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Moose::Cookbook::LegacUseroControbutedkPerlgDocumentated_AttributeMetaclass(3)

NAME
       Moose::Cookbook::Legacy::Labeled_AttributeMetaclass - A meta-attribute,
       attributes with labels

VERSION
       version 2.2005

SYNOPSIS
	 package MyApp::Meta::Attribute::Labeled;
	 use Moose;
	 extends 'Moose::Meta::Attribute';

	 has label => (
	     is	       => 'rw',
	     isa       => 'Str',
	     predicate => 'has_label',
	 );

	 package Moose::Meta::Attribute::Custom::Labeled;
	 sub register_implementation {'MyApp::Meta::Attribute::Labeled'}

	 package MyApp::Website;
	 use Moose;

	 has url => (
	     metaclass => 'Labeled',
	     is	       => 'rw',
	     isa       => 'Str',
	     label     => "The site's URL",
	 );

	 has name => (
	     is	 => 'rw',
	     isa => 'Str',
	 );

	 sub dump {
	     my	$self =	shift;

	     my	$meta =	$self->meta;

	     my	$dump =	'';

	     for my $attribute ( map { $meta->get_attribute($_)	}
		 sort $meta->get_attribute_list	) {

		 if (	$attribute->isa('MyApp::Meta::Attribute::Labeled')
		     &&	$attribute->has_label )	{
		     $dump .= $attribute->label;
		 }
		 else {
		     $dump .= $attribute->name;
		 }

		 my $reader = $attribute->get_read_method;
		 $dump .= ": " . $self->$reader	. "\n";
	     }

	     return $dump;
	 }

	 package main;

	 my $app = MyApp::Website->new(	url => "http://google.com", name => "Google" );

SUMMARY
       WARNING:	Subclassing metaclasses	(as opposed to providing metaclass
       traits) is strongly discouraged.	This recipe is provided	solely for
       reference when encountering older code that does	this.

       In this recipe, we begin	to delve into the wonder of meta-programming.
       Some readers may	scoff and claim	that this is the arena of only the
       most twisted Moose developers. Absolutely not! Any sufficiently twisted
       developer can benefit greatly from going	more meta.

       Our goal	is to allow each attribute to have a human-readable "label"
       attached	to it. Such labels would be used when showing data to an end
       user. In	this recipe we label the "url" attribute with "The site's URL"
       and create a simple method showing how to use that label.

       The proper, modern way to extend	attributes (using a role instead of a
       subclass) is described in Moose::Cookbook::Meta::Recipe3, but that
       recipe assumes you've read and at least tried to	understand this	one.

META-ATTRIBUTE OBJECTS
       All the attributes of a Moose-based object are actually objects
       themselves.  These objects have methods and attributes. Let's look at a
       concrete	example.

	 has 'x' => ( isa => 'Int', is => 'ro' );
	 has 'y' => ( isa => 'Int', is => 'rw' );

       Internally, the metaclass for "Point" has two Moose::Meta::Attribute.
       There are several methods for getting meta-attributes out of a
       metaclass, one of which is "get_attribute_list".	This method is called
       on the metaclass	object.

       The "get_attribute_list"	method returns a list of attribute names. You
       can then	use "get_attribute" to get the Moose::Meta::Attribute object
       itself.

       Once you	have this meta-attribute object, you can call methods on it
       like this:

	 print $point->meta->get_attribute('x')->type_constraint;
	    => Int

       To add a	label to our attributes	there are two steps. First, we need a
       new attribute metaclass that can	store a	label for an attribute.
       Second, we need to create attributes that use that attribute metaclass.

RECIPE REVIEW
       We start	by creating a new attribute metaclass.

	 package MyApp::Meta::Attribute::Labeled;
	 use Moose;
	 extends 'Moose::Meta::Attribute';

       We can subclass a Moose metaclass in the	same way that we subclass
       anything	else.

	 has label => (
	     is	       => 'rw',
	     isa       => 'Str',
	     predicate => 'has_label',
	 );

       Again, this is standard Moose code.

       Then we need to register	our metaclass with Moose:

	 package Moose::Meta::Attribute::Custom::Labeled;
	 sub register_implementation { 'MyApp::Meta::Attribute::Labeled' }

       This is a bit of	magic that lets	us use a short name, "Labeled",	when
       referring to our	new metaclass.

       That was	the whole attribute metaclass.

       Now we start using it.

	 package MyApp::Website;
	 use Moose;
	 use MyApp::Meta::Attribute::Labeled;

       We have to load the metaclass to	use it,	just like any Perl class.

