It's a little bit late to reply but I think the error stems from trying to access the self inside a static func. Instead of declaring writeToPhotoAlbum as a static, declare it without static keyword and call it after creating an instance of ImageDownloader. Like: import Foundation import UIKit class ImageDownloader: NSObject { func writeToPhotoAlbum(imageUrl: String) { // ... } @objc func saveCompleted( _ image: UIImage, didFinishSavingWithError error: Error?, contextInfo: UnsafeRawPointer) { // ... } } Then, var imageDownloader: ImageDownloader? imageDownloader = ImageDownloader() imageDownloader?.writeToPhotoAlbum(imageUrl: imgUrl) These may not be directly compilable but it may give a basic idea about the problem.