callbacks.rb

        callbacks.each do |callback|
          class_eval <<-"end_eval"
            def self.#{callback}(*methods, &block)                             # def self.before_save(*methods, &block)
              callbacks = CallbackChain.build(:#{callback}, *methods, &block)  #   callbacks = CallbackChain.build(:before_save, *methods, &block)
              @#{callback}_callbacks ||= CallbackChain.new                     #   @before_save_callbacks ||= CallbackChain.new
              @#{callback}_callbacks.concat callbacks                          #   @before_save_callbacks.concat callbacks
            end                                                                # end
                                                                               #
            def self.#{callback}_callback_chain                                # def self.before_save_callback_chain
              @#{callback}_callbacks ||= CallbackChain.new                     #   @before_save_callbacks ||= CallbackChain.new
                                                                               #
              if superclass.respond_to?(:#{callback}_callback_chain)           #   if superclass.respond_to?(:before_save_callback_chain)
                CallbackChain.new(                                             #     CallbackChain.new(
                  superclass.#{callback}_callback_chain +                      #       superclass.before_save_callback_chain +
                  @#{callback}_callbacks                                       #       @before_save_callbacks
                )                                                              #     )
              else                                                             #   else
                @#{callback}_callbacks                                         #     @before_save_callbacks
              end                                                              #   end
            end                                                                # end
          end_eval
        end