短い背景タイマーのドリフトを防ぐには？

Question

私はbeginBackgroundTaskWithExpirationHandlerを登録して、バックグラウンドで完了するまで実行される90秒タイマーを持っています。これは簡単ですが、問題はタイマー自体がかなり漂っていることです。 90sタイマーの場合、私は約30-35sのドリフトを得る。つまり、タイマーを起動してアプリをバックグラウンドにしてから90秒後にアプリを開くと、タイマーに残りの30秒が表示されます。

私が90秒間アプリを開いたままにしておけば、ゼロドリフトが発生します。タイマーのtimeIntervalを1秒（私の優先0.05秒ではなく）に落とすと、バックグラウンドのドリフトが消えます。

タイマーの精度を落とさずにバックグラウンドドリフトを除去するにはどうすればよいですか？

class TimerViewController: UIViewController { 

    @IBOutlet weak var startTimerButton: UIButton! 
    @IBOutlet weak var timerLabel: UILabel! 
    @IBOutlet weak var resetTimerButton: UIButton! 

    var timer = NSTimer() 
    let timeInterval:NSTimeInterval = 0.05 
    let timerEnd:NSTimeInterval = 90 
    var timeCount:NSTimeInterval = 0 

    var backgroundTask: UIBackgroundTaskIdentifier = UIBackgroundTaskInvalid 
    var backgroundTaskIdentifier: UIBackgroundTaskIdentifier? 

    override func viewDidLoad() { 
     super.viewDidLoad() 

     resetTimeCount() 
     timerLabel.text = timeString(timeCount) 

     NSNotificationCenter.defaultCenter().addObserver(self, selector: #selector(TimerViewController.reinstateBackgroundTask), name: UIApplicationDidBecomeActiveNotification, object: nil) 
    } 

    deinit { 
     NSNotificationCenter.defaultCenter().removeObserver(self) 
    } 



    override func didReceiveMemoryWarning() { 
     super.didReceiveMemoryWarning() 
     // Dispose of any resources that can be recreated. 
    } 

    // MARK: Actions 

    @IBAction func startTimerButtonTapped(sender: UIButton) { 

     if !timer.valid { //prevent more than one timer on the thread 
      timerLabel.text = timeString(timeCount) 
      timer = NSTimer.scheduledTimerWithTimeInterval(timeInterval, target: self,selector: #selector(TimerViewController.timerDidEnd(_:)),userInfo: nil, repeats: true) 
      schedulePushNotification() 
      registerBackgroundTask() 
     } 

    } 

    @IBAction func resetTimerButtonTapped(sender: UIButton) { 
     timer.invalidate() 
     resetTimeCount() 
     timerLabel.text = timeString(timeCount) 
     cancelAllNotifications() 
    } 

    // MARK: Timer 

    func resetTimeCount(){ 
     timeCount = timerEnd 
    } 

    func timerDidEnd(timer: NSTimer){ 
     //timer that counts down 
     timeCount = timeCount - timeInterval 
     if timeCount <= 0 { //test for target time reached. 
      timerLabel.text = "Time is up!!" 
      timer.invalidate() 
      AudioServicesPlayAlertSound(kSystemSoundID_Vibrate) 
      if backgroundTask != UIBackgroundTaskInvalid { 
       endBackgroundTask() 
       cancelAllNotifications() 
      } 
     } else { //update the time on the clock if not reached 
      timerLabel.text = timeString(timeCount) 
     } 

    } 

    func timeString(time: NSTimeInterval) -> String { 
     let minutes = Int(time)/60 
     let seconds = time - Double(minutes) * 60 
     let secondsFraction = seconds - Double(Int(seconds)) 
     return String(format:"%02i:%02i.%02i",minutes,Int(seconds),Int(secondsFraction * 100.0)) 
    } 

    // MARK: BackgroundTask 

    func registerBackgroundTask() { 
     backgroundTask = UIApplication.sharedApplication().beginBackgroundTaskWithExpirationHandler { 
      [unowned self] in 
      self.endBackgroundTask() 
     } 
     assert(backgroundTask != UIBackgroundTaskInvalid) 
    } 

    func reinstateBackgroundTask() { 
     if timeCount != 0.0 && (backgroundTask == UIBackgroundTaskInvalid) { 
      registerBackgroundTask() 
     } 
    } 

    func endBackgroundTask() { 
     UIApplication.sharedApplication().endBackgroundTask(backgroundTask) 
     backgroundTask = UIBackgroundTaskInvalid 
    } 

    // MARK: Notifications 

    func schedulePushNotification() { 
     let notification = UILocalNotification() 
     notification.alertAction = "Go back to App" 
     notification.alertBody = "The 90s timer is finished!" 
     notification.fireDate = NSDate(timeIntervalSinceNow: timerEnd+1) 
     UIApplication.sharedApplication().scheduleLocalNotification(notification) 

    } 

    func cancelAllNotifications() { 
     UIApplication.sharedApplication().cancelAllLocalNotifications() 
    } 

