swift - 如何防止短背景计时器漂移？

Question

我有一个 90 年代计时器，它在后台运行，通过注册 beginBackgroundTaskWithExpirationHandler 完成。这看起来很简单，但我遇到的问题是计时器本身漂移很大。对于 90 秒计时器，我得到大约 30-35 秒的漂移。即，如果我启动计时器，让应用进入后台，然后在 90 秒后打开应用，计时器会显示剩余 30 秒。

如果我让应用程序在整个 90 年代都处于打开状态，我将获得零漂移。如果我将计时器的时间间隔降低到 1 秒(而不是我首选的 0.05 秒)，那么背景漂移就会消失。

如何在不降低计时器精度的情况下消除背景漂移？

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

我放弃了后台任务并将其替换为 NSUserDefaults 中的存储。漂移消失了。这是最终代码:

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()
}

}