NAME

Moose::Cookbook::Legacy::Debugging_BaseClassReplacement - Providing an alternate base object class

VERSION

version 2.2201

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

WARNING: Replacing the base class entirely, as opposed to applying roles to the base class, is strongly discouraged. This recipe is provided solely for reference when encountering older code that does this.

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.

AUTHORS

Stevan Little <stevan@cpan.org>
Dave Rolsky <autarch@urth.org>
Jesse Luehrs <doy@cpan.org>
Shawn M Moore <sartak@cpan.org>
יובל קוג'מן (Yuval Kogman) <nothingmuch@woobling.org>
Karen Etheridge <ether@cpan.org>
Florian Ragwitz <rafl@debian.org>
Hans Dieter Pearcey <hdp@cpan.org>
Chris Prather <chris@prather.org>
Matt S Trout <mstrout@cpan.org>

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.