hook method_added in Module

gistfile1.rb

class Module
  def method_added(name)
    unless @_admin_only.nil? or @_proxy_method
      @_proxy_method = true
      alias_method "_admin_#{name}", name
      module_eval <<-STRING
        def #{name}(*args, &block)
          _admin_#{name}(*args, &block) if admin?
        end
      STRING
      @_proxy_method = false
    end
  end
 
  def admin_only
    @_admin_only = true
  end
end
 
class User
  def admin?
    [true, false][rand(2)]
  end
 
  admin_only
 
  def update_password
    puts "password updated"
  end
 
  def restart_server
    puts "server restarted"
  end
end

# source: http://www.diegocarrion.com/2010/05/27/ruby-annotations/

More info in the book Pragmatic Metaprogramming in Ruby of Paolo Perrota.
Or you can see the ruby doc of Module#method_added