快速浏览视图控制器时，CountDown计时器不起作用？

Question

The countdown goes well in the background, when I minimize the app timer still countdown. but when I navigate FirstViewController to the SecondViewController and click back button from SecondViewController to go to firstViewController , the timer does not continue countdown. I want, when I am back to the first view controller the timer is still countdown.

here is my code

import UIKit

class FirstViewController: UIViewController {

        @IBOutlet weak var timerLabel: UILabel!

        var countdownTimer: Timer!
        var totalTime = 200


        override func viewDidLoad() {
            super.viewDidLoad()
            startTimer()
        }

    func startTimer() {
        countdownTimer = Timer.scheduledTimer(timeInterval: 1, target: self, selector: #selector(updateTime), userInfo: nil, repeats: true)
    }

    @objc func updateTime() {

        timerLabel.text = "\(timeFormatted(totalTime))"

            if totalTime != 0 {
                totalTime -= 1
            } else {
                endTimer()
            }
        }
        func endTimer() {
            countdownTimer.invalidate()
        }

        func timeFormatted(_ totalSeconds: Int) -> String {
            let seconds: Int = totalSeconds % 60
            let minutes: Int = (totalSeconds / 60) % 60
            //     let hours: Int = totalSeconds / 3600
            return String(format: "%02d:%02d", minutes, seconds)
        }


        @IBAction func nextScreen(_ sender: UIButton) {

            let vc = UIStoryboard(name: "Main", bundle: nil).instantiateViewController(withIdentifier: "SecondViewController")
            navigationController?.pushViewController(vc, animated: true)
        }

}

and here is my code in AppDelegate to make timer still run in background:

import UIKit

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

    var window: UIWindow?
    var backgroundUpdateTask: UIBackgroundTaskIdentifier = UIBackgroundTaskIdentifier(rawValue: 0)

    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {

        return true
    }

    func applicationWillResignActive(_ application: UIApplication) {
        self.backgroundUpdateTask = UIApplication.shared.beginBackgroundTask(expirationHandler: {
            self.endBackgroundUpdateTask()
        })
    }

    func applicationDidEnterBackground(_ application: UIApplication) {
    }

    func applicationWillEnterForeground(_ application: UIApplication) {
        self.endBackgroundUpdateTask()
    }

    func endBackgroundUpdateTask() {
        UIApplication.shared.endBackgroundTask(self.backgroundUpdateTask)
        self.backgroundUpdateTask = UIBackgroundTaskIdentifier.invalid
    }


    func applicationDidBecomeActive(_ application: UIApplication) {
        application.applicationIconBadgeNumber = 0
    }
}

second ViewController:

import UIKit

class SecondViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        // Do any additional setup after loading the view.
    }

    @IBAction func backToFirstScreen(_ sender: UIButton) {

        let vc = UIStoryboard(name: "Main", bundle: nil).instantiateViewController(withIdentifier: "FirstViewController")
        navigationController?.pushViewController(vc, animated: true)
    }

}

this the image view controller in storyboard

Answer 1

@IBAction func backToFirstScreen(_ sender: UIButton) 
{
   navigationController?.popViewController(animated:true)
}

Instead of going back you create another first view controller object which starts another one timer. This is the only issue that I can see in your code

Answer 2

Its because you are not getting back to firstViewController!

You are creating a new instance of firstViewController and pushing it on secondViewController.

For getting back you can pop the secondViewController:

@IBAction func backToFirstScreen(_ sender: UIButton) {

    self.navigationController?.popViewController(animated: true)
}