code: static function instance(string $name) : ?self {
          if (!isset(self::$_instances[$name])) {
              Console::warning("Cron service \"$name\" do not exist.");
              return null;
          }
          return self::$_instances[$name];
      }
docs: 获取计划任务服务实例
  
  @param string $name
  @return Cron|null