Images fluctuating on scroll in ios - ios

Question

i am getting images from url's and displaying it in tableview which happens successfully but when i scroll it automatically and repeatedly changes but after i pause at that point for a couple of seconds (roughly 10 or more which is a pretty long time) the proper image loads.
func load_image(urlString:String)
{
let imgURL: NSURL = NSURL(string: urlString)!
let request: NSURLRequest = NSURLRequest(URL: imgURL)
let session = NSURLSession.sharedSession()
let task = session.dataTaskWithRequest(request){
(data, response, error) -> Void in
if (error == nil && data != nil)
{
func display_image()
{
cell.pic.image = UIImage(data: data!)
}
dispatch_async(dispatch_get_main_queue(), display_image)
}
}
task.resume()
}
load_image(urls[indexPath.row])

Answer 1

First of all, why are You loading images in, I guess, cellForRowAtIndexPath?
You can load it in viewDidLoad and store in some array. But if You need to do this as You are doing...
For better performance You can use NSCache to prevent loading images everytime tableview will show your cell. Try something like that:
let imageCache = NSCache()
func load_image(urlString:String)
{
if let imageFromCache = imageCache.objectForKey(urlString) as? UIImage {
cell.pic.image = imageFromCache
return
}
let imgURL: NSURL = NSURL(string: urlString)!
let request: NSURLRequest = NSURLRequest(URL: imgURL)
let session = NSURLSession.sharedSession()
let task = session.dataTaskWithRequest(request){
(data, response, error) -> Void in
if (error == nil && data != nil)
{
func display_image()
{
let imageToCache = UIImage(data: data!)
cell.pic.image = UIImage(data: data!)
imageCache.setObject(imageToCache!, forKey: urlString)
}
dispatch_async(dispatch_get_main_queue(), display_image)
}
}
task.resume()
}
For more informations about NSCache you can check documentation.

Answer 2

Your cell is being reused when scrolling. The cellForRowAt method will be called on scrolling.
Set imageView.image = nil in cellForRowAt method so no image will be display on the image view until they get downloaded.
We make imageView.image = nil is because if no image available for that cell then no image should be shown for that cell.