test.rb

class SlackResource
  def self.build
    new
  end

  def send_message
    puts("Sending slack message")
  end
end

class ClassA
  def self.build
    new(Notification.build)
  end

  def initialize(notification)
    self.notification = notification
  end

  def call(message)
    notification.call(message)
  end
end

class ClassB < ClassA

end

class ClassB
  class Notification
    def self.build
      new(SlackResource.build)
    end

    def initialize(slack_resource)
      self.slack_resource = slack_resource
    end

    def call(message)
      notify_slack(message)
    end
  end
end

ClassB::Notification.build.call("my message")