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

Question

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-我理解，它应该做什么）？

2. serial队列如何阻止当前正在执行的任务，可以在解锁当前任务之前分配下一个任务（哦，这个神秘的runLoop：beforeDate：）？

我想听到有关此问题的详细信息，因为很多事情都取决于这个问题！

UPDATE：除了已接受的答案之外，此SO主题还对以下问题有很好的答案：Pattern for unit testing async queue that calls main queue on completion

我有以下代码：-（void）test_with_running_runLoop {dispatch_semaphore_t semaphore = dispatch_semaphore_create（0）; NSTimeInterval checkEveryInterval = 0.05; NSLog（@“是主要对象...

因为主线程上的默认runloop具有特殊的行为，即它在运行时还会处理主调度队列。在这种情况下，您实际上并没有阻塞，因为您是在告诉dispatch_semaphore_wait立即超时（此操作正在执行（返回非零值，其在if中的值为真））-因此，您需要执行while循环，您驱动当前的运行循环，从而使排队的块得到执行。

但是我的回答很广泛，因为我不确定您对哪一部分感到惊讶。

Answer 1