class ActiveSupport::Callbacks::Callback

def start(key=nil, object=nil)
  return if key && !object.send("_one_time_conditions_valid_#{@callback_id}?")
  # options[0] is the compiled form of supplied conditions
  # options[1] is the "end" for the conditional
  #
  case @kind
  when :before
    # if condition    # before_save :filter_name, :if => :condition
    #   filter_name
    # end
    <<-RUBY_EVAL
      if !halted && #{@compiled_options}
        # This double assignment is to prevent warnings in 1.9.3 as
        # the `result` variable is not always used except if the
        # terminator code refers to it.
        result = result = #{@filter}
        halted = (#{chain.config[:terminator]})
        if halted
          halted_callback_hook(#{@raw_filter.inspect.inspect})
        end
      end
    RUBY_EVAL
  when :around
    # Compile around filters with conditions into proxy methods
    # that contain the conditions.
    #
    # For `around_save :filter_name, :if => :condition':
    #
    # def _conditional_callback_save_17
    #   if condition
    #     filter_name do
    #       yield self
    #     end
    #   else
    #     yield self
    #   end
    # end
    #
    name = "_conditional_callback_#{@kind}_#{next_id}"
    @klass.class_eval <<-RUBY_EVAL,  __FILE__, __LINE__ + 1
       def #{name}(halted)
        if #{@compiled_options} && !halted
          #{@filter} do
            yield self
          end
        else
          yield self
        end
      end
    RUBY_EVAL
    "#{name}(halted) do"
  end
end