“阻止"主线程(dispatch_get_main_queue())和(或不)定期运行currentRunLoop-有什么区别?

我有以下代码:

- (void)test_with_running_runLoop {
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);

    NSTimeInterval checkEveryInterval = 0.05;

    NSLog(@"Is main queue? : %d", dispatch_get_current_queue() == dispatch_get_main_queue());

    dispatch_async(dispatch_get_main_queue(), ^{
        sleep(1);
        NSLog(@"I will reach here, because currentRunLoop is run");
        dispatch_semaphore_signal(semaphore);
    });

    while (dispatch_semaphore_wait(semaphore, DISPATCH_TIME_NOW))
        [[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate dateWithTimeIntervalSinceNow:checkEveryInterval]];

    NSLog(@"I will see this, after dispatch_semaphore_signal is called");
}

- (void)test_without_running_runLoop {
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);

    NSLog(@"Is main queue? : %d", dispatch_get_current_queue() == dispatch_get_main_queue());

    dispatch_async(dispatch_get_main_queue(), ^{
        sleep(1);
        NSLog(@"I will not reach here, because currentRunLoop is not run");
        dispatch_semaphore_signal(semaphore);
    });

    NSLog(@"I will just hang here...");
    while (dispatch_semaphore_wait(semaphore, DISPATCH_TIME_NOW));

    NSLog(@"I will see this, after dispatch_semaphore_signal is called");
}

产生以下内容:

Starting CurrentTests/test_with_running_runLoop
2012-11-29 08:14:29.781 Tests[31139:1a603] Is main queue? : 1
2012-11-29 08:14:30.784 Tests[31139:1a603] I will reach here, because currentRunLoop is run
2012-11-29 08:14:30.791 Tests[31139:1a603] I will see this, after dispatch_semaphore_signal is called
OK (1.011s)

Starting CurrentTests/test_without_running_runLoop
2012-11-29 08:14:30.792 Tests[31139:1a603] Is main queue? : 1
2012-11-29 08:14:30.797 Tests[31139:1a603] I will just hang here...

我的问题彼此相关:

1. 如果我理解正确，主队列(dispatch_get_main_queue())是一个串行队列.我用dispatch_semaphore_wait阻塞了主队列/主线程，所以为什么在第一个测试用例中看到我会到达这里，因为currentRunLoop正在运行" (第二种情况我可以-在我的理解中，它应该做什么)?

1. If I understand it right, main queue (dispatch_get_main_queue()) is a serial queue. I block the main queue/main thread with dispatch_semaphore_wait, so why do I see the "I will reach here, because currentRunLoop is run" in the first test case (I'm OK with the second case - in my understanding, it does, what it should)?

已阻塞当前正在执行的任务的 serial 队列如何在解锁当前任务之前分配下一个任务(哦，这个神秘的runLoop:beforeDate:)?

How a serial queue, having current executing task blocked, can have a next task (oh, this mysterious runLoop:beforeDate:) dispatched before the current one is unlocked?

我想听到有关详细和综合的答案，因为非常非常多的事情(在这附近也是如此)都取决于这个问题！

I want to hear detailed and comprehensive answer on this, because very, very much things (here around on SO too) depend on this issue!

更新:除了已接受的答案之外，此SO主题还对以下问题提供了很好的答案:

UPDATE: Besides the accepted answer, this SO topic has good answer to this question: Pattern for unit testing async queue that calls main queue on completion