您不应该根据进入后台或恢复的时间进行任何调整，而只是保存您开始或结束的时间（取决于您是向上还是向下计数）。然后，当应用程序再次启动时，您只需在重建计时器时使用该时间/时间即可。
同样，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，执行not做类似的事情seconds++或类似的东西，因为它可能不会在您希望的时候准确地被调用），但总是时不时地回到那个地方。
这是一个倒计时器的例子，它说明了我们不“计算”任何东西。我们也不关心之间流逝的时间appDidEnterBackground and appDidBecomeActive。只需保存停止时间，然后计时器处理程序即可比较目标stopTime和当前时间，并根据您的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，上面还说明了本地通知的使用（以防计时器到期而应用程序当前未运行）。
对于 Swift 2 版本，请参阅此答案的先前修订版 https://stackoverflow.com/revisions/34497360/4.
