信号量 处理 多个异步网络请求后进行操作

    dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
    dispatch_group_t group = dispatch_group_create();
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

    dispatch_group_notify(group, queue, ^{
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
        dispatch_async(dispatch_get_main_queue(), ^{
            // 请求完成 操作
        });
    });

    //网络请求
    dispatch_group_async(group, queue, ^{

    });    

    //网络请求二
    dispatch_group_async(group, queue, ^{

    });