Class::Method::ModifieUserpContributed Perl DocumClass::Method::Modifiers(3pm)
NAME
Class::Method::Modifiers - Provides Moose-like method modifiers
VERSION
version 2.14
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
• Class::Method::Modifiers::Fast
• Moose
• Class::Trigger
• Class::MOP::Method::Wrapped
• MRO::Compat
• CLOS <https://en.wikipedia.org/wiki/Common_Lisp_Object_System>
ACKNOWLEDGEMENTS
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
<https://rt.cpan.org/Public/Dist/Display.html?Name=Class-Method-
Modifiers> (or bug-Class-Method-Modifiers@rt.cpan.org <mailto:bug-
Class-Method-Modifiers@rt.cpan.org>).
AUTHOR
Shawn M Moore <sartak@gmail.com>
CONTRIBUTORS
• Karen Etheridge <ether@cpan.org>
• Shawn M Moore <code@sartak.org>
• Graham Knop <haarg@haarg.org>
• Aaron Crane <arc@cpan.org>
• Peter Rabbitson <ribasushi@cpan.org>
• David Steinbrunner <dsteinbrunner@pobox.com>
• gfx <gfuji@cpan.org>
• Justin Hunter <justin.d.hunter@gmail.com>
• mannih <github@lxxi.org>
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.
perl v5.36.0 2023-01-21 Class::Method::Modifiers(3pm)
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