3种GCD线程同步方式分别是：

dispatch_semaphore_t
dispatch_barrier_async()
dispatch_group_notify()

第一种使用dispatch_semaphore_t实现线程同步

说明：block块异步任务添加到了全局并发队列里，主线程会跳过block块（同时开辟子线程异步执行block块）。
执行block块外的代码dispatch_semaphore_wait，因为semaphore信号量为0，且时间为DISPATCH_TIME_FOREVER，所以会阻塞当前线程（主线程），从而先执行子线程的block块，直到执行block块内部的dispatch_semaphore_signal使得信号量+1。正在被阻塞的线程（主线程）才恢复继续执行。这样保证了线程之间的同步。

- (void)test_dispatch_semaphore {

    __block int j = 0;
    
    // global全局并发队列
    dispatch_queue_t queueu_t = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_semaphore_t semaphore_t  = dispatch_semaphore_create(0);
    dispatch_async(queueu_t, ^{
        for (int i = 0; i < 1000; i++) {
            j++;
        }
        dispatch_semaphore_signal(semaphore_t);
    });
    dispatch_semaphore_wait(semaphore_t, DISPATCH_TIME_FOREVER);
    NSLog(@"j = %d", j);
    
}

2020-05-12 19:37:50.460186+0800 Test_Objective-C[3310:158423] j = 1000

第二种使用dispatch_barrier_async()

- (void)test_barrier_async {
    
    // 新建并发队列
    dispatch_queue_t queue_t_1 = dispatch_queue_create("com.queue_t_1", DISPATCH_QUEUE_CONCURRENT);
    dispatch_async(queue_t_1, ^{
        NSLog(@"异步任务_1");
    });
    dispatch_async(queue_t_1, ^{
        NSLog(@"异步任务_2");
    });
    dispatch_async(queue_t_1, ^{
        NSLog(@"异步任务_3");
    });
    dispatch_barrier_sync(queue_t_1, ^{
        int sum = 0;
        for (int i = 0; i < 100; i++) {
            sum += i;
        }
        NSLog(@"barrier执行任务，sum = %d", sum);
    });
    dispatch_async(queue_t_1, ^{
        NSLog(@"异步任务_4");
    });
    
    
}

2020-05-12 19:36:53.449664+0800 Test_Objective-C[3289:157683] 异步任务_1
2020-05-12 19:36:53.449696+0800 Test_Objective-C[3289:157686] 异步任务_3
2020-05-12 19:36:53.449715+0800 Test_Objective-C[3289:157684] 异步任务_2
2020-05-12 19:36:53.449996+0800 Test_Objective-C[3289:157602] barrier执行任务，sum = 4950
2020-05-12 19:36:53.450136+0800 Test_Objective-C[3289:157684] 异步任务_4