title: Method for visible cell in tableview
body: <p>I want to download an image to my tableViewCell but I only want the image  downloaded to be inside a cell who is VISIBLE in my tableview. I want my tableView to download image when the tableviewcell is visible and during that time to display a loading bar or something until the download is finished and then display that image to the corresponding tableviewcell</p>
  
  <p>This is the code I use to display image to tableviewcell:</p>
  
  <pre><code>override func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -&gt; UITableViewCell {
      let cell = tableView.dequeueReusableCellWithIdentifier("ContentCell", forIndexPath: indexPath)
  
      let contentCell = cell as! ContentCell
  
      contentCell.thumbnail.kf_setImageWithURL(self.contentArray[indexPath.row].thumbnailUrl)
      print("row: \(indexPath.row)")
  
  
      return contentCell
  }
  </code></pre>
  
  <p>What I get is all the image is download all at once. I know this because When I scroll down I find that all the image is instantly displayed without delay even though my logging method print()  only print visible cell's row.</p>
  
  <p>Am I implementing this wrong?</p>