模拟器是否会像物理设备一样杀死不结束后台任务的应用程序？

Question

I'm working on debugging why a count down timer is being killed off in the background.

To familiarize myself better, I made a quick and dirty timer implementation. It starts a background task, starts a standard timer, and adds it to the RunLoop.

Whenever the seconds change on the count down, I print out how many seconds on the count down I have left and how many seconds the OS has given me (ie UIApplication.shared.backgroundTimeRemaining ).

However, when I run this in the simulator, start the timer, and put the app in the background, the timer works just fine and never stops until it counted all the way down.

Some things to keep in mind:

I WANT the timer to not stop until done, even if in background. However, I know the OS typically gives 3-5 mins in the background. This is where my question comes from. If I only have 3-5 mins in the background, then why is my timer running basically as long as it needs to? Does the Simulator not kill off apps in the same time as the physical device in the background?

Additionally, I also set a callback to fire when and if the OS kills my off (ie the expirationHandler callback provided by the beginBackgroundTask(withName: function)

Any insight into this would be helpful: Here's my View Controller class:

class TimerViewController: UIViewController, UIPickerViewDelegate, UIPickerViewDataSource {

    // MARK: - Outlets

    @IBOutlet weak var timeLeftLabel: UILabel!
    @IBOutlet weak var timePicker: UIPickerView!

    // MARK: - Properties

    var timer: Timer?
    var timeLeft: Int = 0 {
        didSet {
            DispatchQueue.main.async {
                self.timeLeftLabel.text = "\(self.timeLeft.description) seconds left"
            }
        }
    }
    var backgroundTask: UIBackgroundTaskIdentifier = .invalid
    let backgroundTaskName = "bgTask"

    // MARK: - Lifecycle

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

    @objc func applicationDidMoveToBackground() {
        print("moved to backgorund")
    }

    @objc func applicationWillMoveToForegraund() {
        print("moved to foreground")
    }

    // MARK: - Setup

    func setupUI() {
        NotificationCenter.default.addObserver(self, selector: #selector(applicationDidMoveToBackground), name: UIApplication.didEnterBackgroundNotification, object: nil)
        NotificationCenter.default.addObserver(self, selector: #selector(applicationWillMoveToForegraund), name: UIApplication.willEnterForegroundNotification, object: nil)
        timePicker.tintColor = .white
        timePicker.backgroundColor = .clear
    }

    func registerBackgroundTask() {
        //end any bg tasks
        endBackgroundTask()
        //start new one
        backgroundTask = UIApplication.shared.beginBackgroundTask(withName: backgroundTaskName, expirationHandler: {
            //times up, do this stuff when ios kills me
            print("background task being ended by expiration handler")
            self.endBackgroundTask()
        })
        assert(backgroundTask != UIBackgroundTaskIdentifier.invalid)

        //actual meat of bg task
        print("starting")
        timePicker.isHidden = true
        timeLeftLabel.isHidden = false
        timeLeft = getCurrentPickerViewSeconds()
        timeLeftLabel.text = "\(timeLeft) seconds left"
        setupTimer()
    }

    func endBackgroundTask() {
        UIApplication.shared.endBackgroundTask(self.backgroundTask)
        self.backgroundTask = .invalid
    }

    func setupTimer() {
        timer = Timer.scheduledTimer(timeInterval: 1.0, target: self, selector: #selector(fire), userInfo: nil, repeats: true)
        if timer != nil {
            RunLoop.current.add(timer!, forMode: .common)
        } else {
            print("timer is nil, didnt add to runloop")
        }
    }

    // MARK: - Helpers

    func getCurrentPickerViewSeconds() -> Int {
        let mins = timePicker.selectedRow(inComponent: 0)
        let seconds = timePicker.selectedRow(inComponent: 1)
        let totalSeconds = seconds + (mins * 60)
        return totalSeconds
    }

    // MARK: - Actions

    @objc func fire() {
        print("current time left: \(timeLeft)")
        print("background time remaining: \(UIApplication.shared.backgroundTimeRemaining)")
        if timeLeft > 0 {
            timeLeft -= 1
        } else {
            print("done")
            stopTimer()
        }
    }

    @IBAction func startTimer() {
        registerBackgroundTask()
    }

    @IBAction func stopTimer() {
        print("stopping")
        endBackgroundTask()
        timePicker.isHidden = false
        timeLeftLabel.isHidden = true
        timer?.invalidate()
        timer = nil
    }

    @IBAction func resetTimer() {
        print("resetting")
        stopTimer()
        startTimer()
    }

    @IBAction func doneTapped() {
        print("done-ing")
        stopTimer()
        dismiss(animated: true, completion: nil)
    }

    // MARK: - Picker View

    func numberOfComponents(in pickerView: UIPickerView) -> Int {
        return 2
    }

    func pickerView(_ pickerView: UIPickerView, numberOfRowsInComponent component: Int) -> Int {
        return 59
    }

    func pickerView(_ pickerView: UIPickerView, attributedTitleForRow row: Int, forComponent component: Int) -> NSAttributedString? {
        var label = ""
        switch component {
        case 0:
            label = "m"
        case 1:
            label = "s"
        default:
            label = ""
        }
        let result = "\(row) \(label)"
        let attributedResult = NSAttributedString(string: result, attributes: [NSAttributedString.Key.foregroundColor : UIColor.white])
        return attributedResult
    }

    func pickerView(_ pickerView: UIPickerView, didSelectRow row: Int, inComponent component: Int) {
        let seconds = row + (60 * component)
        timeLeft = seconds
    }
}

Here's are some screenshots of the output (one from the start of the count down, one from when the expirationHandler was fired, one from the end of the count down):

Answer 1

Well, I figured it out, Stupidly enough. I should have just tried it on a physical device (just didn't have one at the time of posting that question).

Findings:

The simulator does NOT behave in the same way as the physical device because it does NOT kill off apps after the allowed time from the OS is up (ie the UIApplication.shared.backgroundTimeRemaining is basically a lie/non issue when in the simulator).

On the other hand, with a physical device, as soon as backgroundTimeRemaining hit 0, the app was killled off which was expected. Here's a screenshot of the output: