It can be used to track which operation finished if the same completion-handler object is used for multiple operations.

如果相同的对象用于多个操作，其可用于追踪哪个操作已完成。

youdao

It can be used to track which operation finished if the same completion-handler object is used for multiple operations.

如果相同的对象用于多个操作，其可用于追踪哪个操作已完成。

youdao