Continue countdown timer when app is running in background/suspended

Question

I have a functional countdown timer.

The problem is that I need to continue the countdown when the user puts the app in the background. Or suspends the app? I am not sure what the correct terminology is to describe this action.

In other words, the user opens the app and starts the countdown. The user should be able to use other apps but the countdown still operate. Also, the countdown should be able to run forever until it finishes or when the user terminates running the app.

Here is the code for my timer:

    class Timer{
var timer = NSTimer();
// the callback to be invoked everytime the timer 'ticks'
var handler: (Int) -> ();
//the total duration in seconds for which the timer should run to be set by the caller
let duration: Int;
//the amount of time in seconds elapsed so far
var elapsedTime: Int = 0;
var targetController = WaitingRoomController.self

var completionCallback: (() -> Void)!



/**
:param: an integer duration specifying the total time in seconds for which the timer should run repeatedly
:param: handler is reference to a function that takes an Integer argument representing the elapsed time allowing the implementor to process elapsed time and returns void
*/
init(duration: Int , handler : (Int) -> ()){
    self.duration = duration;
    self.handler = handler;
}

/**
Schedule the Timer to run every 1 second and invoke a callback method specified by 'selector' in repeating mode
*/
func start(){
    self.timer = NSTimer.scheduledTimerWithTimeInterval(1, target: self, selector: "onTick", userInfo: nil, repeats: true);
}

/**
invalidate the timer
*/
func stop(){
    println("timer was invaidated from stop()")
    timer.invalidate();
}


/**
Called everytime the timer 'ticks'. Keep track of the total time elapsed and trigger the handler to notify the implementors of the current 'tick'. If the amount of time elapsed is the same as the total duration for the timer was intended to run, stop the timer.
*/


@objc func onTick() {
    //println("onTick")
    //increment the elapsed time by 1 second
    self.elapsedTime++;
    //Notify the implementors of the updated value of elapsed time
    self.handler(elapsedTime);
    //If the amount of elapsed time in seconds is same as the total time in seconds for which this timer was intended to run, stop the timer
    if self.elapsedTime == self.duration {
        self.stop();

    }
}
deinit{
    println("timer was invalidated from deinit()")
    self.timer.invalidate();
}

}

Answer 1

What I suggest is cancel the timer and store a NSDate when the app goes to the background. You can use this notification to detect the app going to the background:

NSNotificationCenter.defaultCenter().addObserver(self, selector: "pauseApp", name: UIApplicationDidEnterBackgroundNotification, object: nil)

Then cancel the timer and store the date:

func pauseApp(){
    self.stop() //invalidate timer
    self.currentBackgroundDate = NSDate()
}

Use this notification to detect the user coming back:

NSNotificationCenter.defaultCenter().addObserver(self, selector: "startApp", name: UIApplicationDidBecomeActiveNotification, object: nil)

Then calculate the difference from the stored date to the current date, update your counter and start the timer again:

func startApp(){
    let difference = self.currentBackgroundDate.timeIntervalSinceDate(NSDate())
    self.handler(difference) //update difference
    self.start() //start timer
}

Answer 2

You shouldn't do that. A running timer keeps the device's processor running at full power, which is bad.

Only certain types of apps are allowed to actually run code indefinitely from the background, e.g. VOIP apps and music apps.

A couple of options:

1. Set up a local notification for a future date (which will send a message to your app, or re-launch it if it was no longer running.)

2. When you start your timer, record the current NSDate. Then, when your app returns to the foreground or gets re-launched, fetch compare the current date to the saved date, figure out the amount of time that's elapsed, and decide if your timer is finished yet.

Answer 3

Try this instead.. this code updates the time when app comes back from background/suspend and in active state..

class ProgressV: UIView {

var timer: NSTimer!
var expirationDate = NSDate()

//Set the time that you want to decrease..
var numSeconds: NSTimeInterval = 36000.0 // Ex:10:00 hrs

func startTimer()
{
    // then set time interval to expirationDate…
    expirationDate = NSDate(timeIntervalSinceNow: numSeconds)
    timer = NSTimer.scheduledTimerWithTimeInterval(1.0, target: self, selector: #selector(updateUI(_:)), userInfo: nil, repeats: true)
    NSRunLoop.currentRunLoop().addTimer(timer, forMode: NSRunLoopCommonModes)

}
func updateUI(timer: NSTimer)
{
  // Call the currentTimeString method which can decrease the time..
   let timeString = currentTimeString()
   labelName.text = timeString
}
func currentTimeString() -> String {
    let unitFlags: NSCalendarUnit = [.Hour, .Minute, .Second]
    let countdown: NSDateComponents = NSCalendar.currentCalendar().components(unitFlags, fromDate: NSDate(), toDate: expirationDate, options: [])

    var timeRemaining: String
    if countdown.hour > 0
    {
        timeRemaining = String(format: "%02d:%02d:%02d", countdown.hour, countdown.minute, countdown.second)
    }
    else {
        timeRemaining = String(format: "%02d:%02d:%02d", countdown.hour, countdown.minute, countdown.second)
    }
    return timeRemaining
}

}

Answer 4

Be sure that you turn on the background mode and set needed values in xCode. It is strange, but even if background mode is turned off, this code works. It seems to be work any timers after setting application.beginBackgroundTask {} in AppDelegate.

I use this code:

In AppDelegate add code below:

func applicationDidEnterBackground(_ application: UIApplication) {

    application.beginBackgroundTask {} // allows to run background tasks
}

And call method below where you want.

func registerBackgroundTask() {
    DispatchQueue.global(qos: .background).asyncAfter(deadline: DispatchTime.now() + 5, qos: .background) {
            print("fasdf")
        }
}