dispatch_set_target_queue让多个串行队列

在好多网络上的文章里面都有提到将多个串行队列使用dispatch_set_target_queue设置一个串行队列Target来保持同步执行

    dispatch_queue_t q1 = dispatch_queue_create("1", DISPATCH_QUEUE_SERIAL);
    dispatch_queue_t q2 = dispatch_queue_create("2", DISPATCH_QUEUE_SERIAL);
    dispatch_queue_t q3 = dispatch_queue_create("3", DISPATCH_QUEUE_SERIAL);
    dispatch_set_target_queue(q1, q3);
    dispatch_set_target_queue(q2, q3);
    dispatch_async(q1, ^{
        sleep(1);
        NSLog(@"1");
    });
    dispatch_async(q2, ^{
        sleep(1);
        NSLog(@"2");
    });

通过上面代码可以发现，两个串行队列q1,q2本来会同时执行，但是添加了target之后，保持了顺序执行。
但是其实这样做会导致其他问题。
情况1：

    dispatch_queue_t q1 = dispatch_queue_create("1", DISPATCH_QUEUE_SERIAL);
    dispatch_queue_t q2 = dispatch_queue_create("2", DISPATCH_QUEUE_SERIAL);
    dispatch_queue_t q3 = dispatch_queue_create("3", DISPATCH_QUEUE_SERIAL);
    dispatch_set_target_queue(q1, q3);
    dispatch_set_target_queue(q2, q3);
    for (int i = 0; i < 2; i ++) {
        dispatch_async(q1, ^{
            NSLog(@"123---%@", [NSThread currentThread]);
        });
        dispatch_async(q2, ^{
            NSLog(@"abc---%@", [NSThread currentThread]);
        });
    }

123---<NSThread: 0x600002e56840>{number = 3, name = (null)}
123---<NSThread: 0x600002e56840>{number = 3, name = (null)}
abc---<NSThread: 0x600002e56840>{number = 3, name = (null)}
abc---<NSThread: 0x600002e56840>{number = 3, name = (null)}

正常情况来说，上面两个任务应该交替执行才对。但是打印结果却是q1任务全部执行完才会执行q2.
所以通过dispatch_set_target_queue这个方法并不能解决让多个串行队列同步执行的问题。
情况2：

    dispatch_queue_t q1 = dispatch_queue_create("1", DISPATCH_QUEUE_SERIAL);
    dispatch_queue_t q2 = dispatch_queue_create("2", DISPATCH_QUEUE_SERIAL);
    dispatch_queue_t q3 = dispatch_queue_create("3", DISPATCH_QUEUE_SERIAL);
    dispatch_set_target_queue(q1, q3);
    dispatch_set_target_queue(q2, q3);
    for (int i = 0; i < 10; i ++) {
        dispatch_async(q1, ^{
            NSLog(@"123---%@", [NSThread currentThread]);
        });
        dispatch_async(q3, ^{
            NSLog(@"sss---%@", [NSThread currentThread]);
        });
        dispatch_async(q2, ^{
            NSLog(@"abc---%@", [NSThread currentThread]);
        });
    }

123---<NSThread: 0x60000029d600>{number = 3, name = (null)}
123---<NSThread: 0x60000029d600>{number = 3, name = (null)}
sss---<NSThread: 0x60000029d600>{number = 3, name = (null)}
abc---<NSThread: 0x60000029d600>{number = 3, name = (null)}
abc---<NSThread: 0x60000029d600>{number = 3, name = (null)}
sss---<NSThread: 0x60000029d600>{number = 3, name = (null)}

根据以上两种情况猜测dispatch_set_target_queue的执行逻辑：

image.png
如果所示，当提交任务到q1时，会将整个q1队列作为q3的任务，同理q2提交任务也一样，而q3是串行队列，所以会将q1队列中所有任务执行完才会执行后续任务。
也就是说当q1任务正在执行时，一直提交任务到q1，那么其他队列的任务都不会执行。