subscribe

# File activesupport/lib/active_support/testing/event_reporter_assertions.rb, line 66
            def subscribe
              return if @subscribed

              @mutex.synchronize do
                unless @subscribed
                  if ActiveSupport.event_reporter
                    ActiveSupport.event_reporter.subscribe(self)
                    @subscribed = true
                  else
                    raise Minitest::Assertion, "No event reporter is configured"
                  end
                end
              end
            end