} 

Answer 1

バックグラウンドタスクを削除し、NSUserDefaultsのストレージに置き換えました。ドリフトはなくなった。最後のコードは次のとおりです。

class TimerViewController: UIViewController { 

@IBOutlet weak var startTimerButton: UIButton! 
@IBOutlet weak var timerLabel: UILabel! 
@IBOutlet weak var resetTimerButton: UIButton! 

var timer = NSTimer() 
let timeInterval:NSTimeInterval = 0.01 
let timerEnd:NSTimeInterval = 90 
var timeCount:NSTimeInterval = 0 

override func viewDidLoad() { 
    super.viewDidLoad() 

    resetTimeCount() 
    timerLabel.text = timeString(timeCount) 

    NSNotificationCenter.defaultCenter().addObserver(self, selector: #selector(TimerViewController.applicationWillResignActive),name: UIApplicationWillResignActiveNotification, object: nil) 
    NSNotificationCenter.defaultCenter().addObserver(self, selector: #selector(TimerViewController.applicationDidBecomeActive),name: UIApplicationDidBecomeActiveNotification, object: nil) 
} 

deinit { 
    NSNotificationCenter.defaultCenter().removeObserver(self) 
} 

override func didReceiveMemoryWarning() { 
    super.didReceiveMemoryWarning() 
    // Dispose of any resources that can be recreated. 
} 

// MARK: Actions 

@IBAction func startTimerButtonTapped(sender: UIButton) { 

    if !timer.valid { //prevent more than one timer on the thread 
     timerLabel.text = timeString(timeCount) 
     timer = NSTimer.scheduledTimerWithTimeInterval(timeInterval, target: self,selector: #selector(TimerViewController.timerDidEnd(_:)),userInfo: nil, repeats: true) 
     schedulePushNotification() 
    } 

} 

@IBAction func resetTimerButtonTapped(sender: UIButton) { 
    timer.invalidate() 
    resetTimeCount() 
    timerLabel.text = timeString(timeCount) 
    cancelAllNotifications() 
} 

// MARK: Timer 

func resetTimeCount(){ 
    timeCount = timerEnd 
} 

func timerDidEnd(timer: NSTimer){ 
    //timer that counts down 
    timeCount = timeCount - timeInterval 
    if timeCount <= 0 { //test for target time reached. 
     timerLabel.text = "Time is up!!" 
     timer.invalidate() 
     AudioServicesPlayAlertSound(kSystemSoundID_Vibrate) 
     cancelAllNotifications() 
    } else { //update the time on the clock if not reached 
     timerLabel.text = timeString(timeCount) 
    } 
} 

func timeString(time: NSTimeInterval) -> String { 
    let minutes = Int(time)/60 
    let seconds = time - Double(minutes) * 60 
    let secondsFraction = seconds - Double(Int(seconds)) 
    return String(format:"%02i:%02i.%02i",minutes,Int(seconds),Int(secondsFraction * 100.0)) 
} 

// MARK: Timer Storage 

struct PropertyKey { 
    static let timeCountKey = "TimerViewController_timeCount" 
    static let timeMeasurementKey = "TimerViewController_timeMeasurement" 
} 

dynamic private func applicationWillResignActive() { 
    if !timer.valid { 
     clearDefaults() 
     NSLog("Clearning defaults") 
    } else { 
     saveDefaults() 
     NSLog("Saving defaults") 
    } 
} 

dynamic private func applicationDidBecomeActive() { 
    if timer.valid { 
     loadDefaults() 
     NSLog("Loading defaults") 
    } 
} 

private func saveDefaults() { 
    let userDefault = NSUserDefaults.standardUserDefaults() 
    userDefault.setObject(timeCount, forKey: PropertyKey.timeCountKey) 
    userDefault.setObject(NSDate().timeIntervalSince1970, forKey: PropertyKey.timeMeasurementKey) 

    userDefault.synchronize() 

} 

private func clearDefaults() { 
    let userDefault = NSUserDefaults.standardUserDefaults() 
    userDefault.removeObjectForKey(PropertyKey.timeCountKey) 
    userDefault.removeObjectForKey(PropertyKey.timeMeasurementKey) 

    userDefault.synchronize() 

} 

private func loadDefaults() { 
    let userDefault = NSUserDefaults.standardUserDefaults() 
    let restoredTimeCount = userDefault.objectForKey(PropertyKey.timeCountKey) as! Double 
    let restoredTimeMeasurement = userDefault.objectForKey(PropertyKey.timeMeasurementKey) as! Double 

    let timeDelta = NSDate().timeIntervalSince1970 - restoredTimeMeasurement 
    print(timeDelta) 
    print(timeCount - restoredTimeCount - timeDelta) 
    timeCount = restoredTimeCount - timeDelta 
} 

// MARK: Notifications 

func schedulePushNotification() { 
    let notification = UILocalNotification() 
    notification.alertAction = "Go back to App" 
    notification.alertBody = "The 90s timer is finished!" 
    notification.fireDate = NSDate(timeIntervalSinceNow: timerEnd+1) 
    UIApplication.sharedApplication().scheduleLocalNotification(notification) 

} 

func cancelAllNotifications() { 
    UIApplication.sharedApplication().cancelAllLocalNotifications() 
} 

}