怎么解决网络请求的依赖关系

[operationB addDependency:operationA]; // 操作B依赖于操作

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, ^{ /*加载图片1 */ });
dispatch_group_async(group, queue, ^{ /*加载图片2 */ });
dispatch_group_async(group, queue, ^{ /*加载图片3 */ }); 
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        // 合并图片… …
});

/* 创建并发队列 */
dispatch_queue_t concurrentQueue = dispatch_queue_create("test.concurrent.queue", DISPATCH_QUEUE_CONCURRENT);
/* 添加两个并发操作A和B，即A和B会并发执行 */
dispatch_async(concurrentQueue, ^(){
    NSLog(@"OperationA");
});
dispatch_async(concurrentQueue, ^(){
    NSLog(@"OperationB");
});
/* 添加barrier障碍操作，会等待前面的并发操作结束，并暂时阻塞后面的并发操作直到其完成 */
dispatch_barrier_async(concurrentQueue, ^(){
    NSLog(@"OperationBarrier!");
});
/* 继续添加并发操作C和D，要等待barrier障碍操作结束才能开始 */
dispatch_async(concurrentQueue, ^(){
    NSLog(@"OperationC");
});
dispatch_async(concurrentQueue, ^(){
    NSLog(@"OperationD");
});

/* 创建一个信号量 */
dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);

/* 任务1 */
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    /* 耗时任务1 */
    NSLog(@"任务1开始");
    [NSThread sleepForTimeInterval:3];
    NSLog(@"任务1结束");
    /* 任务1结束，发送信号告诉任务2可以开始了 */
    dispatch_semaphore_signal(semaphore);
});

/* 任务2 */
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    /* 等待任务1结束获得信号量, 无限等待 */
    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
    /* 如果获得信号量则开始任务2 */
    NSLog(@"任务2开始");
    [NSThread sleepForTimeInterval:3];
    NSLog(@"任务2结束");
});
[NSThread sleepForTimeInterval:10];