Swift countdown timer

CountDownTimer.swift

// MIT License

import Foundation

class CountdownTimer {
    
    struct ClockTime {
        let hours: Int
        let minutes: Int
        let seconds: Int
        let milliseconds: Int
        
        static var zero: ClockTime {
            ClockTime(hours: 0, minutes: 0, seconds: 0, milliseconds: 0)
        }
    }
    
    enum State {
        case started
        case progress(clockTime: ClockTime, timeRemaining: TimeInterval)
        case finished
    }
    
    private var timer: Timer?
    private let timeIntervalInFuture: TimeInterval
    private let countDownInterval: TimeInterval
    private let timerTolerance: TimeInterval
    
    init(timeIntervalInFuture: TimeInterval, countDownInterval: TimeInterval, timerTolerance: TimeInterval = 0) {
        self.timeIntervalInFuture = timeIntervalInFuture
        self.countDownInterval = countDownInterval
        self.timerTolerance = timerTolerance
    }
    
    func start(completionHandler: @escaping (State) -> Void) {
        
        completionHandler(.started)

        let startTime = Date().timeIntervalSince1970

        timer = Timer.scheduledTimer(withTimeInterval: countDownInterval, repeats: true) { (timer) in
            
            let endTime = startTime + self.timeIntervalInFuture
            let timeRemainingInSeconds = endTime - Date().timeIntervalSince1970
   
            if timeRemainingInSeconds <= 0 {
                completionHandler(.progress(clockTime: .zero, timeRemaining: timeRemainingInSeconds))
                completionHandler(.finished)
                timer.invalidate()
                self.timer = nil
            } else {
                let hoursRounded = Int(timeRemainingInSeconds.rounded()) / 3600
                let minutesRounded = Int(timeRemainingInSeconds.rounded()) / 60
                let secondsRounded = Int(timeRemainingInSeconds.rounded()) % 60
                
                let milliseconds = ( (timeRemainingInSeconds - floor(timeRemainingInSeconds)) * 1000 ).rounded()
                let millisecondsRounded = Int(milliseconds)
                
                completionHandler(.progress(clockTime: ClockTime(hours: hoursRounded, minutes: minutesRounded, seconds: secondsRounded, milliseconds: millisecondsRounded), timeRemaining: timeRemainingInSeconds))
            }
        }
        timer?.tolerance = timerTolerance
    }
    
    func stop() {
        self.timer?.invalidate()
        self.timer = nil
    }
}