遇到的问题

多线程异步并行会导致多个线程的顺序随机执行

信号量(Semaphore)

可以控制线程并发的数量,且可以通过信号量来控制信号量执行顺序.

1.创建信号量,参数为初始值

dispatch_semaphore_create（信号量值）

2.等待降低信号量，等待资源使用权

dispatch_semaphore_wait（信号量，等待时间）

3.提高信号量，释放资源使用权

dispatch_semaphore_signal(信号量)

注意:

dispatch_semaphore_signal与dispatch_semaphore_wait一般为配对使用
signal升高信号量+1,wait降低信号量-1
当信号量为0时再执行wait会导致信号量小于0,阻塞线程

未使用信号量时

/**
 2021-07-21 20:36:03.756500+0800 SemaDemo[1331:20883] run task2
 2021-07-21 20:36:03.756510+0800 SemaDemo[1331:20881] run task3
 2021-07-21 20:36:03.756518+0800 SemaDemo[1331:20882] run task4
 2021-07-21 20:36:03.756500+0800 SemaDemo[1331:20885] run task1
 顺序随机
 */
- (void)normalThread{
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_async(queue, ^{
        NSLog(@"run task1");
    });
    dispatch_async(queue, ^{
        NSLog(@"run task2");
    });
    dispatch_async(queue, ^{
        NSLog(@"run task3");
    });
    dispatch_async(queue, ^{
        NSLog(@"run task4");
    });
}

与queue结合可以完成多个接口顺序完成需求.

/**
 
 2021-07-22 12:58:41.810017+0800 SemaDemo[1978:31675] run task1
 2021-07-22 12:58:41.810205+0800 SemaDemo[1978:31675] run task2
 2021-07-22 12:58:41.810353+0800 SemaDemo[1978:31675] run task3
 2021-07-22 12:58:41.810494+0800 SemaDemo[1978:31675] run task4
 顺序执行
 */
- (void)semaphoreThread_0{

    //信号量初始值为0

    dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);

    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);



    dispatch_async(queue, ^{

        NSLog(@"run task1");

        dispatch_semaphore_signal(semaphore);

    });

    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);



    dispatch_async(queue, ^{

        NSLog(@"run task2");

        dispatch_semaphore_signal(semaphore);

    });

    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);



    dispatch_async(queue, ^{

        NSLog(@"run task3");

        dispatch_semaphore_signal(semaphore);

    });

    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);



    dispatch_async(queue, ^{

        NSLog(@"run task4");

        dispatch_semaphore_signal(semaphore);



    });



}

与group,queue结合可以完成多个接口操作完,进行下一步操作的需求.

/**
 2021-07-23 15:28:32.030550+0800 SemaDemo[22090:312053] 2请求成功
 2021-07-23 15:28:32.095009+0800 SemaDemo[22090:312050] 1请求成功
 2021-07-23 15:28:32.095012+0800 SemaDemo[22090:312048] 信号量为0
 2021-07-23 15:28:32.116285+0800 SemaDemo[22090:312053] 3请求成功
 或
 2021-07-23 15:28:32.030550+0800 SemaDemo[22090:312053] 1请求成功
 2021-07-23 15:28:32.095009+0800 SemaDemo[22090:312050] 2请求成功
 2021-07-23 15:28:32.095012+0800 SemaDemo[22090:312048] 信号量为0
 2021-07-23 15:28:32.116285+0800 SemaDemo[22090:312053] 3请求成功
 */
- (void)groupThread{
    //结束请求进行下一步操作,不注重顺序
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
    
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_group_t group = dispatch_group_create();
    
    dispatch_group_async(group, queue, ^{
        NSURLSessionDataTask * dataTask = [[NSURLSession sharedSession] dataTaskWithURL:[NSURL URLWithString:@"https://www.baidu.com"] completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
            dispatch_semaphore_signal(semaphore);
            NSLog(@"1请求成功");
        }];
        [dataTask resume];
    });
    
    dispatch_group_async(group, queue, ^{
        NSURLSessionDataTask * dataTask = [[NSURLSession sharedSession] dataTaskWithURL:[NSURL URLWithString:@"https://www.baidu.com"] completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
            dispatch_semaphore_signal(semaphore);
            NSLog(@"2请求成功");
        }];
        [dataTask resume];
    });
    
    dispatch_group_notify(group, queue, ^{
        //        信号量为0时再wait会阻塞线程
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
        NSLog(@"信号量为0");
        NSURLSessionDataTask * dataTask = [[NSURLSession sharedSession] dataTaskWithURL:[NSURL URLWithString:@"https://www.baidu.com"] completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
            dispatch_semaphore_signal(semaphore);
            NSLog(@"3请求成功");
        }];
        [dataTask resume];
    });
    
}