title: How to download multiple files from multiple url by NSOperationQueue
body: <p>I'm struggling to implement download mechanism for multiple files using AFNetworking. I want to download zip file one after another from multiple url with progression message. I tried like following code, but getting error as -</p>
  
  <pre><code>*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[NSOperationQueue addOperation:]: operation is already enqueued on a queue'
  </code></pre>
  
  <p>Here is my code portion : </p>
  
  <pre><code>- (void) downloadCarContents:(NSArray *)urlArray forContent:(NSArray *)contentPaths {
  
      NSOperationQueue *operationQueue = [[NSOperationQueue alloc] init];
  
      for (int i=0; i&lt; urlArray.count; i++) {
  
          NSString *destinationPath = [self.documentDirectory getDownloadContentPath:contentPaths[i]];
  
          NSLog(@"Dest : %@", destinationPath);
  
          AFHTTPRequestOperationManager *manager = [AFHTTPRequestOperationManager manager];
          AFHTTPRequestOperation *operation = [manager GET:urlArray[i]
                                                parameters:nil
                                                   success:^(AFHTTPRequestOperation *operation, id responseObject) {
  
                                                   } failure:^(AFHTTPRequestOperation *operation, NSError *error) {
  
                                                       NSLog(@"Error : %ld", (long)error.code);
                                                   }];
  
          operation.outputStream = [NSOutputStream outputStreamToFileAtPath:destinationPath append:NO];
  
          [operation setDownloadProgressBlock:^(NSUInteger bytesRead, long long totalBytesRead, long long totalBytesExpectedToRead) {
              float percentage = (float) (totalBytesRead * 100) / totalBytesExpectedToRead;
              [self.delegate downloadProgression:percentage];
          }];
  
          [operationQueue addOperation:operation];
      }
  }
  </code></pre>
  
  <p>Please help.</p>