From: Phlip <phlip2005@...>
Date: 2007-09-15T06:00:16+09:00
Subject: metaprogramming to generate list of class-specific attributes

Rubyists:

Here's a test case illustrates the problem. Then we'll have a candidate fix
and some questions.

  class Kozmik
   def self.acts_as_frog(species)
     @@frog_list << species
   end
   @@frog_list = []
   cattr_accessor :frog_list
 end

 class Toad < Kozmik
   acts_as_frog :bufo
   acts_as_frog :kermit
 end

 class BullFrog < Kozmik
   acts_as_frog :tree
   acts_as_frog :riparian
 end

 def test_fun_with_metaclasses
   assert_equal [:tree, :riparian], BullFrog.frog_list
   assert_equal [:bufo, :kermit], Toad.frog_list
 end

Suppose we need an 'acts_as_' system to generate some methods for our target
classes. And suppose one method is a list of all the arguments passed to
each acts_as_() in each specific class.

The above test fails because @@ occupies the entire inheritance chain. So
both frog_list() class-methods return [:tree, :riparian, :bufo, :kermit].
The fix seems to be to reach into the meta-class for each derived class:

 class Kozmik
   def self.acts_as_frog(species)
     @frog_list ||= []
     @frog_list << species
     self.class.send :attr_accessor, :frog_list
   end
 end

Is that as clean and lean as it could be? Is there some way to do this with
even fewer lines?

Next question: Couldn't class Kozmik be a module instead?

-- 
 Phlip
 http://www.oreilly.com/catalog/9780596510657/
 ^ assert_xpath