From: Raul Parolari <raulparolari@...>
Date: 2007-11-19T13:25:23+09:00
Subject: Re: Composition: Build objects from other objects

Thufir wrote:

> On Sun, 18 Nov 2007 06:20:04 +0900, Raul Parolari wrote:
> 
>> 1) Thufir was right ..

> Well that's very complimentary of you :)

  Just stating a fact in the summary I attempted to do (which does not 
include the interesting and alternative view given by Trans, which came 
after).

> I need to learn more about Delegation..

You can learn what Delegation is very easily; the real work in the 
Forwarding module is done in a few lines; I wrote a mini-version of the 
module removing exception processing and others things not necessary, to 
play with it a bit.

Look at this simplified version (used in the example below):

module Simple_Forwardable

  def def_delegator(accessor, method, ali = method)

    module_eval(<<-EOS)
      def #{ali}(*args, &block)
         #{accessor}.send(:#{method}, *args, &block)
    EOS
  end
end

Believe it or not, these 6 lines are the heart of Forwardable; in short, 
in case you are not familiar with some things:

a) the method 'send' is used to 'send' a method name (eg, a method that 
you have in a string or symbol, for example that you collected from a 
web form or cmd line) to an object.
b) the module_eval allows to define dynamically methods using 
interpolation.
That's it!

If the syntax looks a bit mysterious at first, look at 'accessor' as our 
@engine object and 'method' as 'vroom'. Notice that we can even name an 
alias for 'vroom' if in the Vehicle class we prefer a different name 
(say, 'rev'); and,we can even pass arguments (eg, we can pass the level 
of the vroom we want). Both are done in the file below.

Below is the file that you may want to play with, if you find it useful 
(it combines the 2 examples given by Liam in one file; perhaps there are 
too many options, but you will decide that).

Good luck! and perhaps let us know how your project goes and if stood up 
well to the shock with 'reality' :-)

Raul


module Simple_Forwardable

  def def_delegator(accessor, method, ali= method)
    module_eval(<<-EOS)
      def #{ali}(*args, &block)
  #{accessor}.__send__(:#{method}, *args, &block)
      end
    EOS
  end

end

# from Liam's examples, combined
class Engine
  def vroom(level)
    puts "vroom #{level}"
  end
end

class MustangEngine
  def vroom(level)
    puts "rumble #{level}"
  end
end

class Vehicle
  extend Simple_Forwardable

  def_delegator :@engine, :vroom, :rev
  attr_accessor :engine

  def initialize(engine)
    @engine=engine
  end
end

class Factory
  def self.build_car
    Vehicle.new(Engine.new)
  end
end

falcon = Factory.build_car
falcon.rev('mid')    # => vroom mid

v = Vehicle.new(Engine.new)
v.rev('high')        # => vroom high

# changing the engine
v.engine = MustangEngine.new
v.rev('low')         # => rumble low
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