swift NSTimer in Background

You shouldn't be messing with any adjustments based upon when it enters background or resumes, but rather just save the time that you are counting from or to (depending upon whether you are counting up or down). Then when the app starts up again, you just use that from/to time when reconstructing the timer.

Likewise, make sure your timer handler is not dependent upon the exact timing that the handling selector is called (e.g. do not do anything like seconds++ or anything like that because it may not be called precisely when you hope it will), but always go back to that from/to time.

Here is an example of a count-down timer, which illustrates that we don't "count" anything. Nor do we care about the time elapsed between appDidEnterBackground and appDidBecomeActive. Just save the stop time and then the timer handler just compares the target stopTime and the current time, and shows the elapsed time however you'd like.

For example:

import UIKit
import UserNotifications

private let stopTimeKey = "stopTimeKey"

class ViewController: UIViewController {

    @IBOutlet weak var datePicker: UIDatePicker!
    @IBOutlet weak var timerLabel: UILabel!

    private weak var timer: Timer?
    private var stopTime: Date?

    let dateComponentsFormatter: DateComponentsFormatter = {
        let formatter = DateComponentsFormatter()
        formatter.allowedUnits = [.hour, .minute, .second]
        formatter.unitsStyle = .positional
        formatter.zeroFormattingBehavior = .pad
        return formatter
    }()

    override func viewDidLoad() {
        super.viewDidLoad()

        registerForLocalNotifications()

        stopTime = UserDefaults.standard.object(forKey: stopTimeKey) as? Date
        if let time = stopTime {
            if time > Date() {
                startTimer(time, includeNotification: false)
            } else {
                notifyTimerCompleted()
            }
        }
    }

    @IBAction func didTapStartButton(_ sender: Any) {
        let time = datePicker.date
        if time > Date() {
            startTimer(time)
        } else {
            timerLabel.text = "timer date must be in future"
        }
    }
}

// MARK: Timer stuff

private extension ViewController {
    func registerForLocalNotifications() {
        if #available(iOS 10, *) {
            UNUserNotificationCenter.current().requestAuthorization(options: [.alert, .sound, .badge]) { granted, error in
                guard granted, error == nil else {
                    // display error
                    print(error ?? "Unknown error")
                    return
                }
            }
        } else {
            let types: UIUserNotificationType = [.alert, .sound, .badge]
            let settings = UIUserNotificationSettings(types: types, categories: nil)
            UIApplication.shared.registerUserNotificationSettings(settings)
        }
    }

    func startTimer(_ stopTime: Date, includeNotification: Bool = true) {
        // save `stopTime` in case app is terminated

        UserDefaults.standard.set(stopTime, forKey: stopTimeKey)
        self.stopTime = stopTime

        // start Timer

        timer = Timer.scheduledTimer(timeInterval: 0.1, target: self, selector: #selector(handleTimer(_:)), userInfo: nil, repeats: true)

        guard includeNotification else { return }

        // start local notification (so we're notified if timer expires while app is not running)

        if #available(iOS 10, *) {
            let content = UNMutableNotificationContent()
            content.title = "Timer expired"
            content.body = "Whoo, hoo!"
            let trigger = UNTimeIntervalNotificationTrigger(timeInterval: stopTime.timeIntervalSinceNow, repeats: false)
            let notification = UNNotificationRequest(identifier: "timer", content: content, trigger: trigger)
            UNUserNotificationCenter.current().add(notification)
        } else {
            let notification = UILocalNotification()
            notification.fireDate = stopTime
            notification.alertBody = "Timer finished!"
            UIApplication.shared.scheduleLocalNotification(notification)
        }
    }

    func stopTimer() {
        timer?.invalidate()
    }

    // I'm going to use `DateComponentsFormatter` to update the
    // label. Update it any way you want, but the key is that
    // we're just using the scheduled stop time and the current
    // time, but we're not counting anything. If you don't want to
    // use `DateComponentsFormatter`, I'd suggest considering
    // `Calendar` method `dateComponents(_:from:to:)` to
    // get the number of hours, minutes, seconds, etc. between two
    // dates.

    @objc func handleTimer(_ timer: Timer) {
        let now = Date()

        if stopTime! > now {
            timerLabel.text = dateComponentsFormatter.string(from: now, to: stopTime!)
        } else {
            stopTimer()
            notifyTimerCompleted()
        }
    }

    func notifyTimerCompleted() {
        timerLabel.text = "Timer done!"
    }
}

By the way, the above also illustrates the use of a local notification (in case the timer expires while the app isn't currently running).

For Swift 2 rendition, see previous revision of this answer.

Unfortunately, there is no reliable way to periodically run some actions while in background. You can make use of background fetches, however the OS doesn't guarantee you that those will be periodically executed.

While in background your application is suspended, and thus no code is executed, excepting the above mentioned background fetches.