add_event_subscriber

# File activesupport/lib/active_support/subscriber.rb, line 65
      def add_event_subscriber(event) # :doc:
        return if %{ start finish }.include?(event.to_s)

        pattern = "#{event}.#{namespace}"

        # Don't add multiple subscribers (eg. if methods are redefined).
        return if subscriber.patterns.include?(pattern)

        subscriber.patterns << pattern
        notifier.subscribe(pattern, subscriber)
      end