tick.swift

import UIKit
import PlaygroundSupport

@objc class ViewController: UIViewController {
    public func bbi(_ title: String, _ action: Selector) -> UIBarButtonItem {
        return UIBarButtonItem(title: title, style: .plain, target: self, action: action)
    }
    
    var timer: Timer? = nil
    var timeRemaining: Double = 0
    
    func reset(_ item: UIBarButtonItem?) {
        navigationItem.leftBarButtonItem = nil
        navigationItem.rightBarButtonItem = bbi("Start", #selector(start(_:)))
        timer?.invalidate(); timer = nil
        timeRemaining = 0
        title = ""
    }
    
    func pause(_ item: UIBarButtonItem?) {
        navigationItem.leftBarButtonItem = nil
        navigationItem.rightBarButtonItem = bbi("Resume", #selector(resume(_:)))
        timer?.invalidate(); timer = nil
    }
    
    func tick() {
        guard timeRemaining > 0 else {
            reset(nil); return
        }
        title = "Time remaining: \(timeRemaining)"
        timeRemaining -= 1.0
    }
    
    func resume(_ item: UIBarButtonItem?) {
        navigationItem.rightBarButtonItem = bbi("Pause", #selector(pause(_:)))
        navigationItem.leftBarButtonItem = bbi("Reset", #selector(reset(_:)))
        timer = Timer.scheduledTimer(withTimeInterval: 1.0, repeats: true, block: { _ in self.tick() } )
        tick()
    }
    
    func start(_ item: UIBarButtonItem?) {
        timeRemaining = 10.0
        resume(nil)
    }
    
    public override func viewDidLoad() {
        super.viewDidLoad()
        tick()
    }
}

let vc = ViewController()
let nav = UINavigationController(rootViewController: vc)
PlaygroundPage.current.liveView = nav
PlaygroundPage.current.needsIndefiniteExecution = true

retain cycle in resume?