From: Dave Rolsky <autarch@urth.org>
Date: Tue, 17 Feb 2009 15:41:43 +0000 (+0000)
Subject: Add basics recipe 11, BUILD & BUILDARGS
X-Git-Tag: 0.71~15
X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=commitdiff_plain;h=04d80e2a40cb02740dea7def17777cc0eb27d3d4;p=gitmo%2FMoose.git

Add basics recipe 11, BUILD & BUILDARGS
---

diff --git a/lib/Moose/Cookbook.pod b/lib/Moose/Cookbook.pod
index 030af86..9940871 100644
--- a/lib/Moose/Cookbook.pod
+++ b/lib/Moose/Cookbook.pod
@@ -75,9 +75,10 @@ provide a default attribute value.
 Demonstrates using operator overloading, coercion, and subtypes to
 model how eye color is determined during reproduction.
 
-=item L<Moose::Cookbook::Basics::Recipe11> - BUILD and BUILDARGS (TODO)
+=item L<Moose::Cookbook::Basics::Recipe11> - Using BUILDARGS and BUILD to hook into object construction
 
-We need a good recipe demonstrating how these work.
+This recipe demonstrates the use of C<BUILDARGS> and C<BUILD> to hook
+into object construction.
 
 =back
 
diff --git a/lib/Moose/Cookbook/Basics/Recipe11.pod b/lib/Moose/Cookbook/Basics/Recipe11.pod
new file mode 100644
index 0000000..4c36c80
--- /dev/null
+++ b/lib/Moose/Cookbook/Basics/Recipe11.pod
@@ -0,0 +1,129 @@
+
+=pod
+
+=head1 NAME
+
+Moose::Cookbook::Basics::Recipe11 - Using BUILDARGS and BUILD to hook into object construction
+
+=head1 SYNOPSIS
+
+  package Person;
+
+  has 'ssn' => (
+      is        => 'ro',
+      isa       => 'Str',
+      predicate => 'has_ssn',
+  );
+
+  has 'country_of_residence' => (
+      is      => 'ro',
+      isa     => 'Str',
+      default => 'usa'
+  );
+
+  has 'first_name' => (
+      is  => 'ro',
+      isa => 'Str',
+  );
+
+  has 'last_name' => (
+      is  => 'ro',
+      isa => 'Str',
+  );
+
+  sub BUILDARGS {
+      my $class = shift;
+
+      if ( @_ == 1 && ! ref $_[0] ) {
+          return { ssn => $_[0] };
+      }
+      else {
+          return $class->SUPER::BUILDARGS(@_);
+      }
+  }
+
+  sub BUILD {
+      my $self = shift;
+
+      if ( $self->country_of_residence eq 'usa' ) {
+          die 'Cannot create a Person who lives in the USA without an ssn.'
+              unless $self->has_ssn;
+      }
+  }
+
+=head1 DESCRIPTION
+
+This recipe demonstrates the use of C<BUILDARGS> and C<BUILD>. By
+defining these methods, we can hook into the object construction
+process without overriding C<new>.
+
+The C<BUILDARGS> method is called I<before> an object has been
+created. It is called as a class method, and receives all of the
+parameters passed to the C<new> method. It is expected to do something
+with these arguments and return a hash reference. The keys of the hash
+must be attribute C<init_arg>s.
+
+The primary purpose of C<BUILDARGS> is to allow a class to accept
+something other than named arguments. In the case of our C<Person>
+class, we are allowing it to be called with a single argument, a
+social security number:
+
+  my $person = Person->new('123-45-6789');
+
+The key part of our C<BUILDARGS> is this conditional:
+
+      if ( @_ == 1 && ! ref $_[0] ) {
+          return { ssn => $_[0] };
+      }
+
+By default, Moose constructors accept a list of key-value pairs, or a
+hash reference. We need to make sure that C<$_[0]> is not a reference
+before assuming it is a social security number.
+
+We call C<< $class->SUPER::BUILDARGS(@_) >> to handle all the other
+cases. You should always do this in your own C<BUILDARGS> methods,
+since L<Moose::Object> provides its own C<BUILDARGS> method that
+handles hash references and a list of key-value pairs.
+
+The C<BUILD> method is called I<after> the object is constructed, but
+before it is returned to the caller. The C<BUILD> method provides an
+opportunity to check the object state as a whole. This is a good place
+to put logic that cannot be expressed as a type constraint on a single
+object.
+
+In the C<Person> class, we need to check the relationship between two
+attributes, C<ssn> and C<country_of_residence>. We throw an exception
+if the object is not logically consistent.
+
+=head1 MORE CONSIDERATIONS
+
+This recipe is made significantly simpler because all of the
+attributes are read-only. If the C<country_of_residence> attribute
+were settable, we would need to check that a Person had an C<ssn> if
+the new country was C<usa>. This could be done with a C<before>
+modifier.
+
+=head1 CONCLUSION
+
+We have repeatedly discouraged overriding C<new> in Moose
+classes. This recipe shows how you can use C<BUILDARGS> and C<BUILD>
+to hook into object construction without overriding C<new>
+
+The C<BUILDARGS> method lets us expand on Moose's built-in parameter
+handling for constructors. The C<BUILD> method lets us implement
+logical constraints across the whole object after it is created.
+
+=head1 AUTHOR
+
+Dave Rolsky E<lt>autarch@urth.orgE<gt>
+
+=head1 COPYRIGHT AND LICENSE
+
+Copyright 2006-2009 by Infinity Interactive, Inc.
+
+L<http://www.iinteractive.com>
+
+This library is free software; you can redistribute it and/or modify
+it under the same terms as Perl itself.
+
+=cut