NSNotification: Attempt to present UIAlertController on ViewController whose view is not in the window hierarchy

Question

I'm trying to show a UIAlertController in my ViewController in a function that's been called via an NSNotification. However I'm getting the error:

Attempt to present <UIAlertController: 0x7fe013d05d40> on <submarine.ViewController: 0x7fe011f20370> whose view is not in the window hierarchy!

The NSNotification is posted from a completion block (callback I guess) from something else in my UI. Because it's a callback it's failing to display. Hence I thought I'd try NSNotificationCentre to get around the problem without using the rootViewController to display the alert.

My code is:

override func viewDidAppear(animated: Bool) {
    // Handle onboarding
    if needsOnboarding() {
        handleOnboarding() // This create the completion block that posts the NSNotification
    }

    NSNotificationCenter.defaultCenter().addObserver(self, selector: "showTermsAlert:", name:"showTermsAlert", object: nil)
}

func showTermsAlert(notification: NSNotification) {
    let termsAlert:UIAlertController = UIAlertController(title: "Terms And Conditions", message: "Please view the terms below before accepting them.", preferredStyle: UIAlertControllerStyle.Alert)

    termsAlert.addAction(UIAlertAction(title: "View Terms", style: .Default, handler: { (action: UIAlertAction!) in
        UIApplication.sharedApplication().openURL(NSURL(string: "my_terms_url")!)
    }))

    termsAlert.addAction(UIAlertAction(title: "I Agree to the Terms", style: .Default, handler: { (action: UIAlertAction!) in
        self.onboardingFinished()
    }))

    self.presentViewController(termsAlert, animated: true, completion: nil)
}

Has anyone got an idea why this is happening? I don't see why it's not in the window hierarchy - it's being presented from the self viewController and is created in a top-level function inside the VC.

Thanks!

EDIT: original code inside the handleOnboarding():

Library used: Onboard

func handleOnboarding() {

        let secondPage = OnboardingContentViewController(title: "What's going on?", body: "Submarine routes your data through our network, around any filters and restrictions, giving you unrestricted and unmonitored internet access.", image: UIImage(named: "back"), buttonText: "Next") { () -> Void in
            // do something here when users press the button, like ask for location services permissions, register for push notifications, connect to social media, or finish the onboarding process
        }
        secondPage.movesToNextViewController = true

        let thirdPage = OnboardingContentViewController(title: "Terms of Use", body: "You must agree to our Terms of Use to use Submarine.\nIf you don't, please close Submarine.", image: UIImage(named: "back"), buttonText: "View Terms") { () -> Void in

            let termsAlert:UIAlertController = UIAlertController(title: "Terms And Conditions", message: "Please view the terms below before accepting them.", preferredStyle: UIAlertControllerStyle.Alert)

            termsAlert.addAction(UIAlertAction(title: "View Terms", style: .Default, handler: { (action: UIAlertAction!) in
                UIApplication.sharedApplication().openURL(NSURL(string: "my_policy_url")!)
            }))

            termsAlert.addAction(UIAlertAction(title: "I Agree to the Terms", style: .Default, handler: { (action: UIAlertAction!) in
                self.onboardingFinished()
            }))

            self.presentViewController(termsAlert, animated: true, completion: nil)

//            NSNotificationCenter.defaultCenter().postNotificationName("showTermsAlert", object: nil)
        }

        // Image
        let onboardingVC = OnboardingViewController(backgroundImage: UIImage(named: "back"), contents: [secondPage, thirdPage])

        self.navigationController?.presentViewController(onboardingVC, animated: false, completion: nil)

    }

Answer 1

This happen when the presenting view controller is no longer part of the controller hierarchy, and it's view is no longer in the view hierarchy of any window. Most likely, the controller was dismissed or popped, but it heard the notification and attempted to present the alert controller.

You should manage your controller states more carefully. Perhaps remove observer when the controller is dismissed or popped from your controller hierarchy.