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Moose::Manual::MethodModifiers(3)    User Contributed Perl Documentation   Moose::Manual::MethodModifiers(3)



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

VERSION
       version 2.0205

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 methods without modifying the definition of those
       methods.

   BEFORE and AFTER modifiers
       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.

   AROUND modifiers
       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);
         };

   Wrapping multiple methods at once
       "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!";
             };
         }

   Using regular expressions to select methods to wrap
       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.  Bear in mind that if your regular expression
       matches certain Perl and Moose reserved method names with a special meaning to Moose or Perl, such as
       "meta", "new", "BUILD", "DESTROY", "AUTOLOAD", etc, this could cause unintended (and hard to debug)
       problems and is best avoided.

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 { };

CAVEATS
       These method modification features do not work well with multiple inheritance, due to how method
       resolution is performed in Perl. Experiment with a test program to ensure your class hierarchy works
       as expected, or more preferably, don't use multiple inheritance (roles can help with this)!

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.



perl v5.12.5                                     2011-09-06                Moose::Manual::MethodModifiers(3)