callbacks.rb

module Rack
  class Callbacks < ::Array
    DUMMY_APP = proc {|env| }

    def initialize(app, &block)
      @app = app
      instance_eval(&block) if block_given?
    end

    def use(middleware, *args, &block)
      if block_given?
        push middleware.new(DUMMY_APP, *args, &block)
      else
        push middleware.new(DUMMY_APP, *args)
      end
    end

    def call(env)
      each {|c| c.call(env) }
      @app.call(env)
    end
  end
end

spec_rack_callbacks.rb

require 'rack/mock'
require 'rack/contrib/callbacks'

class Flame
  def initialize(app)
    @app = app
  end

  def call(env)
    env['flame'] = 'F Lifo..'
    @app.call(env)
  end
end

class Pacify
  def initialize(app, with)
    @app = app
    @with = with
  end

  def call(env)
    env['peace'] = @with
    @app.call(env)
  end
end

context "Rack::Callbacks" do
  specify "supports blocks on use" do

    app = Rack::Builder.new do
      use Rack::Callbacks do
        use Flame
        use Pacify, "with love"
      end

      run lambda { |env| [200, {}, env['flame'] + env['peace']] }
    end.to_app

    response = Rack::MockRequest.new(app).get("/")
    response.body.to_s.should.equal 'F Lifo..with love'
  end
end