多个请求

GCD ：

   dispatch_queue_t serielQueue = dispatch_queue_create("myQueue", DISPATCH_QUEUE_SERIAL);

   dispatch_sync(serielQueue, ^{

         [self loadDetailData];  //请求A

  });
   dispatch_sync(serielQueue, ^{

          [self loadHotCommentData]; //请求B

   });
   dispatch_sync(serielQueue, ^{

        [self loadOtherCommentData]; //请求C

   });
  dispatch_sync(serielQueue, ^{

       [self loadPhotoData];  //请求D

  });

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NSOperationQueue ：

// 创建一个队列
NSOperationQueue *queue = [[NSOperationQueue alloc]init];

// 设置最大线程数
queue.maxConcurrentOperationCount = 1;

// 创建一个A操作
NSBlockOperation *operationA = [NSBlockOperation blockOperationWithBlock:^{
    [self loadDetailData];
}];

// 创建一个B操作
NSBlockOperation *operationB = [NSBlockOperation blockOperationWithBlock:^{
   [self loadHotCommentData];
}];
// 创建一个C操作
NSBlockOperation *operationC = [NSBlockOperation blockOperationWithBlock:^{
    [self loadOtherCommentData];
}];
// 创建一个D操作
NSBlockOperation *operationD = [NSBlockOperation blockOperationWithBlock:^{
    [self loadPhotoData];
}];

/ /添加依赖
[operationB addDependency:operationA];
[operationC addDependency:operationB];
[operationD addDependency:operationC];

// 分别加入到队列中
[queue addOperation:operationA];
[queue addOperation:operationB];
[queue addOperation:operationC];
[queue addOperation:operationD];