title: How can I specify the return types in GCD blocks
body: <p>How can i specify the return types in GCD blocks? Here I want to return my result as <code>NSData</code> ...  </p>
  
  <pre><code>- (NSData *)returnData:(NSString *)urlString{
      dispatch_queue_t concurrentQueue = 
      dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
  
      dispatch_sync(concurrentQueue, ^{   // here return type showing error
  
          NSString *strUrl = urlString;
          if([strUrl length] == 0)
              return nil; // from this point i want to stop the execution of the block and return nil .
  
          // otherwise only continue the left operations
  
  
  
      });         
  }
  </code></pre>