iOS UITableView: what's the different between “cellForRowAtIndexPath” and “willDisplayCell: forRowAtIndexPath:”

Question

Just as the question\'s title mentions: What\'s the difference between \"cellForRowAtIndexPath\" and \"willDisplayCell: forRowAtIndexPath

Answer 1

Despite what might seem intuitive, willDisplay cell: is called immediately after cellForRowAt indexPath: is called.

I had an app where images and video would be loaded in from either a URLCache or downloaded and displayed in the cell. I noticed whenever I'd start my application all the videos and images would load before I could see them and I noticed this because I could hear the audio from the last video in the tableView while looking at the first item in the tableView. This is with 8 items in the tableView. I printed to the console to get a better idea of what the order of the delegate functions calls were.

The results:

• Created cell at row 0
  • Drew feed cell at row 0
• Created cell at row 1
  • Drew feed cell at row 1
• Created cell at row 2
  • Drew feed cell at row 2
• Created cell at row 3
  • Drew feed cell at row 3
• Created cell at row 4
  • Drew feed cell at row 4
• Created cell at row 5
  • Drew feed cell at row 5
• Created cell at row 6
  • Drew feed cell at row 6
• Created cell at row 7
  • Drew feed cell at row 7
• Stopped showing cell at row 2
• Stopped showing cell at row 3
• Stopped showing cell at row 4
• Stopped showing cell at row 5
• Stopped showing cell at row 6
• Stopped showing cell at row 7

The Code:

    override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
    let cell = tableView.dequeueReusableCell(withIdentifier: "feedCell", for: indexPath) as! FeedCell
    print("Created cell at row \(indexPath.row)")
    let post = posts[indexPath.row]
    cell.post = post
    cell.viewController = self
    cell.configureCell()
    return cell
}

override func tableView(_ tableView: UITableView, willDisplay cell: UITableViewCell, forRowAt indexPath: IndexPath) {
    if let feedCell = cell as? FeedCell{
        print("Drew feed cell at row \(indexPath.row)")
        // only want download task to start when the cell will display
        if feedCell.post.isVideo {
            feedCell.loadVideo()
        } else {
            feedCell.loadImage()
        }
    }
}

override func tableView(_ tableView: UITableView, didEndDisplaying cell: UITableViewCell, forRowAt indexPath: IndexPath) {
    print("Stopped showing cell at row \(indexPath.row)")
    if let feedCell = cell as? FeedCell{
        feedCell.player?.pause()
    }
}