forwardable - Support for the Delegation Pattern

$Release Version: 1.1$
$Revision: 16857 $
$Date: 2008-06-06 17:05:24 +0900 (Fri, 06 Jun 2008) $
by Keiju ISHITSUKA(keiju@ishitsuka.com)

Documentation by James Edward Gray II and Gavin Sinclair

Introduction

This library allows you delegate method calls to an object, on a method by method basis. You can use Forwardable to setup this delegation at the class level, or SingleForwardable to handle it at the object level.

Notes

Be advised, RDoc will not detect delegated methods.

forwardable.rb provides single-method delegation via the def_delegator() and def_delegators() methods. For full-class delegation via DelegateClass(), see delegate.rb.

Examples

Forwardable

Forwardable makes building a new class based on existing work, with a proper interface, almost trivial. We want to rely on what has come before obviously, but with delegation we can take just the methods we need and even rename them as appropriate. In many cases this is preferable to inheritance, which gives us the entire old interface, even if much of it isn’t needed.

class Queue
  extend Forwardable

  def initialize
    @q = [ ]    # prepare delegate object
  end

  # setup preferred interface, enq() and deq()...
  def_delegator :@q, :push, :enq
  def_delegator :@q, :shift, :deq

  # support some general Array methods that fit Queues well
  def_delegators :@q, :clear, :first, :push, :shift, :size
end

q = Queue.new
q.enq 1, 2, 3, 4, 5
q.push 6

q.shift    # => 1
while q.size > 0
  puts q.deq
end

q.enq "Ruby", "Perl", "Python"
puts q.first
q.clear
puts q.first

Prints:

2
3
4
5
6
Ruby
nil

SingleForwardable

printer = String.new
printer.extend SingleForwardable        # prepare object for delegation
printer.def_delegator "STDOUT", "puts"  # add delegation for STDOUT.puts()
printer.puts "Howdy!"

Prints:

Howdy!