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Moose-2.0008
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/ Moose::Manual::MethodModifiers

NAME

Moose::Manual::MethodModifiers - Moose's method modifiers

VERSION

version 2.0008

WHAT IS A METHOD MODIFIER?

Moose provides a feature called "method modifiers". You can also think of these as "hooks" or "advice".

It's probably easiest to understand this feature with a few examples:

package Example;

use Moose;

sub foo {
    print "    foo\n";
}

before 'foo' => sub { print "about to call foo\n"; };
after 'foo'  => sub { print "just called foo\n"; };

around 'foo' => sub {
    my $orig = shift;
    my $self = shift;

    print "  I'm around foo\n";

    $self->$orig(@_);

    print "  I'm still around foo\n";
};

Now if I call Example->new->foo I'll get the following output:

about to call foo
  I'm around foo
    foo
  I'm still around foo
just called foo

You probably could have figured that out from the names "before", "after", and "around".

Also, as you can see, the before modifiers come before around modifiers, and after modifiers come last.

When there are multiple modifiers of the same type, the before and around modifiers run from the last added to the first, and after modifiers run from first added to last:

before 2
 before 1
  around 2
   around 1
    primary
   around 1
  around 2
 after 1
after 2

WHY USE THEM?

Method modifiers have many uses. They are often used in roles to alter the behavior of methods in the classes that consume the role. See Moose::Manual::Roles for more information about roles.

Since modifiers are mostly useful in roles, some of the examples below are a bit artificial. They're intended to give you an idea of how modifiers work, but may not be the most natural usage.

BEFORE, AFTER, AND AROUND

Method modifiers can be used to add behavior to a method that Moose generates for you, such as an attribute accessor:

has 'size' => ( is => 'rw' );

before 'size' => sub {
    my $self = shift;

    if (@_) {
        Carp::cluck('Someone is setting size');
    }
};

Another use for the before modifier would be to do some sort of prechecking on a method call. For example:

before 'size' => sub {
    my $self = shift;

    die 'Cannot set size while the person is growing'
        if @_ && $self->is_growing;
};

This lets us implement logical checks that don't make sense as type constraints. In particular, they're useful for defining logical rules about an object's state changes.

Similarly, an after modifier could be used for logging an action that was taken.

Note that the return values of both before and after modifiers are ignored.

An around modifier is more powerful than either a before or after modifier. It can modify the arguments being passed to the original method, and you can even decide to simply not call the original method at all. You can also modify the return value with an around modifier.

An around modifier receives the original method as its first argument, then the object, and finally any arguments passed to the method.

around 'size' => sub {
    my $orig = shift;
    my $self = shift;

    return $self->$orig()
        unless @_;

    my $size = shift;
    $size = $size / 2
        if $self->likes_small_things();

    return $self->$orig($size);
};

before, after, and around can also modify multiple methods at once. The simplest example of this is passing them as a list:

before [qw(foo bar baz)] => sub {
    warn "something is being called!";
};

This will add a before modifier to each of the foo, bar, and baz methods in the current class, just as though a separate call to before was made for each of them. The list can be passed either as a bare list, or as an arrayref. Note that the name of the function being modified isn't passed in in any way; this syntax is only intended for cases where the function being modified doesn't actually matter. If the function name does matter, use something like this:

for my $func (qw(foo bar baz)) {
    before $func => sub {
        warn "$func was called!";
    };
}

In addition, you can specify a regular expression to indicate the methods to wrap, like so:

after qr/^command_/ => sub {
    warn "got a command";
};

This will match the regular expression against each method name returned by "get_method_list" in Class::MOP::Class, and add a modifier to each one that matches. The same caveats apply as above. Using regular expressions to determine methods to wrap is quite a bit more powerful than the previous alternatives, but it's also quite a bit more dangerous. In particular, you should make sure to avoid wrapping methods with a special meaning to Moose or Perl, such as meta, new, BUILD, DESTROY, AUTOLOAD, etc., as this could cause unintended (and hard to debug) problems.

INNER AND AUGMENT

Augment and inner are two halves of the same feature. The augment modifier provides a sort of inverted subclassing. You provide part of the implementation in a superclass, and then document that subclasses are expected to provide the rest.

The superclass calls inner(), which then calls the augment modifier in the subclass:

package Document;

use Moose;

sub as_xml {
    my $self = shift;

    my $xml = "<document>\n";
    $xml .= inner();
    $xml .= "</document>\n";

    return $xml;
}

Using inner() in this method makes it possible for one or more subclasses to then augment this method with their own specific implementation:

package Report;

use Moose;

extends 'Document';

augment 'as_xml' => sub {
    my $self = shift;

    my $xml = "  <report>\n";
    $xml .= inner();
    $xml .= "  </report>\n";

    return $xml;
};

When we call as_xml on a Report object, we get something like this:

<document>
  <report>
  </report>
</document>

But we also called inner() in Report, so we can continue subclassing and adding more content inside the document:

package Report::IncomeAndExpenses;

use Moose;

extends 'Report';

augment 'as_xml' => sub {
    my $self = shift;

    my $xml = '    <income>' . $self->income . '</income>';
    $xml .= "\n";
    $xml .= '    <expenses>' . $self->expenses . '</expenses>';
    $xml .= "\n";

    $xml .= inner() || q{};

    return $xml;
};

Now our report has some content:

<document>
  <report>
    <income>$10</income>
    <expenses>$8</expenses>
  </report>
</document>

What makes this combination of augment and inner() special is that it allows us to have methods which are called from parent (least specific) to child (most specific). This inverts the normal inheritance pattern.

Note that in Report::IncomeAndExpenses we call inner() again. If the object is an instance of Report::IncomeAndExpenses then this call is a no-op, and just returns false. It's a good idea to always call inner() to allow for future subclassing.

OVERRIDE AND SUPER

Finally, Moose provides some simple sugar for Perl's built-in method overriding scheme. If you want to override a method from a parent class, you can do this with override:

package Employee;

use Moose;

extends 'Person';

has 'job_title' => ( is => 'rw' );

override 'display_name' => sub {
    my $self = shift;

    return super() . q{, } . $self->title();
};

The call to super() is almost the same as calling $self->SUPER::display_name. The difference is that the arguments passed to the superclass's method will always be the same as the ones passed to the method modifier, and cannot be changed.

All arguments passed to super() are ignored, as are any changes made to @_ before super() is called.

SEMI-COLONS

Because all of these method modifiers are implemented as Perl functions, you must always end the modifier declaration with a semi-colon:

after 'foo' => sub { };

AUTHOR

Stevan Little <stevan@iinteractive.com>

COPYRIGHT AND LICENSE

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.