man Class::Delegator (3): Simple and fast object-oriented delegation

Synopsis

  package Car;
  use Class::Delegator
      send => 'start',
        to => '{engine}',
      send => 'power',
        to => 'flywheel',
        as => 'brake',
      send => [qw(play pause rewind fast_forward shuffle)],
        to => 'ipod',
      send => [qw(accelerate decelerate)],
        to => 'brakes',
        as => [qw(start stop)],
      send => 'drive',
        to => [qw(right_rear_wheel left_rear_wheel)],
        as => [qw(rotate_clockwise rotate_anticlockwise)]
  ;

Description

This module provides a subset of the functionality of Damian Conway's lovely Class::Delegation module. Why a subset? Well, I didn't need all of the fancy matching semantics, just string string specifications to map delegations. Furthermore, I wanted it to be fast (See Benchmarks). And finally, since Class::Delegation uses an "INIT" block to do its magic, it doesn't work in persistent environments that don't execute "INIT" blocks, such as in mod_perl.

However the specification semantics of Class::Delegator differ slightly from those of Class::Delegation, so this module isn't a drop-in replacement for Class::Delegation. Read on for details.

Specifying methods to be delegated

The names of methods to be redispatched can be specified using the "send" parameter. This parameter may be specified as a single string or as an array of strings. A single string specifies a single method to be delegated, while an array reference is a list of methods to be delegated.

Specifying attributes to be delegated to

Use the "to" parameter to specify the attribute(s) or accessor method(s) to which the method(s) specified by the "send" parameter are to be delegated. The semantics of the "to" parameter are a bit different from Class::Delegation. In order to ensure the fastest performance possible, this module simply installs methods into the calling class to handle the delegation. There is no use of $AUTOLOAD or other such trickery. But since the new methods are installed by "eval"ing a string, the "to" parameter for each delegation statement must be specified in the manner appropriate to accessing the underlying attribute. For example, to delegate a method call to an attribute stored in a hash key, simply wrap the key in braces:

  use Class::Delegator
      send => 'start',
        to => '{engine}',
  ;

To delegate to a method, simply name the method:

  use Class::Delegator
      send => 'power',
        to => 'flywheel',
  ;

If your objects are array-based, wrap the appropriate array index number in brackets:

  use Class::Delegator
      send => 'idle',
        to => '[3]',
  ;

And so on.

Specifying the name of a delegated method

Sometimes it's necessary for the name of the method that's being delegated to be different from the name of the method to which you're delegating execution. For example, your class might already have a method with the same name as the method to which you're delegating. The "as" parameter allows you translate the method name or names in a delegation statement. The value associated with an "as" parameter specifies the name of the method to be invoked, and may be a string or an array (with the number of elements in the array matching the number of elements in a corresponding "send" array).

If the attribute is specified via a single string, that string is taken as the name of the attribute to which the associated method (or methods) should be delegated. For example, to delegate invocations of "$self->power(...)" to "$self->{flywheel}->brake(...)":

  use Class::Delegator
      send => 'power',
        to => '{flywheel}',
        as => 'brake',
  ;

If both the "send" and the "as" parameters specify array references, each local method name and deleted method name form a pair, which is invoked. For example:

  use Class::Delegator
      send => [qw(accelerate decelerate)],
        to => 'brakes',
        as => [qw(start stop)],
  ;

In this example, the "accelerate" method will be delegated to the "start" method of the "brakes" attribute and the "decelerate" method will be delegated to the "stop" method of the "brakes" attribute.

Delegation to multiple attributes in parallel

An array reference can be used as the value of the "to" parameter to specify the a list of attributes, all of which are delegated to---in the same order as they appear in the array. In this case, the "send" parameter must be a scalar value, not an array of methods to delegate.

For example, to distribute invocations of "$self->drive(...)" to both "$self->{left_rear_wheel}->drive(...)" and "$self->{right_rear_wheel}->drive(...)":

  use Class::Delegator
      send => 'drive',
        to => ["{left_rear_wheel}", "{right_rear_wheel}"]
  ;

Note that using an array to specify parallel delegation has an effect on the return value of the delegation method specified by the "send" parameter. In a scalar context, the original call returns a reference to an array containing the (scalar context) return values of each of the calls. In a list context, the original call returns a list of array references containing references to the individual (list context) return lists of the calls. So, for example, if the "cost" method of a class were delegated like so:

  use Class::Delegator
      send => 'cost',
        to => ['supplier', 'manufacturer', 'distributor']
  ;

then the total cost could be calculated like this:

  use List::Util 'sum';
  my $total = sum @{$obj->cost()};

If both the "to" key and the "as" parameters specify multiple values, then each attribute and method name form a pair, which is invoked. For example:

  use Class::Delegator
      send => 'escape',
        to => ['{flywheel}', '{smokescreen}'],
        as => ['engage',   'release'],
  ;

would sequentially call, within the "escape()" delegation method:

  $self->{flywheel}->engage(...);
  $self->{smokescreen}->release(...);

Benchmarks

I whipped up a quick script to compare the performance of Class::Delegator to Class::Delegation and a manually-installed delegation method (the control). I'll let the numbers speak for themselves:

  Benchmark: timing 1000000 iterations of Class::Delegation, Class::Delegator, Manually...
  Class::Delegation: 106 wallclock secs (89.03 usr +  2.09 sys = 91.12 CPU) @ 10974.54/s  (n=1000000)
  Class::Delegator:    3 wallclock secs ( 3.44 usr +  0.02 sys =  3.46 CPU) @ 289017.34/s (n=1000000)
           Control:    3 wallclock secs ( 3.01 usr +  0.02 sys =  3.03 CPU) @ 330033.00/s (n=1000000)

Bugs

Please send bug reports to <[email protected]> or report them via the CPAN Request Tracker at <http://rt.cpan.org/NoAuth/Bugs.html?Dist=Class-Delegator>.

Author

David Wheeler <[email protected]>

Copyright and License

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