background run timer swift - swift

Question

I want the timer to run even when I close the application. I want it to work in the background counter. the timer goes back one second when I run it.(counter) How can I do that?
class TimerViewController: UIViewController {
var selectedDay: String?
var seconds =
var timer = Timer()
#IBAction func start(_ sender: AnyObject) {
timer = Timer.scheduledTimer(timeInterval: 1, target: self, selector: #selector(TimerViewController.counter), userInfo: nil, repeats: true)
sliderOutlet.isHidden = true
startOutlet.isHidden = true
}
#objc func counter() {
seconds -= 1
favoriteDayTextField.text = String(seconds) + " Seconds"
var bgTask = UIBackgroundTaskIdentifier(rawValue: seconds)
bgTask = UIApplication.shared.beginBackgroundTask(expirationHandler: {
UIApplication.shared.endBackgroundTask(bgTask)
})
if (seconds == 0) {
timer.invalidate()
if self.button.isOn {
updateState()
} else {
updateState1()
}
}
}
}

Answer 1

I am not clear what you want to achieve. Suppose you want to update the label after the timer has started each 1 second. Then one approach will be:-
Start the timer in view did load if the duration is remaining.
Register for applicationWillTerminate
In application will terminate save the passed duration and terminated time to calculate remaining time in next launch.
var remainingDuration: TimeInterval!
override func viewDidLoad() {
super.viewDidLoad()
let remainingDurationFromLastLaunch = UserDefaults.standard.object(forKey: "duration") as? TimeInterval ?? 0
let lastTerminatedTime = UserDefaults.standard.object(forKey: "lastTerminatedDate") as? Date ?? Date()
if Date().timeInterval(since: lastTerminatedTime) > remainingDurationFromLastLaunch {
remainingDuration = remainingDurationFromLastLaunch - Date().timeInterval(since: lastTerminatedTime)
timer = Timer.scheduledTimer(timeInterval: 1, target: self, selector: #selector(TimerViewController.counter), userInfo: nil, repeats: true)
NotificationCenter.default.addObserver(self, selector: #selector(TimerViewController.applicationWillTerminate), name: NSNotification.Name.UIApplicationWillTerminate, object: nil)
} else { //Duration is passed....Do whatever you want
}
}
#objc func counter() {
remainingDuration -= 1
if remainingDuration == 0 { //Duration is passed....Do whatever you want
timer.invalidate()
timer = nil
} else {
favoriteDayTextField.text = String(remainingDuration) + " Seconds"
}
}
#objc func applicationWillTerminate() {
if timer != nil {
backgroundTask = UIApplication.shared.beginBackgroundTask { [weak self] in
UserDefaults.standard.set(remainingDuration, forKey: "duration")
UserDefaults.standard.set(Date(), forKey: "lastTerminatedDate")
}
self?.endBackgroundTask()
}
}
func endBackgroundTask() {
UIApplication.shared.endBackgroundTask(backgroundTask)
backgroundTask = UIBackgroundTaskInvalid
}

Answer 2

The only way for your iOS application to perform some action even while it is in the background is to use Background Modes .
However you cannot perform anything and everything while your
application is in background
There are certain limitations to the type of tasks that you can perform . I have attached a really good article for your reference
Background Modes Tutorial
However, I am not sure if you can initiate and continue a timer sort of functionality while your application is in background
Though, keep in mind , once your application is closed (i.e. by double tapping the home button and swiping the application window up to close it completely) , not even Background modes work at that point because the user does not want to run your app anymore, even in the background