From: Trans <transfire@...>
Date: 2006-09-28T10:15:10+09:00
Subject: Multi-Module Alternative to the Factory Pattern?

I've been using a factory pattern for a set of packaging classes
--classes that generate different package formats (eg. Debian, RPM,
etc.), but it's not quite the right fit b/c it leads to repetition of
the staging step. Yuk! So I think a better fit is to have one class
(Packager) that calls on external modules (or classes) for each
different format. But, unfortuately there are quite a few attributes
needed by each from the Packager, and it's just doesn't seem right to
pass that many variables. So it seems like it would be better to just
include the needed module into the Packager class as needed. But here
again I can't un-include a format module once done with it in order to
use another format. So I'm right back to using a factory pattern.

Then I had this idea. and I'm wondering what others think of it.

First we need this little extension from Facets:

  class Object

    # Like super but skips to a specific ancestor module or class.
    #
    #   class A
    #     def x ; 1 ; end
    #   end
    #
    #   class B < A
    #     def x ; 2 ; end
    #   end
    #
    #   class C < B
    #     def x ; superior(A) ; end
    #   end
    #
    #   C.new.x  #=> 1
    #
    def superior(klass=self.class.superclass, *args, &blk)
      unless self.class.ancestors.include?(klass)
        raise ArgumentError
      end
      called = /\`([^\']+)\'/.match(caller(1).first)[1].to_sym
      klass.instance_method(called).bind(self).call(*args,&blk)
    end

  end

Then:

  module Debian
    def pack ; "Debian package"; end
  end

  module RPM
    def pack ; "RPM package"; end
  end

  class Packager
    include Debian, RPM

    def pack( type )
      superior(type)
    end
 end

  pkgr = Packager.new
  pkgr.pack(Debian)  #=> "Debian package"
  pkgr.pack(RPM)      #=> "RPM package"

What do you think?

Thanks,
T.