ios - NSCache Doesn't work with all images when loading for the first time

Question

I'm woking on a project in swift 3.0 where I cache the response from the server by using NSCache as to populate them in a UITableView. However for some reason I'm only seeing few images loading when the app loads for the first time, but if If i scroll and come back I see everything (end of retrieving the response from the server I reload my tableview too, but seems that not the case). I'm not sure what I''m exactly missing here, the code as bellow as to show how I cache the images.

let imageCache = NSCache<AnyObject, AnyObject>()
var imageURLString : String?

extension UIImageView {


    public func imageFromServerURL(urlString: String) {
        imageURLString = urlString

        if let url = URL(string: urlString) {

            image = nil


            if let imageFromCache = imageCache.object(forKey: urlString as AnyObject) as? UIImage {

                self.image = imageFromCache

                return
            }

            URLSession.shared.dataTask(with: url, completionHandler: { (data, response, error) in

                if error != nil{
                    print(error as Any)


                    return
                }

                DispatchQueue.main.async(execute: {

                    if let imgaeToCache = UIImage(data: data!){

                        if imageURLString == urlString {
                            self.image = imgaeToCache
                        }

                        imageCache.setObject(imgaeToCache, forKey: urlString as AnyObject)// calls when scrolling
                    }
                })
            }) .resume()
        }
    }
}

Answer 1

I think this would be a better approach using subclassing rather than extension, (taking help from Jageen's comment, as we cannot contain stored properties inside extension so we use the idea of encapsulation)

let imageCache = NSCache<AnyObject, AnyObject>()


    class CustomImageView: UIImageView {

            var imageUrlString: String?
            func loadImageUsingUrlString(_ urlString: String) {
                    let url = URL(string: urlString)
                    imageUrlString = urlString
                    image = nil

                    if let imageFromCache = imageCache.object(forKey: urlString as AnyObject) as? UIImage {
                            self.image = imageFromCache
                            return
                    }

                    URLSession.shared.dataTask(with: url!) { (data, response, error) in
                            if error != nil {
                                    print(error!)
                                    return

                            }

                            DispatchQueue.main.async {

                                    let imageToCache = UIImage(data: data!)
                                    if self.imageUrlString == urlString {
                                           self.image = imageToCache  
                                    }
                                    imageCache.setObject(imageToCache!, forKey: urlString as AnyObject)

                            }

                            }.resume()

            }
    }

-Now use this subclass as the type of imageViews that you are showing on the screen