您不应该根据进入后台或恢复的时间来进行任何调整，而只需节省您要数的时间（取决于您是在递增还是递减）。然后，当应用再次启动时，您只需在重构计时器时使用该时间即可。

同样，请确保您的计时器处理程序不依赖于调用处理选择器的确切时间（例如， 不要
执行seconds++任何类似操作，因为您可能希望调用时不会准确地调用它），但请务必返回从/到时间。

这是一个倒数计时器的示例，它说明我们不对任何东西进行“计数”。我们也不在乎介于appDidEnterBackground和之间的时间appDidBecomeActive。只需保存停止时间，然后计时器处理程序就可以将目标stopTime时间与当前时间进行比较，并根据需要显示经过的时间。

例如：

import UIKit
import UserNotifications

private let stopTimeKey = "stopTimeKey"

class ViewController: UIViewController {

    @IBOutlet weak var datePicker: UIDatePicker!
    @IBOutlet weak var timerLabel: UILabel!

    private weak var timer: Timer?
    private var stopTime: Date?

    let dateComponentsFormatter: DateComponentsFormatter = {
        let formatter = DateComponentsFormatter()
        formatter.allowedUnits = [.hour, .minute, .second]
        formatter.unitsStyle = .positional
        formatter.zeroFormattingBehavior = .pad
        return formatter
    }()

    override func viewDidLoad() {
        super.viewDidLoad()

        registerForLocalNotifications()

        stopTime = UserDefaults.standard.object(forKey: stopTimeKey) as? Date
        if let time = stopTime {
            if time > Date() {
                startTimer(time, includeNotification: false)
            } else {
                notifyTimerCompleted()
            }
        }
    }

    @IBAction func didTapStartButton(_ sender: Any) {
        let time = datePicker.date
        if time > Date() {
            startTimer(time)
        } else {
            timerLabel.text = "timer date must be in future"
        }
    }
}

// MARK: Timer stuff

private extension ViewController {
    func registerForLocalNotifications() {
        if #available(iOS 10, *) {
            UNUserNotificationCenter.current().requestAuthorization(options: [.alert, .sound, .badge]) { granted, error in
                guard granted, error == nil else {
                    // display error
                    print(error ?? "Unknown error")
                    return
                }
            }
        } else {
            let types: UIUserNotificationType = [.alert, .sound, .badge]
            let settings = UIUserNotificationSettings(types: types, categories: nil)
            UIApplication.shared.registerUserNotificationSettings(settings)
        }
    }

    func startTimer(_ stopTime: Date, includeNotification: Bool = true) {
        // save `stopTime` in case app is terminated

        UserDefaults.standard.set(stopTime, forKey: stopTimeKey)
        self.stopTime = stopTime

        // start Timer

        timer = Timer.scheduledTimer(timeInterval: 0.1, target: self, selector: #selector(handleTimer(_:)), userInfo: nil, repeats: true)

        guard includeNotification else { return }

        // start local notification (so we're notified if timer expires while app is not running)

        if #available(iOS 10, *) {
            let content = UNMutableNotificationContent()
            content.title = "Timer expired"
            content.body = "Whoo, hoo!"
            let trigger = UNTimeIntervalNotificationTrigger(timeInterval: stopTime.timeIntervalSinceNow, repeats: false)
            let notification = UNNotificationRequest(identifier: "timer", content: content, trigger: trigger)
            UNUserNotificationCenter.current().add(notification)
        } else {
            let notification = UILocalNotification()
            notification.fireDate = stopTime
            notification.alertBody = "Timer finished!"
            UIApplication.shared.scheduleLocalNotification(notification)
        }
    }

    func stopTimer() {
        timer?.invalidate()
    }

    // I'm going to use `DateComponentsFormatter` to update the
    // label. Update it any way you want, but the key is that
    // we're just using the scheduled stop time and the current
    // time, but we're not counting anything. If you don't want to
    // use `DateComponentsFormatter`, I'd suggest considering
    // `Calendar` method `dateComponents(_:from:to:)` to
    // get the number of hours, minutes, seconds, etc. between two
    // dates.

    @objc func handleTimer(_ timer: Timer) {
        let now = Date()

        if stopTime! > now {
            timerLabel.text = dateComponentsFormatter.string(from: now, to: stopTime!)
        } else {
            stopTimer()
            notifyTimerCompleted()
        }
    }

    func notifyTimerCompleted() {
        timerLabel.text = "Timer done!"
    }
}

顺便说一句，以上内容还说明了本地通知的使用（以防定时器在应用当前未运行时到期）。