callbacks.rb

module Rack
  class Callbacks
    def initialize(&block)
      @before = []
      @after  = []
      instance_eval(&block) if block_given?
    end

    def before(middleware, *args, &block)
      if block_given?
        @before << middleware.new(*args, &block)
      else
        @before << middleware.new(*args)
      end
    end

    def after(middleware, *args, &block)
      if block_given?
        @after << middleware.new(*args, &block)
      else
        @after << middleware.new(*args)
      end
    end

    def run(app)
      @app = app
    end

    def call(env)
      @before.each {|c| c.call(env) }
      response = @app.call(env)
      @after.each {|c| c.call(env) }
      response
    end
  end
end

spec_rack_callbacks.rb

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

class Flame
  def call(env)
    env['flame'] = 'F Lifo..'
  end
end

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

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

class Finale
  def call(env)
    $hax_logger = 'lol'
  end
end

context "Rack::Callbacks" do
  specify "works for love and small stack trace" do
    callback_app = Rack::Callbacks.new do
      before Flame
      before Pacify, "with love"

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

      after Finale
    end

    app = Rack::Builder.new do
      run callback_app
    end.to_app

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