From: David Masover <ninja@...>
Date: 2008-08-29T13:45:32+09:00
Subject: Re: Difficult Inheritance Problem

On Wednesday 27 August 2008 06:44:46 James Coglan wrote:
> >
> > The easiest way I can think of is:
> >
> >  class Page
> >    class << self
> >      attr_accessor :difference_mapping
> >    end
> >    def self.inherited(c)
> >      c.difference_mapping = difference_mapping
> >    end
> >  end
> >
> >  Page.difference_mapping =  { :blah => :blah }
> >
> >  class Next < Page
> >  end
> >
> >  p Next.difference_mapping   # { :blah => :blah }
> 
> 
> 
> Depends whether you only need the definition done once. This technique will
> not cause the subclass to reflect the parent class' value if it is changed,

In that case, let me take a shot:

class Page

  class << self
    attr_accessor :difference_mapping
  end

  module DescendantClassMethods
    def difference_mapping
      Page.difference_mapping
    end
    def difference_mapping= value
      Page.difference_mapping = value
    end
  end

  def self.inherited klass
    klass.extend DescendantClassMethods
  end
end


Since I'm in a late-night hacking mood, here's a way that's even more meta:

# because << drives me crazy
require 'metaid'

# name it whatever you want...
module ClassVariables
  module BaseClassMethods
    def class_variable name
      self.meta_eval do
        attr_accessor name
      end
      superclass = self
      (self::InheritableClassVariables ||= Module.new).module_eval do
        define_method name do
          superclass.send name
        end
        assignment = :"#{name}="
        define_method assignment do |value|
          superclass.send assignment, value
        end
      end
    end

    def inherited klass
      mod = self::InheritableClassVariables
      # I think this, instead of extend, will let subclasses override
      # these methods properly.
      klass.meta_eval do
        include mod
      end
    end
  end

  def self.included mod
    mod.extend BaseClassMethods
  end
end



Hmm. That's... mostly complete. Usage would be:

class Page
  include ClassVariable
  class_variable :difference_mapping
end

But I haven't so much as executed it to check for syntax errors.