add_event_subscriber

# File activesupport/lib/active_support/subscriber.rb, line 85
        def add_event_subscriber(event) # :doc:
          return if invalid_event?(event)

          pattern = prepare_pattern(event)

          # Don't add multiple subscribers (e.g. if methods are redefined).
          return if pattern_subscribed?(pattern)

          subscriber.patterns[pattern] = notifier.subscribe(pattern, subscriber)
        end