From: Pat Maddox <pergesu@...>
Date: 2007-11-18T07:08:26+09:00
Subject: Re: Composition: Build objects from other objects

On Nov 16, 2007 9:36 PM, Raul Parolari <raulparolari@gmail.com> wrote:
> Thufir wrote:
> >
> > then can you make the vw_bug go "vroooooom" and so forth?  What would be
> > the usual way to do this in ruby?  When a car is instantiated, should an
> > engine object get passed to the initializer method?
> >
> > thanks,
> >
> > Thufir
>
> I do not like (at first view) the idea of this design; it leads to, as
> Konrad has correctly observed, to start the car by saying:
>
>     vw_bug.engine.vroom()
>
> Should the car user know that he has to tell the car to ask the engine
> to rev up? Uhm..
>
> Another take is the following; let's use a module Engine:
>
> module Engine
>   attr_accessor :type
>
>   def initialize
>     @type = "generic engine"
>   end
>
>   def vroom
>     puts "vroooooom"
>   end
> end
>
> class Vehicle
>   include Engine
>
>   def initialize
>     super
>   end
> end
>
> class Car < Vehicle
>   def initialize
>     super
>   end
> end
>
> vw_bug = Car.new
> vw_bug.vroom
>
> I am not crazy of the 'Engine' Module, but I definitely like to ask the
> VW to wroom, without worrying about what is inside,
>
> Raul
>
> --
> Posted via http://www.ruby-forum.com/.
>
>


I think this is a pretty bad approach.  Consider the following example
(keep in mind I know nothing about cars):

module Engine
  attr_reader :engine

  def initialize
    @engine = :generic
  end

  def vroom
    puts "vrooooooom"
  end
end

module Transmission
  attr_reader :transmission

  def initialize
    @transmission = :silky_smooth
  end

  def vroom
    puts "smooth like butta"
  end
end

class Car
  include Engine
  include Transmission

  def initialize
    super
  end
end

By "composing" it with modules like that, we end up with a seriously
broken car - one with no engine!  Clearly this approach won't work.
Part of it is due to how you set stuff up in initialize...but anyway,
the point is that you don't really use modules for this kind of
composition.

A far better approach would be to have different engine and
transmission objects.

class Engine
  def vroom
    "vrooooooom"
  end
end

class Transmission
  def vroom
    "smooth like butta"
  end
end

class Car
  def initialize(engine, transmission)
    @engine = engine
    @transmission = transmission
  end

  def vroom
    "we go #{@engine.vroom}, #{@transmission.vroom}"
  end
end

c = Car.new Engine.new, Transmission.new
c.vroom   # => "we go vrooooooom, smooth like butta"

if you want to create a car with its parts already initialized, you
can create a factory method:

def Car.basic
  self.new Engine.new, Transmission.new
end

The basic idea is that you don't really want to make the car object
responsible for a bunch of stuff...you want a bunch of little objects
that know how to do one thing well.

Pat