object();

ok( $broken, 'made anon class with Restartable::ButBroken role' );

$broken->start();

is( $cnt, 1, '... start called explode' );

$broken->stop();

is( $cnt, 2, '... stop also called explode' );
}

=end testing

=head1 AUTHOR

Moose is maintained by the Moose Cabal, along with the help of many contributors. See L<Moose/CABAL> and L<Moose/CONTRIBUTORS> for details.

=head1 COPYRIGHT AND LICENSE

This is free software; you can redistribute it and/or modify it under
the same terms as the Perl 5 programming language system itself.

=cut