code: private static void subscribeIfNotDone(EventCallBackStruct eventCallBackStruct,
                                            String callbackKey) throws DevFailed{
  
          eventCallBackStruct.consumer.subscribe_event(
                  eventCallBackStruct.device,
                  eventCallBackStruct.attr_name,
                  eventCallBackStruct.event_type,
                  eventCallBackStruct.callback,
                  eventCallBackStruct.max_size,
                  eventCallBackStruct.filters,
                  false);
          failed_event_callback_map.remove(callbackKey);
      }
docs: ===============================================================