iOS 如何解决并发请求时，只接受最后一个请求返回的结果

 1 + (AFHTTPRequestOperation *)requestSellerWithCompletion:(requestFinishedCompletionBlock)successBlock
 2 {
 3     AFHTTPRequestOperationManager *manager = [AFHTTPRequestOperationManager manager];
 4 
 5     AFHTTPRequestOperation * operation = [manager GET:kRequestSellerURL parameters:nil success:^(AFHTTPRequestOperation *operation, id responseObject) {
 6  
 7         if (operation.isCancelled) {
 8          //若果operation已经被cancel了，则不对返回数据进行处理
 9             NSLog(@"operation is Canceled");
10         }else{
11      //如果没有cancel 掉，则进行正常的数据解析
12             NSArray *sellerArray = [MTLJSONAdapter modelsOfClass:[Seller class] fromJSONArray:responseObject[@"data"] error:nil];
13             if (successBlock) {
14               //  NSLog(@"current operation count is %d",[manager.operationQueue operations].count);
15                 successBlock(sellerArray);
16             }
17         }
18 
19 
20     } failure:^(AFHTTPRequestOperation *operation, NSError *error) {
21 
22     }];

然后在for循环里这么做.

for (int i = 1; i <= 10; i++) {

        if (operation) {
            [operation cancel];
        }
      operation = [Seller requestSellerWithCompletion:^(id object) {
            NSLog(@"finished download %d",i);

        }];

    }