Execute a func when the device time change - ios

Question

I'm trying to execute a func when the device clock change.
This func will return the next course of a student, all the courses are in a array.
if I understand correctly we can not execute a func when the clock of the device change.
I read some topics where people say to do a timer of 60s or other but if the user launch the app a 08:05:07 the func will execute with 7s of late.
I thought to use a do while but I think it will use the CPU a lot and so the battery too. no ?
Does anyone have an idea ?

Answer 1

If you’re just saying that you want to fire a timer at some specific future Date, you should just calculate the amount of time between now and then (using timeIntervalSince), and then use that.
For example, it’s currently "2019-01-20 17:11:59 +0000”, but if I want it to fire at 17:15, you can do:
weak var timer: Timer?
func startTimer() {
let futureDate = ISO8601DateFormatter().date(from: "2019-01-20T17:15:00Z")!
let elapsed = futureDate.timeIntervalSince(Date()) // will be roughly 180.56 in this example at this moment of time
timer?.invalidate() // invalidate prior timer, if any
timer = Timer.scheduledTimer(withTimeInterval: elapsed, repeats: false) { [weak self] _ in
// whatever you want to do at 17:15
}
}
Clearly, however you come up with futureDate will be different in your case, but it illustrates the idea. Just calculate the elapsed time between the future target date and now, and use that for the timer.
Now, if you’re really worried about changes to clock, in iOS you might observe significantTimeChangeNotification, e.g.,
NotificationCenter.default.addObserver(forName: UIApplication.significantTimeChangeNotification, object: nil, queue: .main) { [weak self] _ in
// do something, maybe `invalidate` existing timer and create new one
self?.startTimer()
}
I thought to use a do while but I think it will use the CPU a lot and so the battery too. no ?
Yes, spinning in a loop, waiting for some time to elapse, is always a bad idea. Generally you’d just set a timer.
This func will return the next course of a student, all the courses are in a array.
This begs the question whether the app will be running in the foreground or not. If you want to notify the user at some future time, whether they’re running the app right now or not, consider "user notifications”. E.g. request permission for notification:
UNUserNotificationCenter.current().requestAuthorization(options: [.alert, .badge, .sound]) { granted, _ in
if !granted {
// warn the user that they won't be notified after the user leaves the app unless they grant permission for notifications
DispatchQueue.main.async {
let alert = UIAlertController(title: nil, message: "We need permission to notify you of your class", preferredStyle: .alert)
if let url = URL(string: UIApplication.openSettingsURLString) {
alert.addAction(UIAlertAction(title: "Settings", style: .default) { _ in
UIApplication.shared.open(url)
})
}
alert.addAction(UIAlertAction(title: "Cancel", style: .default))
self.present(alert, animated: true)
}
}
}
And then, assuming that permissions have been granted, then schedule a notification:
let content = UNMutableNotificationContent()
content.title = "ClassTime"
content.body = "Time to go to math class"
let components = Calendar.current.dateComponents([.year, .month, .day, .hour, .minute], from: futureDate)
let trigger = UNCalendarNotificationTrigger(dateMatching: components, repeats: false)
let request = UNNotificationRequest(identifier: "Math 101", content: content, trigger: trigger)
UNUserNotificationCenter.current().add(request)

Answer 2

I did this
private var secondesActuelles : Int = 0
private var timer = Timer()
private func récuperLesSecondes(date : Date) -> Int {
let date = date
let calendar = Calendar.current
let second = calendar.component(.second, from: date)
return second
}
override func viewDidLoad() {
super.viewDidLoad()
secondesActuelles = récuperLesSecondes(date: Date())
timer = Timer.scheduledTimer(withTimeInterval: TimeInterval(60 - secondesActuelles), repeats: false, block: { (timer) in
print("l'heure a changé")
self.secondesActuelles = self.récuperLesSecondes(date: Date())
print("secondesActuelles : \(self.secondesActuelles)")
})
}
It work once. but after the timeInterval don't change.