多个请求按顺序执行

1.dispatch_semaphore 信号量方式 https://www.aliyun.com/jiaocheng/362267.html
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);
});

2.gcd调度组 https://www.jianshu.com/p/698f7c89c32e
// 1. 调度组
dispatch_group_t group = dispatch_group_create();

// 2. 队列
dispatch_queue_t q = dispatch_get_global_queue(0, 0);

// 3. 将任务添加到队列和调度组
dispatch_group_async(group, q, ^{
    [NSThread sleepForTimeInterval:1.0]; 
    NSLog(@"任务 1 %@", [NSThread currentThread]);
});
dispatch_group_async(group, q, ^{
    NSLog(@"任务 2 %@", [NSThread currentThread]);
});
dispatch_group_async(group, q, ^{
    NSLog(@"任务 3 %@", [NSThread currentThread]);
});

// 4. 监听所有任务完成
dispatch_group_notify(group, q, ^{
    NSLog(@"OVER %@", [NSThread currentThread]);
});

3.dispatch_barrier_async 栅栏 https://blog.csdn.net/willluckysmile/article/details/61195571

iOS简单优雅的实现复杂情况下的串行需求https://blog.csdn.net/spicyshrimp/article/details/70845822

iOS基础知识系列https://blog.csdn.net/spicyShrimp/article/details/62218521