From: Phrogz <phrogz@...>
Date: 2008-02-13T06:14:55+09:00
Subject: Re: How to add a instance variable through a mixin?

On Feb 12, 12:54 pm, "MrBana...@googlemail.com"
<MrBana...@googlemail.com> wrote:
> > If you mix the module in after some instances of MyClass have already
> > been created, do you want it to dynamically create the instance
> > variables in those instances?
>
> > If you mix the module in before some instances of MyClass have been
> > created, do you want it to run code each time a new instance is
> > created?
>
> I would be interested in both cases, but for my particularly case it s
> the second one.

Here's a way to do it:

module Rands
  def self.included( klass )
    # Modify existing instances
    ObjectSpace.each_object( klass ){ |inst|
      inst.initialize_rands
    }

    # Replace the initialization with your own
    klass.class_eval{
      # Beware name clashes
      alias_method :init_pre_rand, :initialize
      def initialize( *args )
        init_pre_rand( *args )
        initialize_rands
      end
    }
  end

  def initialize_rands
    @foo = "%.2f" % rand
  end
end

class Foo
  def initialize( id )
    @id = id
  end
end

f1 = Foo.new( 1 )
f2 = Foo.new( 2 )
class Foo
  include Rands
end
f3 = Foo.new( 3 )
f4 = Foo.new( 4 )

p f1, f2, f3, f4
#=> #<Foo:0x7ffa696c @id=1, @foo="0.92">
#=> #<Foo:0x7ffa691c @id=2, @foo="0.57">
#=> #<Foo:0x7ffa6908 @id=3, @foo="0.40">
#=> #<Foo:0x7ffa68cc @id=4, @foo="0.41">