app.rb

module Hook
  def self.included(klass)
    klass.extend DSLMethod
  end

  def initialize(app = nil)
    @app = app
  end

  def call(env)
    path = env["PATH_INFO"] ||= "/"
    puts self.class
    ["200", { "Content-Type" => "text/html" }, ["Hello, %s via %s" % [path,self.class]]]
  end

  module DSLMethod
    def self.extended(klass)
     klass.instance_exec do
        @builder = Rack::Builder.new
        @builder.run(klass.new)
      end
    end

    def use(*args, &block)
      @builder.use(*args, &block)
    end

    def map(*args, &block)
      print "maped ", args
      @builder.map(*args, &block)
    end
  end
end

class Admin
  include Hook
end

class Main
  include Hook
  map "/admin" do
    use Admin
    run Admin.new
  end
end

config.ru

require './app.rb'
run Main.new

想实现以 admin 开始的都由 class Admin 来处理 但是实际上都是 class Main 处理，Rack::Builder.map 不起作用