Best practice to safely load image from url - error-handling

Question

I have the following code snippet to load an image from an url:
let url = NSURL(string: imageUrl)
let data = NSData(contentsOfURL: url!)
let image = UIImage(data: data!)
In case that my variable imageUrl has a valid string value, what is the most secure way to protect this code against possible edge cases?
Following code seems not to be very handy:
if let url = NSURL(string: imageUrl) {
if let data = NSData(contentsOfURL: url) {
if let image = UIImage(data: data) {
// success -> do something with the image...
}
else {
// print error message
}
}
else {
// print error message
}
}
else {
// print error message
}

Answer 1

The best practice is not to use a synchronous method like contentsOfURL to load data from over the network.
The recommended way is NSURLSession which works asynchronously.
This is a simple example with a completion block and an enum with associated types,
it catches all possible errors
enum Result {
case Success(UIImage), Failure(NSString)
}
func loadImage(string : String, completion: (Result) -> ()) {
guard let url = NSURL(string: string) else {
completion(.Failure("Bad URL"))
return
}
NSURLSession.sharedSession().dataTaskWithURL(url) { (data, response, error) in
if error != nil {
completion(.Failure(error!.localizedDescription))
} else {
guard let image = UIImage(data: data!) else {
completion(.Failure("Could not load image data"))
return
}
completion(.Success(image))
}
}.resume()
}
Call it with:
loadImage("http://myserver.com/path/to/image.png") { result in
switch result {
case .Success(let image) :
// do something with the image
case .Failure(let error) :
print(error)
}
}