middleware_builder_access.rb

module Rack
  class Builder
    def use(middleware, *args, &block)
      middleware.instance_variable_set "@rack_builder", self
      def middleware.rack_builder
        @rack_builder
      end
      @ins << lambda { |app| middleware.new(app, *args, &block) }
    end
    
    def leaf_app
      @ins.last
    end
  end

  class Flash
    def initialize(app, opts={})
      rack_app = self.class.rack_builder.leaf_app
      def rack_app.flash
        env['rack-flash']
      end
      @app, @opts = app, opts
    end
  end
end