Set value to an cached image ends in crash

Question

I am downloading an image and add it to my cache. That works fine, but when I want to add a value to my downloaded image in the cache, my code returns the error "Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[<NSCache 0x600001cfa3c0> setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key XXX".

 let cache = NSCache<NSString, UIImage>()

 func downloadImage(withURL url:URL, completion: @escaping (_ image:UIImage?)->()) {
        let dataTask = URLSession.shared.dataTask(with: url) { data, responseURL, error in

            var downloadedImage:UIImage?

            if let data = data {
                downloadedImage = UIImage(data: data)
                print("downloadedImage")
            }
            DispatchQueue.main.async {
                completion(downloadedImage)
            }
            if downloadedImage != nil {

                let date = Date() // calling the date
                var calendar = Calendar.current // calling the calendar
                if let timeZone = TimeZone(identifier: "GMT-4") { 
                    calendar.timeZone = timeZone
                }
                let day = calendar.component(.day, from: date)
                
                self.cache.setObject(downloadedImage!, forKey: url.absoluteString as NSString)
                self.cache.setValue(day, forKey: url.absoluteString) // this line runs into an error
            }
            
        }
        dataTask.resume()
    }

Answer 1

The error occurs because Int is not a subclass of an NSObject

But there are two other serious issues.

1. The cache is declared as <NSString, UIImage> so the value must be an UIImage, however day is Int.
2. It makes no sense to overwrite the image with the integer anyway

---

You could append the day value to the URL string

let day = calendar.component(.day, from: date)
let key = "\(url.absoluteString)-\(day)"            
self.cache.setObject(downloadedImage!, forKey: key as NSString)