https://developer.apple.com/library/content/documentation/General/Conceptual/ConcurrencyProgrammingGuide/OperationQueues/OperationQueues.html#//apple_ref/doc/uid/TP40008091-CH102-SW25
Dispatch groups are a way to block a thread until one or more tasks finish executing. You can use this behavior in places where you cannot make progress until all of the specified tasks are complete. For example, after dispatching several tasks to compute some data, you might use a group to wait on those tasks and then process the results when they are done. Another way to use dispatch groups is as an alternative to thread joins. Instead of starting several child threads and then joining with each of them, you could add the corresponding tasks to a dispatch group and wait on the entire group.
dispatch_group_enter(group);
dispatch_group_leave(group);
用于任务本身是异步的情况；
dispatch_group_wait(group, dispatch_time(DISPATCH_TIME_NOW, 4 * NSEC_PER_SEC));
用于阻塞当前线程等待任务结束。
dispatch_group_notify：
用于异步等待任务结束。