快速NSTimer在背景中

Question

关于如何在堆栈或其他地方处理背景中的NSTimer,我遇到了很多问题.我已经尝试了其中一个实际上有意义的选项..当应用程序进入后台时停止计时器

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

和

    NSNotificationCenter.defaultCenter().addObserver(self, selector: "appDidBecomeActive", name: UIApplicationWillEnterForegroundNotification, object: nil)

起初我认为我的问题已经解决了,我只是节省了应用程序进入后台的时间并计算了应用程序进入前景时的差异..但后来我注意到时间实际上推迟了3,4,5秒.它实际上是不一样的..我把它与另一台设备上的秒表进行了比较.

真的有任何SOLID解决方案在后台运行NSTimer吗？

Answer 1

你不应该根据它进入后台或恢复的时间来搞乱任何调整,而只是节省你从或计算的时间(取决于你是否向上或向下计数).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.

同样,确保你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,不要做任何类似seconds++或类似的事情,因为它可能无法在你希望的时候准确调用),但总是回到从/到时间.

这是一个倒计时器的示例,它说明我们不"计算"任何东西.我们也不关心时间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只比较目标stopTime和当前时间,并显示您想要的经过时间.

在Swift 3中:

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!"
    }
}

或者在Swift 2中:

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!"
    }
}

顺便说一下,上面还说明了本地通知的使用(如果计时器在app当前没有运行时到期).