/ 
Class-Method-Modifiers-2.15
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/ Class::Method::Modifiers

NAME

Class::Method::Modifiers - Provides Moose-like method modifiers

VERSION

version 2.15

SYNOPSIS

package Child;
use parent 'MyParent';
use Class::Method::Modifiers;

sub new_method { }

before 'old_method' => sub {
    carp "old_method is deprecated, use new_method";
};

around 'other_method' => sub {
    my $orig = shift;
    my $ret = $orig->(@_);
    return $ret =~ /\d/ ? $ret : lc $ret;
};

after 'private', 'protected' => sub {
    debug "finished calling a dangerous method";
};

use Class::Method::Modifiers qw(fresh);

fresh 'not_in_hierarchy' => sub {
    warn "freshly added method\n";
};

DESCRIPTION

Method modifiers are a convenient feature from the CLOS (Common Lisp Object System) world.

In its most basic form, a method modifier is just a method that calls $self->SUPER::foo(@_). I for one have trouble remembering that exact invocation, so my classes seldom re-dispatch to their base classes. Very bad!

Class::Method::Modifiers provides three modifiers: before, around, and after. before and after are run just before and after the method they modify, but can not really affect that original method. around is run in place of the original method, with a hook to easily call that original method. See the "MODIFIERS" section for more details on how the particular modifiers work.

One clear benefit of using Class::Method::Modifiers is that you can define multiple modifiers in a single namespace. These separate modifiers don't need to know about each other. This makes top-down design easy. Have a base class that provides the skeleton methods of each operation, and have plugins modify those methods to flesh out the specifics.

Parent classes need not know about Class::Method::Modifiers. This means you should be able to modify methods in any subclass. See Term::VT102::ZeroBased for an example of subclassing with Class::Method::Modifiers.

In short, Class::Method::Modifiers solves the problem of making sure you call $self->SUPER::foo(@_), and provides a cleaner interface for it.

As of version 1.00, Class::Method::Modifiers is faster in some cases than Moose. See benchmark/method_modifiers.pl in the Moose distribution.

Class::Method::Modifiers also provides an additional "modifier" type, fresh; see below.

MODIFIERS

All modifiers let you modify one or multiple methods at a time. The names of multiple methods can be provided as a list or as an array-reference. Examples:

before 'method' => sub { ... };
before 'method1', 'method2' => sub { ... };
before [ 'method1', 'method2' ] => sub { ... };

before method(s) => sub { ... };

before is called before the method it is modifying. Its return value is totally ignored. It receives the same @_ as the method it is modifying would have received. You can modify the @_ the original method will receive by changing $_[0] and friends (or by changing anything inside a reference). This is a feature!

after method(s) => sub { ... };

after is called after the method it is modifying. Its return value is totally ignored. It receives the same @_ as the method it is modifying received, mostly. The original method can modify @_ (such as by changing $_[0] or references) and after will see the modified version. If you don't like this behavior, specify both a before and after, and copy the @_ during before for after to use.

around method(s) => sub { ... };

around is called instead of the method it is modifying. The method you're overriding is passed in as the first argument (called $orig by convention). Watch out for contextual return values of $orig.

You can use around to:

Pass $orig a different @_
around 'method' => sub {
    my $orig = shift;
    my $self = shift;
    $orig->($self, reverse @_);
};
Munge the return value of $orig
around 'method' => sub {
    my $orig = shift;
    ucfirst $orig->(@_);
};
Avoid calling $orig -- conditionally
around 'method' => sub {
    my $orig = shift;
    return $orig->(@_) if time() % 2;
    return "no dice, captain";
};

fresh method(s) => sub { ... };

(Available since version 2.00)

Unlike the other modifiers, this does not modify an existing method. Ordinarily, fresh merely installs the coderef as a method in the appropriate class; but if the class hierarchy already contains a method of the same name, an exception is thrown. The idea of this "modifier" is to increase safety when subclassing. Suppose you're writing a subclass of a class Some::Base, and adding a new method:

package My::Subclass;
use base 'Some::Base';

sub foo { ... }

If a later version of Some::Base also adds a new method named foo, your method will shadow that method. Alternatively, you can use fresh to install the additional method into your subclass:

package My::Subclass;
use base 'Some::Base';

use Class::Method::Modifiers 'fresh';

fresh 'foo' => sub { ... };

Now upgrading Some::Base to a version with a conflicting foo method will cause an exception to be thrown; seeing that error will give you the opportunity to fix the problem (perhaps by picking a different method name in your subclass, or similar).

Creating fresh methods with install_modifier (see below) provides a way to get similar safety benefits when adding local monkeypatches to existing classes; see http://aaroncrane.co.uk/talks/monkey_patching_subclassing/.

For API compatibility reasons, this function is exported only when you ask for it specifically, or for :all.

install_modifier $package, $type, @names, sub { ... }

install_modifier is like before, after, around, and fresh but it also lets you dynamically select the modifier type ('before', 'after', 'around', 'fresh') and package that the method modifiers are installed into. This expert-level function is exported only when you ask for it specifically, or for :all.

NOTES

All three normal modifiers; before, after, and around; are exported into your namespace by default. You may use Class::Method::Modifiers () to avoid modifying your namespace. I may steal more features from Moose, namely super, override, inner, augment, and whatever the Moose folks come up with next.

Note that the syntax and semantics for these modifiers is directly borrowed from Moose (the implementations, however, are not).

Class::Trigger shares a few similarities with Class::Method::Modifiers, and they even have some overlap in purpose -- both can be used to implement highly pluggable applications. The difference is that Class::Trigger provides a mechanism for easily letting parent classes to invoke hooks defined by other code. Class::Method::Modifiers provides a way of overriding/augmenting methods safely, and the parent class need not know about it.

:lvalue METHODS

When adding before or after modifiers, the wrapper method will be an lvalue method if the wrapped sub is, and assigning to the method will propagate to the wrapped method as expected. For around modifiers, it is the modifier sub that determines if the wrapper method is an lvalue method.

CAVEATS

It is erroneous to modify a method that doesn't exist in your class's inheritance hierarchy. If this occurs, an exception will be thrown when the modifier is defined.

It doesn't yet play well with caller. There are some TODO tests for this. Don't get your hopes up though!

Applying modifiers to array lvalue methods is not fully supported. Attempting to assign to an array lvalue method that has an after modifier applied will result in an error. Array lvalue methods are not well supported by perl in general, and should be avoided.

MAJOR VERSION CHANGES

This module was bumped to 1.00 following a complete reimplementation, to indicate breaking backwards compatibility. The "guard" modifier was removed, and the internals are completely different.

The new version is a few times faster with half the code. It's now even faster than Moose.

Any code that just used modifiers should not change in behavior, except to become more correct. And, of course, faster. :)

SEE ALSO

ACKNOWLEDGMENTS

Thanks to Stevan Little for Moose, I would never have known about method modifiers otherwise.

Thanks to Matt Trout and Stevan Little for their advice.

SUPPORT

Bugs may be submitted through the RT bug tracker (or [email protected]).

AUTHOR

Shawn M Moore <[email protected]>

CONTRIBUTORS

COPYRIGHT AND LICENSE

This software is copyright (c) 2007 by Shawn M Moore.

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