NSURL error for broken links in Swift - macos

Question

I coded a function for OSX 10.10 that is willing to open a text file from an URL and display its content.
Everything is working but if the URL cannot be reach then the App will crash. How could I handle this type of Error?
I guess it comes from the completionHandler closure but I am not sure.
here is my code
#IBAction func checkAdminMessage(sender: NSMenuItem) {
let messageURL = NSURL(string: "http://www.xxxxxx.com/text.txt")
// The Network stuff will be handled in a background thread
let sharedSession = NSURLSession.sharedSession()
let downloadTask: NSURLSessionDownloadTask = sharedSession.downloadTaskWithURL(messageURL!,
completionHandler: {
(location: NSURL!, response: NSURLResponse!, error: NSError!) -> Void in
var urlContents = NSString(contentsOfURL: location, encoding: NSUTF8StringEncoding, error: nil)
// Check if text.txt has NULL as content
if urlContents! == "NULL" {
// Have to use Grand Central Dispatch to put NSAlert in the main thread
let noMessage = NSLocalizedString("Nothing there", comment: "Text to dislay when the file is empty" )
dispatch_async(dispatch_get_main_queue()) { () -> Void in
self.displayAlertNotification(notification: noMessage)
}
} else {
// If the file is not empty then we display the content of this file
dispatch_async(dispatch_get_main_queue()) { () -> Void in
self.displayAlertNotification(notification: urlContents!)
}
}
})
downloadTask.resume()
}
Thank you
EDIT: Here is the updated code but the App still crashed
#IBAction func checkAdminMessage(sender: NSMenuItem) {
if let messageURL = NSURL(string: "http://www.xxxxxx.com/text.txt") {
let sharedSession = NSURLSession.sharedSession()
let downloadTask: NSURLSessionDownloadTask = sharedSession.downloadTaskWithURL(messageURL,
completionHandler: {
(location: NSURL!, response: NSURLResponse!, error: NSError!) -> Void in
var urlContents = NSString(contentsOfURL: location, encoding: NSUTF8StringEncoding, error: nil )
if urlContents == "NULL" {
println(urlContents)
// Have to use Grand Central Dispatch to put NSAlert in the main thread
let noMessage = NSLocalizedString("Nothing there", comment: "Text to dislay when the file is empty" )
dispatch_async(dispatch_get_main_queue()) { () -> Void in
self.displayAlertNotification(notification: noMessage)
}
}
else {
dispatch_async(dispatch_get_main_queue()) { () -> Void in
self.displayAlertNotification(notification: urlContents!)
}
}
})
downloadTask.resume()
}
else {
println("Error")
}
}

Answer 1

NSURL(string: ...) returns an optional, so the result may be nil due to several reasons.
Wrap your code in a conditional unwrap:
if let messageURL = NSURL(string: "http://www.xxxxxx.com/text.txt") {
// success
...
}
else {
// error
}

Answer 2

I figured it out with the helps of the people that commented my question.
I was getting a nil from 'location' in downloadTaskWithUrl, then the var urlContents was receiving a nil as well.
The solution is to check if 'location' is nil :
let downloadTask: NSURLSessionDownloadTask = sharedSession.downloadTaskWithURL(messageURL,
completionHandler: {
(location: NSURL!, response: NSURLResponse!, error: NSError!) -> Void in
**if (location != nil) {**
var urlContents = NSString(contentsOfURL: location, encoding: NSUTF8StringEncoding, error: nil ) ...