How can I make a countdown with NSTimer?

Question

How can I make a countdown with an NSTimer using Swift?

Answer 1

Add this to the end of your code... and call startTimer function with a parameter of where you want to count down to... For example (2 hours to the future of the date right now) -> startTimer(for: Date().addingTimeInterval(60*60*2))

Click here to view a screenshot of iPhone Simulator of how it'll look

extension ViewController
{
    func startTimer(for theDate: String)
    {
        let todaysDate = Date()
        let tripDate = Helper.getTripDate(forDate: theDate)
        let diffComponents = Calendar.current.dateComponents([.hour, .minute], from: Date(), to: tripDate)
        if let hours = diffComponents.hour
        {
            hoursLeft = hours
        }
        if let minutes = diffComponents.minute
        {
            minutesLeft = minutes
        }
        if tripDate > todaysDate
        {
            timer = Timer.scheduledTimer(timeInterval: 1.00, target: self, selector: #selector(onTimerFires), userInfo: nil, repeats: true)
        }
        else
        {
            timerLabel.text = "00:00:00"
        }
    }
    
    @objc func onTimerFires()
    {
        secondsLeft -= 1
        
        //timerLabel.text = "\(hoursLeft):\(minutesLeft):\(secondsLeft)"
        timerLabel.text = String(format: "%02d:%02d:%02d", hoursLeft, minutesLeft, secondsLeft)
        if secondsLeft <= 0 {
            if minutesLeft != 0
            {
                secondsLeft = 59
                minutesLeft -= 1
            }
        }
        
        if minutesLeft <= 0 {
            if hoursLeft != 0
            {
                minutesLeft = 59
                hoursLeft -= 1
            }
        }
        
        if(hoursLeft == 0 && minutesLeft == 0 && secondsLeft == 0)
        {
            timer.invalidate()
        }
        
    }
}