Moose::Cookbook::Extending::Recipe3(3)

NAME
Moose::Cookbook::Extending::Recipe3 - Providing an alternate base object class

VERSION
version 2.0205

SYNOPSIS
  package MyApp::Base;
  use Moose;

  extends 'Moose::Object';

  before 'new' => sub { warn "Making a new " . $_[0] };

  no Moose;

  package MyApp::UseMyBase;
  use Moose ();
  use Moose::Exporter;

  Moose::Exporter->setup_import_methods( also => 'Moose' );

  sub init_meta {
      shift;
      return Moose->init_meta( @_, base_class => 'MyApp::Base' );
  }

DESCRIPTION
A common extension is to provide an alternate base class. One way to do that is to make a "MyApp::base" and add "extends 'MyApp::Base'" to every class in your application. That's pretty tedious. Instead, you can create a Moose-alike module that sets the base object class to "MyApp::Base" for you.

Then, instead of writing "use Moose" you can write "use MyApp::UseMyBase".

In this particular example, our base class issues some debugging output every time a new object is created, but you can think of some more interesting things to do with your own base class.

This uses the magic of Moose::Exporter. When we call "Moose::Exporter->setup_import_methods( also => 'Moose' )" it builds "import" and "unimport" methods for you. The "also => 'Moose'" bit says that we want to export everything that Moose does.

The "import" method that gets created will call our "init_meta" method, passing it "for_caller => $caller" as its arguments. The $caller is set to the class that actually imported us in the first place.

See the Moose::Exporter docs for more details on its API.

USING MyApp::UseMyBase
To actually use our new base class, we simply use "MyApp::UseMyBase" instead of "Moose". We get all the Moose sugar plus our new base class.

  package Foo;

  use MyApp::UseMyBase;

  has 'size' => ( is => 'rw' );

  no MyApp::UseMyBase;

CONCLUSION
This is an awful lot of magic for a simple base class. You will often want to combine a metaclass trait with a base class extension, and that's when this technique is useful.

AUTHOR
Stevan Little <stevan@iinteractive.com>

COPYRIGHT AND LICENSE
This software is copyright (c) 2011 by Infinity Interactive, Inc..

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