From: Trans <transfire@...>
Date: 2005-10-21T08:31:59+09:00
Subject: Re: Reflection, observ(er|able) instance vars?

Use Module, or better yet your own mixin. And just do the class eval
yourself w/o metaid, it's pretty easy

module HashAttr

   def attr_hashing( *arr )
     attr_reader *arr
     arr.each do |var|
       class_eval %{
         def #{var}(val) ; @var ; end
       }
       class_eval %{
         def #{var}=(val)
           @var=val
           @thehash = nil  #reset
         end
       }
     end
   end

 end

Then do

  class Wombat
    extend HashAttr

    attr_hashing :wom, :bat
    attr_reader :thehash

    def initialize
      @wom = "wom"
      @bat = "bat"
      @thehash = nil
    end

    def hash
      puts "hash called"
      @thehash ||= (@wom.to_s + @bat.to_s).hash
    end

    def to_s
      @wom.inspect + @bat.inspect
    end

    # compare
    def ==( other )
      hash == other.hash
    end
  end 

HTH,
T.