annotations.rb

class Class
  def with_attr_reader(*args, &b)
    with_attr(:attr_reader, *args, &b)
  end
  
  def with_attr_writer(*args, &b)
    with_attr(:attr_writer, *args, &b)
  end
  
  def with_attr_accessor(*args, &b)
    with_attr(:attr_accessor, *args, &b)
  end
  
  private
  
  def with_attr(type, *args, &b)
    @annotations ||= {}
    ann = Annotations.new
    ann.instance_eval(&b)
    self.send(type, *args)
    args.each do |arg|
      ann.copy_to(@annotations[arg] ||= {})
      ann.run_procs(self, arg)
    end
    
    unless respond_to?(:annotations) || @annotations.length == 0
      (class << self; self; end).class_eval do
        define_method :annotations do
          @annotations
        end
      end
    end
  end
end

class Annotations
  def registered
    (@registered ||= {})
  end
  
  def copy_to(original)
    registered.each do |k, v|
      (original[k] ||= []).concat(v)
    end
  end
  
  def run_procs(context, method)
    procs.each { |p| p.call(context, method) }
  end
  
  private
  
  def procs
    @procs ||= []
  end
  
  def register(ann)
    if ann.class == Proc
      procs.push(ann)
    elsif ann.class == Hash
      ann.each do |k, v|
          (registered[k] ||= []).push(v)
      end
    end
  end
end

class Annotations
  def make_awesome(at_what)
    register :awesome_at => at_what
  end
  
  def make_crappy(at_what)
    register :crappy_at => at_what 
  end
  
  def shadow_attr(name)
    register lambda { |context, a|
      context.send(:attr_reader, "#{a}_#{name}")
    }
  end
end

class TestWithAnnotations
  with_attr_reader :bird do
    make_awesome :flying
    make_crappy :swimming
    shadow_attr :shadow
    shadow_attr :splash
  end
end