Moose::Cookbook::Basics::Recipe6 - The augment/inner example

version 2.0010

  package Document::Page;
  use Moose;

  has 'body' => ( is => 'rw', isa => 'Str', default => sub {''} );

  sub create {
      my $self = shift;
      $self->open_page;
      inner();
      $self->close_page;
  }

  sub append_body {
      my ( $self, $appendage ) = @_;
      $self->body( $self->body . $appendage );
  }

  sub open_page  { (shift)->append_body('<page>') }
  sub close_page { (shift)->append_body('</page>') }

  package Document::PageWithHeadersAndFooters;
  use Moose;

  extends 'Document::Page';

  augment 'create' => sub {
      my $self = shift;
      $self->create_header;
      inner();
      $self->create_footer;
  };

  sub create_header { (shift)->append_body('<header/>') }
  sub create_footer { (shift)->append_body('<footer/>') }

  package TPSReport;
  use Moose;

  extends 'Document::PageWithHeadersAndFooters';

  augment 'create' => sub {
      my $self = shift;
      $self->create_tps_report;
      inner();
  };

  sub create_tps_report {
      (shift)->append_body('<report type="tps"/>');
  }

  # <page><header/><report type="tps"/><footer/></page>
  my $report_xml = TPSReport->new->create;

This recipe shows how the augment method modifier works. This modifier reverses the normal subclass to parent method resolution order. With an augment modifier the least specific method is called first. Each successive call to inner descends the inheritance tree, ending at the most specific subclass.

The augment modifier lets you design a parent class that can be extended in a specific way. The parent provides generic wrapper functionality, and the subclasses fill in the details.

In the example above, we've created a set of document classes, with the most specific being the TPSReport class.

We start with the least specific class, Document::Page. Its create method contains a call to inner():

  sub create {
      my $self = shift;
      $self->open_page;
      inner();
      $self->close_page;
  }

The inner function is exported by Moose, and is like super for augmented methods. When inner is called, Moose finds the next method in the chain, which is the augment modifier in Document::PageWithHeadersAndFooters. You'll note that we can call inner in our modifier:

  augment 'create' => sub {
      my $self = shift;
      $self->create_header;
      inner();
      $self->create_footer;
  };

This finds the next most specific modifier, in the TPSReport class.

Finally, in the TPSReport class, the chain comes to an end:

  augment 'create' => sub {
      my $self = shift;
      $self->create_tps_report;
      inner();
  };

We do call the inner function one more time, but since there is no more specific subclass, this is a no-op. Making this call means we can easily subclass TPSReport in the future.

The augment modifier is a powerful tool for creating a set of nested wrappers. It's not something you will need often, but when you do, it is very handy.

Stevan Little <stevan@iinteractive.com>

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.