IOS 如何处理多个网络请求的先后(依赖)关系

函数介绍

dispatch_semaphore_create(M)

创建一个值为M的信号量

dispatch_semaphore_wait(信号量，等待时间)

如果该信号量的值大于0，则使其信号量的值-1，否则，阻塞线程直到该信号量的值大于0或者达到等待时间。

dispatch_semaphore_signal(信号量)

释放信号量，使得该信号量的值加1

dispatch_group_t group = dispatch_group_create();
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
    dispatch_queue_t queue = dispatch_queue_create(NULL, DISPATCH_QUEUE_SERIAL);
    for (int j = 0; j < 4; j ++) {
        dispatch_group_async(group, queue, ^{

         if (j == 0) {
                [self loadOne:semaphore];
            }else if (j == 1){
                [self loadTwe:semaphore];
            }else if (j == 2){
                [self loadThree:semaphore];
            }else{
                [self loadFour:semaphore];
            }
            dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
        });
    }
    
    dispatch_group_notify(group, queue, ^{
        //所有请求返回数据后执行
        NSLog(@"完成");
    });