callbacks.rb

      def set_callback(name, *filter_list, &block)
        mapped = nil

        __update_callbacks(name, filter_list, block) do |target, chain, type, filters, options|
          mapped ||= filters.map do |filter|
            Callback.new(chain, filter, type, options.dup, self)
          end

          filters.each do |filter|
            chain.delete_if {|c| c.matches?(type, filter) }
          end

          options[:prepend] ? chain.unshift(*(mapped.reverse)) : chain.push(*mapped)

          target.send("_#{name}_callbacks=", chain)
        end
      end
      def __update_callbacks(name, filters = [], block = nil) #:nodoc:
        type = filters.first.in?([:before, :after, :around]) ? filters.shift : :before
        options = filters.last.is_a?(Hash) ? filters.pop : {}
        filters.unshift(block) if block

        ([self] + ActiveSupport::DescendantsTracker.descendants(self)).reverse.each do |target|
          chain = target.send("_#{name}_callbacks")
          yield target, chain.dup, type, filters, options
          target.__reset_runner(name)
        end
      end