subscribe

# File activesupport/lib/active_support/notifications/fanout.rb, line 68
      def subscribe(pattern = nil, callable = nil, monotonic: false, &block)
        subscriber = Subscribers.new(pattern, callable || block, monotonic)
        synchronize do
          case pattern
          when String
            @string_subscribers[pattern] << subscriber
            clear_cache(pattern)
          when NilClass, Regexp
            @other_subscribers << subscriber
            clear_cache
          else
            raise ArgumentError,  "pattern must be specified as a String, Regexp or empty"
          end
        end
        subscriber
      end