Class::Delegator(3pm) User Contributed Perl Documentation Class::Delegator(3pm)
Name
Class::Delegator - 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 <bug-class-delegator@rt.cpan.org> or report them via the CPAN Request Tracker at
<http://rt.cpan.org/NoAuth/Bugs.html?Dist=Class-Delegator>.
Author
David Wheeler <david@kineticode.com>
See Also
Class::Delegation
Damian Conway's brilliant module does ten times what this one does--and does it ten times slower.
Class::Delegate
Kurt Starsinic's module uses inheritance to manage delegation, and has a somewhat more complex interface.
Class::HasA
Simon Cozen's delegation module takes the same approach as this module, but provides no method for resolving method name clashes the
way this module's "as" parameter does.
Copyright and License
Copyright (c) 2005-2008 David Wheeler. Some Rights Reserved.
This module is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
perl v5.10.0 2008-07-23 Class::Delegator(3pm)