1. 一个接口的请求，依赖于另一个请求的结果

• 使用GCD组队列中的dispatch_group_async和dispatch_group_notify

- (void)dispatchGroup{
    dispatch_queue_t queue = dispatch_queue_create("com.test", DISPATCH_QUEUE_CONCURRENT);
    dispatch_group_t group = dispatch_group_create();
    dispatch_group_async(group, queue, ^{
        // 请求1
        NSLog(@"11111");
    });
    
    dispatch_group_notify(group, queue, ^{
        // 请求2
        NSLog(@"2222222");
    });
}

• 使用GCD的栅栏函数dispatch_barrier_async

- (void)dispatchBarrier{
    dispatch_queue_t queue = dispatch_queue_create("com.test", DISPATCH_QUEUE_CONCURRENT);
    dispatch_async(queue, ^{
        // 请求1
        NSLog(@"11111");
    });
    
    dispatch_barrier_async(queue, ^{
        // 请求2
        NSLog(@"22222");
    });
}

• 使用GCD的信号量dispatch_semaphore，信号量的初始值可以用来控制线程并发访问的最大数量。如果设置为1则为串行执行，达到线程同步的目的

- (void)dispatchSemaphore{
    dispatch_queue_t queue = dispatch_queue_create("com.test", DISPATCH_QUEUE_CONCURRENT);
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
    
    dispatch_async(queue, ^{
        NSLog(@"11111");
        // 请求1结束后 增加信号个数,dispatch_semaphore_signal()信号加1
        dispatch_semaphore_signal(semaphore);
    });
    
    dispatch_async(queue, ^{
        // 无限等待,直到请求1结束 增加了信号量,大于0后才开始,dispatch_semaphore_wait()信号减1
        // 如果信号量的值 > 0，就让信号量的值减1，然后继续往下执行代码
        // 如果信号量的值 <= 0，就会休眠等待，直到信号量的值变成>0，就让信号量的值减1，然后继续往下执行代码
        // DISPATCH_TIME_FOREVER 这个是一直等待的意思
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
        // 请求2开始
        NSLog(@"22222");
    });
}

2. 在异步线程中return一个字符串

• 模拟异步线程方法，需要return字符串@"小熊"

- (void)asyncRequestMethod:(void(^)(NSString *name))successBlock{
    dispatch_queue_t queue = dispatch_queue_create("com.test", DISPATCH_QUEUE_SERIAL);
    dispatch_async(queue, ^{
        if (successBlock) {
            successBlock(@"小熊");
        }
    });
}

• while循环等待阻塞线程方法

- (NSString *)nameStr{
    __block NSString *nameStr = @"小关";
    __block BOOL isSleep = YES;

    [self asyncRequestMethod:^(NSString *name) {
        nameStr = name;
        NSLog(@"1");
        isSleep = NO;
        NSLog(@"3");
    }];

    //while循环等待阻塞线程
    while (isSleep) {
        NSLog(@"4");
    }

    NSLog(@"2");

    return nameStr;
}

• 用信号量控制，异步改同步

- (NSString *)nameStr{
    __block NSString *nameStr = @"小关";
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);

    [self asyncRequestMethod:^(NSString *name) {
        nameStr = name;
        dispatch_semaphore_signal(semaphore);
    }];

    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

    return nameStr;
}

• 利用dispatch_group_t调度组，异步改同步

- (NSString *)nameStr{
    __block NSString *nameStr = @"小关";
    dispatch_group_t group = dispatch_group_create();
    dispatch_group_enter(group);

    [self asyncRequestMethod:^(NSString *name) {
        nameStr = name;
        dispatch_group_leave(group);
    }];

    dispatch_group_wait(group, DISPATCH_TIME_FOREVER);

    return nameStr;
}

附：GrandCentralDispatchDemo