Retain countdown timer value when switching between view controllers - ios

Question

I'm working on a trip countdown app that also has weather forecast built in.
Things are working pretty well, but when I switch from countdown timer to weather forecast, then come back, the countdown timer is reset.
Here's my countdown view controller code:
import UIKit
class CountdownVC: UIViewController {
var timer = Timer()
let userCalendar = Calendar.current
let requestedComponent: Set<Calendar.Component> = [.day,.hour,.minute,.second]
var departureDateTime: Date?
#IBOutlet weak var imageView: UIImageView!
let images = [
UIImage(named: "MK.png")!,
UIImage(named: "SSE1.png")!,
UIImage(named: "TofT.png"),
UIImage(named: "TofL1.png")!]
var index = 0
let animationDuration: TimeInterval = 0.25
let switchingInterval: TimeInterval = 15
func animateImageView()
{
CATransaction.begin()
CATransaction.setAnimationDuration(animationDuration)
CATransaction.setCompletionBlock {
DispatchQueue.main.asyncAfter(deadline: .now() + self.switchingInterval) {
self.animateImageView()
}
}
let transition = CATransition()
transition.type = kCATransitionFade
imageView.layer.add(transition, forKey: kCATransition)
imageView.image = images[index]
imageView.contentMode = .scaleAspectFill
imageView.alpha = 0.5
CATransaction.commit()
index = index < images.count - 1 ? index + 1 : 0
}
#IBOutlet weak var departureDateTimePicker: UIDatePicker!
#IBAction func departureDateTimePicker(_ sender: Any) {
let dateFormatter = DateFormatter()
dateFormatter.dateFormat = "MM/dd/yy H:mm:ss"
departureDateTimePicker.addTarget(self, action: #selector(handler), for: UIControlEvents.valueChanged)
}
#IBOutlet weak var selectDepartureDateTimeLabel: UILabel!
#IBOutlet weak var daysLabel: UILabel!
#IBOutlet weak var hoursLabel: UILabel!
#IBOutlet weak var minutesLabel: UILabel!
#IBOutlet weak var secondsLabel: UILabel!
func handler(sender: UIDatePicker) {
let dateFormatter = DateFormatter()
dateFormatter.dateFormat = "MM/dd/yy H:mm:ss"
let departureDateTimeString = dateFormatter.string(from: departureDateTimePicker.date)
departureDateTime = dateFormatter.date(from: departureDateTimeString)
}
func printTime() {
let dateFormatter = DateFormatter()
dateFormatter.dateFormat = "MM/dd/yy H:mm:ss"
let startTime = Date()
let endTime = departureDateTime
let timeDifference = userCalendar.dateComponents(requestedComponent, from: startTime, to: endTime!)
let startTimeDouble: Double = startTime.timeIntervalSinceReferenceDate
var endTimeDouble: Double?
endTimeDouble = (endTime?.timeIntervalSinceReferenceDate)
if endTimeDouble! > startTimeDouble {
daysLabel.text = "\(timeDifference.day!) Days"
hoursLabel.text = "\(timeDifference.hour!) Hours"
minutesLabel.text = "\(timeDifference.minute!) Minutes"
secondsLabel.text = "\(timeDifference.second!) Seconds"
} else {
timer.invalidate()
daysLabel.text = ""
hoursLabel.text = ""
minutesLabel.text = ""
secondsLabel.text = ""
}
}
func runTimer() {
let timer = Timer.scheduledTimer(timeInterval: 1.0, target: self, selector: #selector(printTime), userInfo: nil, repeats: true)
timer.fire()
}
#IBOutlet weak var startBtn: UIButton!
#IBAction func startBtnPressed(_ sender: Any) {
handler(sender: departureDateTimePicker)
printTime()
runTimer()
selectDepartureDateTimeLabel.text = "Time to departure"
departureDateTimePicker.isHidden = true
startBtn.isHidden = true
}
#IBOutlet weak var resetBtn: UIButton!
#IBAction func resetBtnPressed(_ sender: Any) {
timer.invalidate()
selectDepartureDateTimeLabel.text = "Select departure date & time"
departureDateTimePicker.isHidden = false
startBtn.isHidden = false
let dateFormatter = DateFormatter()
dateFormatter.dateFormat = "MM/dd/yy H:mm:ss"
let startTime = Date()
let endTime = startTime
let timeDifference = userCalendar.dateComponents(requestedComponent, from: startTime, to: endTime)
daysLabel.text = "\(timeDifference.day!) Days"
hoursLabel.text = "\(timeDifference.hour!) Hours"
minutesLabel.text = "\(timeDifference.minute!) Minutes"
secondsLabel.text = "\(timeDifference.second!) Seconds"
}
#IBAction func goToWeatherBtnPressed(_ sender: Any) {
performSegue(withIdentifier: "goToWeatherVC", sender: self)
}
override func viewDidLoad() {
super.viewDidLoad()
super.viewDidLoad()
imageView.image = images[index+1]
animateImageView()
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
// Dispose of any resources that can be recreated.
}
}