Wait for async method before segue - multithreading

Question

I have a button and when I press it I´m calling an Async function:
func check(url : String){
let url = NSURL(string: url)
print("Checking")
dispatch_async(dispatch_get_main_queue(), {
let task = NSURLSession.sharedSession().dataTaskWithURL(url!) {(data, response, error) in
if error == nil {
self.isWorking = true
}
else{
self.isWorking = false
}
}
}
task.resume()
})
}
So when I press my button I do the following:
override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject!){
let web = segue.destinationViewController as! ViewController()
check(url)
dispatch_async(dispatch_get_main_queue(), {
if (isWorking){
// Do stuff
}
})
}
The problem is that isWorking is called before the check method is completed.
How can I make sure that check is completed before I make my check for isWorking?

Answer 1

You could use a "completion handler":
func check(url : String, completion: (isWorking: Bool)->()) {
let url = NSURL(string: url)
print("Checking")
let task = NSURLSession.sharedSession().dataTaskWithURL(url!) {(data, response, error) in
if error == nil {
completion(isWorking: true)
}
else{
completion(isWorking: false)
}
}
task.resume()
}
And you call it like this, with a trailing closure:
check(url) { (isWorking) in
if isWorking {
// do stuff
} else {
// not working
}
}

Answer 2

This will do the job:
import UIKit
class AnyViewController: UIViewController
{
var isWorking = false
func check(url : String)
{
let url = NSURL(string: url)
print("Checking")
let task = NSURLSession.sharedSession().dataTaskWithURL(url!) {(data, response, error) in
if error == nil
{
self.performSegueWithIdentifier("mySegueID", sender: nil)
}
else{
self.isWorking = false
}
}
task.resume()
}
}

Answer 3

You can post a notification to a listener to perform the segue only when the task is complete with NSNotificationCenter.defaultCenter().postNotificationName("name", object: nil):
override func viewDidLoad() {
super.viewDidLoad()
// Register a notification listener
NSNotificationCenter.defaultCenter().addObserver(self, selector: "customSegue:", name:"segue", object: nil)
}
func customSegue(notification: NSNotification){
// Do stuff
}
override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject!){
check(url)
}
func check(url : String){
let url = NSURL(string: url)
print("Checking")
dispatch_async(dispatch_get_main_queue(), {
let task = NSURLSession.sharedSession().dataTaskWithURL(url!) {(data, response, error) in
if error == nil {
self.isWorking = true
// Send a notification to the listener to perform stuff
NSNotificationCenter.defaultCenter().postNotificationName("segue", object: nil)
}
else{
self.isWorking = false
}
}
}
task.resume()
})
}
This way the code in customSegue func will only be executed when error == nil