private_classes.rb

class PostsController
  def new
    @post = Post.new
  end

  def create
      service = post_service.new
      service.delegate = self
      service.create_new_post(params[:post])
  end

  # PostServiceProtocol begins
  def new_post_created_successfully(post)
      redirect_to posts_path, :notice => "Post created succesfully!"
  end

  def new_post_not_created_because_of(errors)
    render :new
  end
  # PostServiceProtocol ends

  private

  # I don't like this way
  def post_service
    eval("
    class PostService
      attr_writer :delegate

      def create_new_post(params)
          post = Post.new(params)

          if post.save
            self.delegate.new_post_created_successfully(post)
          else
            self.delegate.new_post_not_created_because_of(post.errors)
          end
      end

      protected
      def delegate
        @delegate ||= NullPostServiceDelegate.new
      end
    end

    class NullPostServiceDelegate
      def new_post_created_successfully(post)
      # no-op
      end

      def new_post_not_created_because_of(errors)
        # no-op
      end
    end
    ")

    PostService
  end

  # I actually like this way.
  def post_service_two
    require_relative 'public_classes'
    PostService
  end
end

puts "PostsController.constants: #{PostsController.constants}"
puts "PostService: #{PostService}"

public_controller.rb

class PostsController
  def new
    @post = Post.new
  end

  def create
      service = post_service.new
      service.delegate = self
      service.create_new_post(params[:post])
  end

  # PostServiceProtocol begins
  def new_post_created_successfully(post)
      redirect_to posts_path, :notice => "Post created succesfully!"
  end

  def new_post_not_created_because_of(errors)
    render :new
  end
  # PostServiceProtocol ends

  private

  # I don't like this way
  def post_service
    eval("
    class PostService
      attr_writer :delegate

      def create_new_post(params)
          post = Post.new(params)

          if post.save
            self.delegate.new_post_created_successfully(post)
          else
            self.delegate.new_post_not_created_because_of(post.errors)
          end
      end

      protected
      def delegate
        @delegate ||= NullPostServiceDelegate.new
      end
    end

    class NullPostServiceDelegate
      def new_post_created_successfully(post)
      # no-op
      end

      def new_post_not_created_because_of(errors)
        # no-op
      end
    end
    ")

    PostService
  end

  # I actually like this way.
  def post_service_two
    require_relative 'public_classes'
    PostService
  end
end

puts "PostsController.constants: #{PostsController.constants}"
puts "PostService: #{PostService}"