当dispatch\u async任务完成时,如何通知我?

完成回调

完成回调可以通过对dispatch\u async()函数的嵌套调用来完成。它是 重要的是要记住在第一次调用dispatch\u async()之前保留目标队列,并在完成回调结束时释放该队列,以确保在完成回调挂起时不释放目标队列。例如:

 void
 async_read(object_t obj,
         void *where, size_t bytes,
         dispatch_queue_t destination_queue,
         void (^reply_block)(ssize_t r, int err))
 {
         // There are better ways of doing async I/O.
         // This is just an example of nested blocks.

         dispatch_retain(destination_queue);

         dispatch_async(obj->queue, ^{
                 ssize_t r = read(obj->fd, where, bytes);
                 int err = errno;

                 dispatch_async(destination_queue, ^{
                         reply_block(r, err);
                 });
                 dispatch_release(destination_queue);
         });
 }

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