Make sure reused cell presents the right picture with GCD

gistfile1.txt

id imageSource = cell.imageSource;
dispatch_async(queue, ^{
   //block captures imageSource
   UIImage *thumbNail = getImageFromSource(imageSource);
  dispatch_sync(dispatch_get_main_queue(), ^{
  //block captures cell
  //if imageSource does not match cell.imageSource, it means the cell is resued before the background thread returns the image, 
  // and the image requested for the already invisible cell may be returned after the current cell obtains its image, in which case
  // the returned image should not be used.
  if ([imagaeSource isEqualToArray: cell.imageSource])
  {
      cell.imageView.image = thumbNail;
  }
});