Crash in Swift 6 when using UNUserNotification

You are receiving the notification in an arbitrary background thread. Make didReceive nonisolated and do whatever you do inside it, in a MainActor isolated task. Do not change willPresent to nonisolated. It needs to be called on MainActor since it is a function about the user interface.

If you are posting the notification yourself, I recommend posting it on MainActor to get rid of most concurrency related issues. Be aware that the system and frameworks will continue posting from an arbitrary thread.

Thanks for the answer.

didReceive was already non isolated. So problem likely does not come from here.

For willPresent, if I don't make it nonIsolated, I get an Xcode 16.4 compiler error:

Main actor-isolated instance method 'userNotificationCenter(_:willPresent:withCompletionHandler:)' cannot be used to satisfy nonisolated requirement from protocol 'UNUserNotificationCenterDelegate'

n Xcode 26, no error in func declaration if I remove nonIsolated.

But error at UIViewController class definition level

Conformance of 'CalcViewController' to protocol 'UNUserNotificationCenterDelegate' crosses into main actor-isolated code and can cause data races

I would be very helpful if you were able to reproduce this on an actual device and attach a full symbolicated crash log to the bug report.

Also, if you are able to reproduce this with a small sample project, do attach that as well to help diagnose the issue.

@Engineer Thanks for reply. Some more crash info. On simulator (did not test on device at this stage).

Here is crash report:

Error occurs in #4 notifyMe IBAction

    @IBAction func notifyMe(_ sender: UIButton) {
        
        let center = UNUserNotificationCenter.current()
        center.delegate = self 
        center.getNotificationSettings(completionHandler: { (settings) in
            if settings.authorizationStatus == .notDetermined {
                center.requestAuthorization(options: [.alert, .sound, .badge]) { (granted, error) in
                    
                    DispatchQueue.main.async {  
                        if !granted {
					                 	// Alert user
                        }
                    }
                }
            } else if settings.authorizationStatus == .denied {
                DispatchQueue.main.async {
					        	// Alert user
                }
            } else if settings.authorizationStatus == .authorized {
                DispatchQueue.main.async {
					        self.alarmButton.isHidden = self.duree <= 120 
                }
            }
        })

        var alertStyle = UIAlertController.Style.alert
        
        let alertController = UIAlertController(
            title: NSLocalizedString("M'avertir", comment: ""),
            message: NSLocalizedString("Attention", comment: ""), preferredStyle: alertStyle)
        var title = NSLocalizedString("5'", comment: "") 
        let ok = UIAlertAction(title: title, style: .default, handler: {(action) -> Void in
            self.sendInitialNotification(beforeEnd: 5)
            DispatchQueue.main.async {  sender.isHidden = true  }
        })
        alertController.addAction(ok)
                
        let cancelAction = UIAlertAction(title: NSLocalizedString("Non", comment: ""), style: .default, handler: { (action) -> Void in
        
        alertController.addAction(cancelAction)

        present(alertController, animated: true, completion: nil)
         
    }

Crash apparently occurs on calling userNotificationCenter:

nonisolated func userNotificationCenter(_ center: UNUserNotificationCenter, willPresent notification: UNNotification, withCompletionHandler completionHandler: @escaping (UNNotificationPresentationOptions) -> Void) {
        
        if notification.request.identifier == requestIdentifier {
            completionHandler( [.alert,.sound,.badge])
        }
    }