Swift Images change to wrong images while scrolling after async image loading to a UITableViewCell

Question

I'm trying to async load pictures inside my FriendsTableView (UITableView) cell. The images load fine but when I'll scroll the table the images will change a few times and wrong images are getting assigned to wrong cells.

I've tried all methods I could find in StackOverflow including adding a tag to the raw and then checking it but that didn't work. I'm also verifying the cell that should update with indexPath and check if the cell exists. So I have no idea why this is happening.

Here is my code:

    func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
        let cell = tableView.dequeueReusableCellWithIdentifier("friendCell", forIndexPath: indexPath) as! FriendTableViewCell
        var avatar_url: NSURL
        let friend = sortedFriends[indexPath.row]

        //Style the cell image to be round
        cell.friendAvatar.layer.cornerRadius = 36
        cell.friendAvatar.layer.masksToBounds = true

        //Load friend photo asyncronisly
        avatar_url = NSURL(string: String(friend["friend_photo_url"]))!
        if avatar_url != "" {
                getDataFromUrl(avatar_url) { (data, response, error)  in
                    dispatch_async(dispatch_get_main_queue()) { () -> Void in
                        guard let data = data where error == nil else { return }
                        let thisCell = tableView.cellForRowAtIndexPath(indexPath)
                        if (thisCell) != nil {
                            let updateCell =  thisCell as! FriendTableViewCell
                            updateCell.friendAvatar.image = UIImage(data: data)
                        }
                    }
                }
        }
        cell.friendNameLabel.text = friend["friend_name"].string
        cell.friendHealthPoints.text = String(friend["friend_health_points"])
        return cell
    }

Answer 1

On cellForRowAtIndexPath:

1) Assign an index value to your custom cell. For instance,

cell.tag = indexPath.row

2) On main thread, before assigning the image, check if the image belongs the corresponding cell by matching it with the tag.

dispatch_async(dispatch_get_main_queue(), ^{
   if(cell.tag == indexPath.row) {
     UIImage *tmpImage = [[UIImage alloc] initWithData:imgData];
     thumbnailImageView.image = tmpImage;
   }});
});

Answer 2

This is because UITableView reuses cells. Loading them in this way causes the async requests to return at different time and mess up the order.

I suggest that you use some library which would make your life easier like Kingfisher. It will download and cache images for you. Also you wouldn't have to worry about async calls.

https://github.com/onevcat/Kingfisher

Your code with it would look something like this:

func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
        let cell = tableView.dequeueReusableCellWithIdentifier("friendCell", forIndexPath: indexPath) as! FriendTableViewCell
        var avatar_url: NSURL
        let friend = sortedFriends[indexPath.row]

        //Style the cell image to be round
        cell.friendAvatar.layer.cornerRadius = 36
        cell.friendAvatar.layer.masksToBounds = true

        //Load friend photo asyncronisly
        avatar_url = NSURL(string: String(friend["friend_photo_url"]))!
        if avatar_url != "" {
            cell.friendAvatar.kf_setImageWithURL(avatar_url)
        }
        cell.friendNameLabel.text = friend["friend_name"].string
        cell.friendHealthPoints.text = String(friend["friend_health_points"])
        return cell
    }