objective-c-blocks - 如何使用 dispatch_queue_set_specific() 和 dispatch_get_specific()

Question

我很难找到关于如何使用这些函数的好例子。

static void * kQueue1Key = "key1";
static void * kQueue2Key = "key2";

dispatch_queue_t queue1 = dispatch_queue_create("com.company.queue1", DISPATCH_QUEUE_SERIAL);
dispatch_queue_t queue2 = dispatch_queue_create("com.company.queue2", DISPATCH_QUEUE_SERIAL);

dispatch_queue_set_specific(queue1, kQueue1Key, (void *)kQueue1Key, NULL);
dispatch_queue_set_specific(queue2, kQueue2Key, (void *)kQueue2Key, NULL);

dispatch_sync(queue1, ^{
    if(dispatch_get_specific(kQueue1Key))
    {
        NSLog(@"I'm expecting this line to run (A)");

        dispatch_sync(queue2, ^{

            NSLog(@"I'm expecting this line to run (B)");

            if(dispatch_get_specific(kQueue2Key))
            {
                if(dispatch_get_specific(kQueue1Key))
                {
                    NSLog(@"I'm expecting this line to run (C)");
                }
                else
                {
                    [NSException raise:NSInternalInconsistencyException format:@"Should not end up here (C)"];
                }
            }
            else
            {
                [NSException raise:NSInternalInconsistencyException format:@"Should not end up here (B)"];
            }
        });
    }
    else
    {
        [NSException raise:NSInternalInconsistencyException format:@"Should not end up here (A)"];
    }
});

结果

I'm expecting this line to run (A)
I'm expecting this line to run (B)
*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Should not end up here (C)'

这是预期的行为吗？如果我要 dispatch_sync 到 queue1 因为我不在队列中，我会死锁。我错过了什么？

Answer 1

哦，在这里，它突然出现在我的脑海中，为什么你得到了你得到的。注意事项:

dispatch_sync(queue1, ^{

当你到达这一点时，“当前队列”是 queue1

    if(dispatch_get_specific(kQueue1Key))

您正在向当前队列询问它对 kQueue1Key 的值，你早点设置，所以它把它还给你。

    {
        NSLog(@"I'm expecting this line to run (A)");

        dispatch_sync(queue2, ^{

当你到达这一点时，“当前队列”现在是 queue2

            NSLog(@"I'm expecting this line to run (B)");

            if(dispatch_get_specific(kQueue2Key))

您正在向当前队列询问它对 kQueue2Key 的值，你早点设置，所以它把它还给你。

            {

                if(dispatch_get_specific(kQueue1Key))

您现在正在向当前队列询问它对 kQueue1Key 的值。 .由于当前队列是 queue2并且您从不使用 kQueue1Key 设置值在 queue2你回来 NULL .

                {
                    NSLog(@"I'm expecting this line to run (C)");
                }
                else
                {
                    [NSException raise:NSInternalInconsistencyException format:@"Should not end up here (C)"];
                }

这里的误解是 dispatch_get_specific不遍历嵌套队列的堆栈，它遍历目标沿袭的队列。例如，如果你这样做，

static void * kQueue1Key = (void*)"key1";
static void * kQueue2Key = (void*)"key2";

dispatch_queue_t queue1 = dispatch_queue_create("com.company.queue1", DISPATCH_QUEUE_SERIAL);
dispatch_queue_t queue2 = dispatch_queue_create("com.company.queue2", DISPATCH_QUEUE_SERIAL);

dispatch_queue_set_specific(queue1, kQueue1Key, (void *)kQueue1Key, NULL);
dispatch_queue_set_specific(queue2, kQueue2Key, (void *)kQueue2Key, NULL);

// Set Queue2 to target Queue1
dispatch_set_target_queue(queue2, queue1);

dispatch_sync(queue2, ^{

    if(dispatch_get_specific(kQueue1Key))
    {
        NSLog(@"I'm expecting this line to run (A)");
    }
    else
    {
        [NSException raise:NSInternalInconsistencyException format:@"Should not end up here (C)"];
    }

    if(dispatch_get_specific(kQueue2Key))
    {
        NSLog(@"I'm expecting this line to run (B)");
    }
    else
    {
        [NSException raise:NSInternalInconsistencyException format:@"Should not end up here (C)"];
    }
});

...定位关系是被遍历的关系，而不是堆栈关系。如果有什么东西可以遍历堆栈关系就好了，但我什么都不知道(您不必自己实现)。