How does dispatch_set_target_queue work?

Question

Due to the lack of material on `dispatch_set_target_queue`, I have come here for help, so thanks! Here is my test code: ``` dispatch_queue_t mySerialDispatchQueue1 = dispatch_queue_create("come.itenyh", NULL); dispatch_queue_t mySerialDispatchQueue2 = dispatch_queue_create("come.itenyh1", NULL); dispatch_set_target_queue(mySerialDispatchQueue1, mySerialDispatchQueue2); dispatch_async(mySerialDispatchQueue1, ^{[self task:@"s1"];}); dispatch_async(mySerialDispatchQueue2, ^{[self task:@"p1"];}); dispatch_async(mySerialDispatchQueue1, ^{[self task:@"s2"];}); - (void)task:(NSString *)taskid { NSLog(@"Now executing taskid:%@", taskid); [NSThread sleepForTimeInterval:5]; } ``` Now if I set ``` dispatch_set_target_queue(mySerialDispatchQueue2, mySerialDispatchQueue1); ``` then the result is: ``` 2014-04-16 22:23:49.581 ITGCDLearning[66758:1303] Now executing taskid:s1 2014-04-16 22:23:54.585 ITGCDLearning[66758:1303] Now executing taskid:s2 2014-04-16 22:23:59.586 ITGCDLearning[66758:1303] No…

Accepted Answer

According to me when you set a target queue to other you are synchronizing the task of both the queue so In first case:

```
dispatch_set_target_queue(mySerialDispatchQueue2, mySerialDispatchQueue1);
```

`mySerialDispatchQueue1` is target queue so all the task added in `mySerialDispatchQueue2` is also enqueued from the `mySerialDispatchQueue1`. As it is target queue.So `mySerialDispatchQueue1` has already two task so other one from queue `mySerialDispatchQueue2` is added later.

In second Case:

```
dispatch_set_target_queue(mySerialDispatchQueue1, mySerialDispatchQueue2);
```

your target queue is `mySerialDispatchQueue2` so in start when there is no task then task from `mySerialDispatchQueue1` is added in the `mySerialDispatchQueue2` then its own task is in queue. So in this way task has been added.

How does dispatch_set_target_queue work?

Solution

Problem