iOS多个线程发起相同请求，避免重复

 @synchronized (self) {//加锁，避免数组重复创建添加等问题
         static NSMutableArray * successBlocks;//用数组保存回调
         static NSMutableArray * failureBlocks;
         static dispatch_once_t onceToken;
         dispatch_once(&onceToken, ^{//仅创建一次数组
            successBlocks = [NSMutableArray new];
            failureBlocks = [NSMutableArray new];
         });
         if (success) {//每调用一次此函数，就把回调加进数组中
            [successBlocks addObject:success];
         }
         if (failure) {
            [failureBlocks addObject:failure];
         }
         
         static BOOL isProcessing = NO;
         if (isProcessing == YES) {//如果已经在请求了，就不再发出新的请求
            return;
         }
         isProcessing = YES;
         [self callerPostTransactionId:transactionId parameters:dic showActivityIndicator:showActivityIndicator showErrorAlterView:showErrorAlterView success:^(id responseObject) {
            @synchronized (self) {//网络请求的回调也要加锁，这里是另一个线程了
               for (successBlock eachSuccess in successBlocks) { //遍历回调数组，把结果发给每个调用者
                  eachSuccess(responseObject);
               }
               [successBlocks removeAllObjects];
               [failureBlocks removeAllObjects];
               isProcessing = NO;
            }
         } failure:^(id data) {
            @synchronized (self) {
               for (failureBlock eachFailure in failureBlocks) {
                  eachFailure(data);
               }
               [successBlocks removeAllObjects];
               [failureBlocks removeAllObjects];
               isProcessing = NO;
            }
         }];
      }