app.rb

class App
  REQUEST_METHOD = 'REQUEST_METHOD'.freeze
  PATH_INFO = 'PATH_INFO'.freeze

  attr_reader :env

  def initialize(routes)
    @routes = routes
    @h = Hart.new(@routes)
  end

  def call(env)
    @env = env
    controller_object = controller_class.new(env)
    action_result = controller_object.send(action)
    
    controller_object.res
  end

  private

  def action
    hart_result[1]
  end

  def controller
    hart_result[0]
  end

  def controller_class
    Object.const_get(controller)
  end

  def hart_result
    @hart_result ||= @h.call(env[REQUEST_METHOD], env[PATH_INFO]).split('#')
  end
end

config.ru

require 'hart'
require_relative './app'
require_relative './controllers/controller'
require_relative './controllers/pages_controller'

routes = {
  default: "PagesController#not_found",

  GET: "PagesController#index",
  "foo" => {
    GET: "PagesController#foo"
  }
}

run App.new(routes)

controller.rb

class Controller
  attr_reader :env, :req, :res

  def initialize(env)
    @env = env
    @req = Rack::Request.new(@env)
    @res = Rack::Response.new
  end
end

pages_controller.rb

class PagesController < Controller
  def foo
    res.write('foo!!')
  end

  def index
    res.write('Hello!!')
  end

  def not_found
    res.status = 404
    res.write('Not found!')
  end
end