       Finally,	we use it for an attribute:

	 has url => (
	     metaclass => 'Labeled',
	     is	       => 'rw',
	     isa       => 'Str',
	     label     => "The site's URL",
	 );

       This looks like a normal	attribute declaration, except for two things,
       the "metaclass" and "label" parameters. The "metaclass" parameter tells
       Moose we	want to	use a custom metaclass for this	(one) attribute. The
       "label" parameter will be stored	in the meta-attribute object.

       The reason that we can pass the name "Labeled", instead of
       "MyApp::Meta::Attribute::Labeled", is because of	the
       "register_implementation" code we touched on previously.

       When you	pass a metaclass to "has", it will take	the name you provide
       and prefix it with "Moose::Meta::Attribute::Custom::". Then it calls
       "register_implementation" in the	package. In this case, that means
       Moose ends up calling
       "Moose::Meta::Attribute::Custom::Labeled::register_implementation".

       If this function	exists,	it should return the real metaclass package
       name. This is exactly what our code does, returning
       "MyApp::Meta::Attribute::Labeled". This is a little convoluted, and if
       you don't like it, you can always use the fully-qualified name.

       We can access this meta-attribute and its label like this:

	 $website->meta->get_attribute('url')->label()

	 MyApp::Website->meta->get_attribute('url')->label()

       We also have a regular attribute, "name":

	 has name => (
	     is	 => 'rw',
	     isa => 'Str',
	 );

       This is a regular Moose attribute, because we have not specified	a new
       metaclass.

       Finally,	we have	a "dump" method, which creates a human-readable
       representation of a "MyApp::Website" object. It will use	an attribute's
       label if	it has one.

	 sub dump {
	     my	$self =	shift;

	     my	$meta =	$self->meta;

	     my	$dump =	'';

	     for my $attribute ( map { $meta->get_attribute($_)	}
		 sort $meta->get_attribute_list	) {

		 if (	$attribute->isa('MyApp::Meta::Attribute::Labeled')
		     &&	$attribute->has_label )	{
		     $dump .= $attribute->label;
		 }

       This is a bit of	defensive code.	We cannot depend on every meta-
       attribute having	a label. Even if we define one for every attribute in
       our class, a subclass may neglect to do so. Or a	superclass could add
       an attribute without a label.

       We also check that the attribute	has a label using the predicate	we
       defined.	We could instead make the label	"required". If we have a
       label, we use it, otherwise we use the attribute	name:

		 else {
		     $dump .= $attribute->name;
		 }

		 my $reader = $attribute->get_read_method;
		 $dump .= ": " . $self->$reader	. "\n";
	     }

	     return $dump;
	 }

       The "get_read_method" is	part of	the Moose::Meta::Attribute API.	It
       returns the name	of a method that can read the attribute's value, when
       called on the real object (don't	call this on the meta-attribute).

CONCLUSION
       You might wonder	why you'd bother with all this.	You could just
       hardcode	"The Site's URL" in the	"dump" method. But we want to avoid
       repetition. If you need the label once, you may need it elsewhere,
       maybe in	the "as_form" method you write next.

       Associating a label with	an attribute just makes	sense! The label is a
       piece of	information about the attribute.

       It's also important to realize that this	was a trivial example. You can
       make much more powerful metaclasses that	do things, as opposed to just
       storing some more information. For example, you could implement a
       metaclass that expires attributes after a certain amount	of time:

	  has site_cache => (
	      metaclass	    => 'TimedExpiry',
	      expires_after => { hours => 1 },
	      refresh_with  => sub { get( $_[0]->url ) },
	      isa	    => 'Str',
	      is	    => 'ro',
	  );

       The sky's the limit!

AUTHORS
       o   Stevan Little <stevan.little@iinteractive.com>

       o   Dave	Rolsky <autarch@urth.org>

       o   Jesse Luehrs	<doy@tozt.net>

       o   Shawn M Moore <code@sartak.org>

       o   xxxx	x<section>xx'xx	(Yuval Kogman) <nothingmuch@woobling.org>

       o   Karen Etheridge <ether@cpan.org>

       o   Florian Ragwitz <rafl@debian.org>

       o   Hans	Dieter Pearcey <hdp@weftsoar.net>

       o   Chris Prather <chris@prather.org>

       o   Matt	S Trout	<mst@shadowcat.co.uk>

COPYRIGHT AND LICENSE
       This software is	copyright (c) 2006 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.

perl v5.24.1		Moose::Cookbook::Legacy::Labeled_AttributeMetaclass(3)

NAME | VERSION | SYNOPSIS | SUMMARY | META-ATTRIBUTE OBJECTS | RECIPE REVIEW | CONCLUSION | AUTHORS | COPYRIGHT AND LICENSE

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