TimerViewController.swift

import WatchKit
import Foundation
 import NotificationCenter

class TimerController: WKInterfaceController {
    @IBOutlet weak var timerOutlet: WKInterfaceTimer!
    
    var myTimer : Timer?
    var duration : TimeInterval = 45.0 //arbitrary number. 45 seconds
    
    var isPaused = false //flag to determine if it is paused or not
    var elapsedTime : TimeInterval = 0.0 //time that has passed between
    
    var startTime = NSDate()
    override func awake(withContext context: Any?) {
        super.awake(withContext: context)
        start_timer()
        timerOutlet.setTextColor(UIColor.red)
        
        // Configure interface objects here.
    }
    
    func start_timer() {
        myTimer = Timer.scheduledTimer(timeInterval: duration, target:
            self,selector: Selector(("timerDone")), userInfo: nil, repeats:
            false)
        timerOutlet.setDate(NSDate(timeIntervalSinceNow: duration ) as
            Date)
        timerOutlet.start()
    }
    func timerDone(){
        //timer done counting down
    }
    @IBAction func pauseResumePressed() {
        //timer is paused. so unpause it and resume countdown
        if isPaused{
            isPaused = false
            myTimer = Timer.scheduledTimer(timeInterval: duration -
                elapsedTime, target: self, selector:
                Selector(("timerDone")), userInfo:
                nil, repeats: false)
            timerOutlet.setDate(NSDate(timeIntervalSinceNow: duration -
                elapsedTime) as Date)
            timerOutlet.start()
            startTime = NSDate()
            //pauseResumeButton.setTitle("Pause")
            
        }
            //pause the timer
        else{
            isPaused = true
            
            //get how much time has passed before they paused it
            let paused = NSDate()
            elapsedTime += paused.timeIntervalSince(startTime as Date)
            
            //stop watchkit timer on the screen
            timerOutlet.stop()
            
            //stop the ticking of the internal timer
            myTimer!.invalidate()
            
            //do whatever UI changes you need to
            //pauseResumeButton.setTitle("Resume")
        }
    }
    
    override func willActivate() {
        // This method is called when watch view controller is about to
        super.willActivate()
        
        NotificationCenter.default.addObserver(self, selector: #selector(stop_timer(notification:)), name: .stopTimer, object: nil)

        
    }
    
    override func didDeactivate() {
        // This method is called when watch view controller is no
        super.didDeactivate()
    }
    
    
   
 
   
    @objc func stop_timer(notification:NSNotification) {
        
        // Timer is paused. so unpause it and resume countdown
        if isPaused {
            isPaused = false
            timerOutlet.start()
            startTime = NSDate()
            
        } else {
            isPaused = true
            
            // Stop watchkit timer on the screen
            timerOutlet.stop()
            
            // Stop the ticking of the internal timer
            myTimer!.invalidate()
            
            // Do whatever UI changes you need to
            
        }
        
    }
    
    
}

extension Notification.Name {
    static let stopTimer = Notification.Name("stopTimer")
}