Guarding against non-200 responses when loading images in Swift 4 - swift

Question

When loading in external images with Swift 4, I'm having trouble when a URL responds with a non 200 response in the code block below. For instance, when serving an image that is sending a 401, the app crashes on line 2 of this example with the following error:
Thread 29: Fatal error: Unexpectedly found nil while unwrapping an Optional value
How can I catch this problem and serve up a different URL?
let imageUrl:URL = URL(string: imageUrlString)!
let imageData:NSData = NSData(contentsOf: imageUrl)!
let imageView = UIImageView(frame: CGRect(x:11, y:90, width:80, height:80))
// When from background thread, UI needs to be updated on main_queue
DispatchQueue.main.async {
let image = UIImage(data: imageData as Data)
imageView.image = image
imageView.layer.zPosition = 1
imageView.contentMode = UIViewContentMode.scaleAspectFit
self.view.addSubview(imageView)
}

Answer 1

From the NSData(contentsOf: documentation (unfortunately this note is missing in Data):
So please take this note seriously and use URLSession
guard let imageUrl = URL(string: imageUrlString) else { // ... handle the `nil` case }
let task = URLSession.shared.dataTask(with: imageUrl) { data, response, error in
if let error = error { print(error); return }
guard let response = response as? HTTPURLResponse else { print("Response is not HTTPResponse"); return }
if response.statusCode != 200 { print("response error:", response.statusCode); return }
DispatchQueue.main.async {
let image = UIImage(data: data!) // `data` can be unwrapped safely if `error` is nil
imageView.image = image
imageView.layer.zPosition = 1
imageView.contentMode = .scaleAspectFit
self.view.addSubview(imageView)
}
}
task.resume()
You can handle error and response separately. If error is nil then data is non-nil and vice versa.

Answer 2

NSData(contentsOf: url) is a failable initialiser which means it returns an optional NSData instance and one instance in which it fails is for a non 200 response.
Your code then tried to force unwrap it using the '!' character which is why you get an error.
First you need to decide what to do if it does fail, i.e. are you going to not show an image, show a default image, etc.
Then change your code to take account of that. So something like this:
let imageUrl:URL = URL(string: imageUrlString)!
if let imageData = NSData(contentsOf: imageUrl) {
let imageView = UIImageView(frame: CGRect(x:11, y:90, width:80, height:80))
// When from background thread, UI needs to be updated on main_queue
DispatchQueue.main.async {
let image = UIImage(data: imageData as Data)
imageView.image = image
imageView.layer.zPosition = 1
imageView.contentMode = UIViewContentMode.scaleAspectFit
self.view.addSubview(imageView)
}
} else {
// Do whatever you want if getting the image fails, e.g. non 200 response.
}
EDIT
As has been pointed out NSData(contentsOf:) should not be used for network related queries however even for local queries it can still fail so I will leave the answer up as that is the cause of the original error.