build

# File actionpack/lib/action_controller/metal.rb, line 34
    def build(action, app=nil, &block)
      app  ||= block
      action = action.to_s
      raise "MiddlewareStack#build requires an app" unless app

      reverse.inject(app) do |a, middleware|
        middleware.valid?(action) ?
          middleware.build(a) : a
      end
    end