iOS多线程面试题：两个线程交替打印

之前遇到的面试题：如何用两个线程交替打印1-100的数字。

思路是使用信号量来控制两个线程的打印顺序。

代码如下：

    /// 使用两个信号量来分别控制两个线程
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(1); // wait该信号量的线程先输出
    dispatch_semaphore_t semaphore_another = dispatch_semaphore_create(0);
    
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    __block int num = 0;
    dispatch_async(queue, ^{
        while (YES) {
            dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
            if (num >= 100) {
                /// 当满足退出条件时，要先把另一个wait状态的信号量释放掉，否则会crash
                dispatch_semaphore_signal(semaphore_another);
                break;
            }
            NSLog(@"num: %i, thread: %@", num, [NSThread currentThread]);
            num++;
            dispatch_semaphore_signal(semaphore_another);
        }
    });

    dispatch_async(queue, ^{
        while (YES) {
            dispatch_semaphore_wait(semaphore_another, DISPATCH_TIME_FOREVER);
            if (num >= 100) {
                dispatch_semaphore_signal(semaphore);
                break;
            }
            NSLog(@"num: %i, thread: %@", num, [NSThread currentThread]);
            num++;
            dispatch_semaphore_signal(semaphore);
        }
    });