信号量的简单使用

有些时候我们会遇见需要限制访问数量或者按一定顺序执行方法，就可以使用到。

- (void)test {
    dispatch_semaphore_t sem = dispatch_semaphore_create(1);
    dispatch_queue_t queue = dispatch_queue_create("testBlock", NULL);
    dispatch_async(queue, ^{
        dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
            NSLog(@"1");
            dispatch_semaphore_signal(sem);
        });
    });
    
    dispatch_async(queue, ^{
        dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);
        NSLog(@"2");
        dispatch_semaphore_signal(sem);
    });
    
    dispatch_async(queue, ^{
        dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
            NSLog(@"3");
            dispatch_semaphore_signal(sem);
        });
    });
    
    dispatch_async(queue, ^{
        dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);
        NSLog(@"4");
        dispatch_semaphore_signal(sem);
    });
}

如上面的代码，我们可以dispatch_semaphore_t sem = dispatch_semaphore_create(1);这时候信号值为1，那么下面的方法就会按顺序执行，所以结果会按顺序1234输出。