Swift NSTiimer not following specified Interval - ios

Question

I am trying to create a quiz app which has a timer for each question when the timer expires (i.e. 10 seconds and I want Timer to have an interval of 1 sec) it resets it self and next question is fetched and Timer again restart from 10... But my issue is the timer doesn't follow a fixed interval when first question is loaded it shows interval of 2 ... i.e. 10,8,6 .. and then for second question it makes jump for 3 secs interval and similarly the interval increases.
import UIKit
class ViewController: UIViewController {
let allQuestions = QuestionsBundle()
var pickedAnswer : Int = 0
var questionCounter = 0
var score : Int = 0
var timer: Timer!
#IBOutlet weak var questionLabel: UILabel!
#IBOutlet weak var countDownLabel: UILabel!
#IBOutlet weak var ansLbl1: UILabel!
#IBOutlet weak var ansLbl2: UILabel!
#IBOutlet weak var ansLbl3: UILabel!
#IBOutlet weak var ansLbl4: UILabel!
#IBOutlet weak var checkBox1: CheckBox!
#IBOutlet weak var checkBox2: CheckBox!
#IBOutlet weak var checkBox3: CheckBox!
#IBOutlet weak var checkBox4: CheckBox!
var checkBoxlist : [CheckBox] = []
#IBAction func correct_Answer_Checbox_Btn(_ sender: AnyObject) {
//print("\(sender.tag) <==> \(String(describing: question?.correctAnswer))")
updateCheckBoxes(sender: sender)
if sender.tag == question?.correctAnswer{
question?.isAnswerCorrect = true
question?.selectedAnswer = sender.tag
//score = score + 1
}
else {
question?.isAnswerCorrect = false
}
}
func updateCheckBoxes(sender: AnyObject){
for checkBoxItem in checkBoxlist{
if checkBoxItem.tag != sender.tag {
checkBoxItem.isChecked = false
}
}
}
#IBOutlet weak var nextButton: UIButton!
#IBAction func nextBtnClicked(_ sender: AnyObject) {
do{
try handleNextQuestion()
}
catch{
moveToResultView()
}
}
func handleNextQuestion() throws {
nextQuestion()
if questionCounter == allQuestions.list.count-1{
finishButton.isHidden = false
nextButton.isHidden = true
//scoreLbl.text = "\(score)"
}
}
var question : Question?
var countTime = 10.0
override func viewDidLoad() {
super.viewDidLoad()
finishButton?.isHidden = true
checkBoxlist = fetchCheckBoxList()
question = fetchQuestion()
setQuizView(question: question!)
// Do any additional setup after loading the view, typically from a nib.
}
// set all questions in a function
#objc func update() {
if(countTime > 0) {
countTime = countTime - 1
self.countDownLabel.text = String(countTime)
}else{
timer.invalidate()
countTime = 10.0
do{
try handleNextQuestion()
}
catch{
moveToResultView()
}
}
}
func startTimer() {
timer = Timer.scheduledTimer(timeInterval: 1.0, target: self, selector: #selector(ViewController.update), userInfo: nil, repeats: true)
}
func setQuizView(question:Question) {
self.countDownLabel.text = "10"
startTimer()
questionLabel.text = question.questionText
ansLbl1.text = question.answer1
ansLbl2.text = question.answer2
ansLbl3.text = question.answer3
ansLbl4.text = question.answer4
if question.selectedAnswer == Constants.DEFAULT_ANSWER {
for checkBoxItem in checkBoxlist{
checkBoxItem.isChecked = false
}
}
}
#IBOutlet weak var finishButton: UIButton!
// prepare segue
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if segue.identifier == resultScreenIdentifier{
let vc = segue.destination as! ResultViewController
vc.data = sender as! String
}
}
let resultScreenIdentifier = "resultScreenSegue"
func moveToResultView(){
performSegue(withIdentifier: resultScreenIdentifier, sender: score)
}
#IBAction func finishButtonClicked(_ sender: UIButton) {
//perform segue
let score = "\(calculateScore())"
moveToResultView()
}
// calculate the score of quiz using loop
func calculateScore()->Int{
var numOfCorrectAnswers = 0
for question in allQuestions.list{
if question.isAnswerCorrect {
numOfCorrectAnswers = numOfCorrectAnswers + 1
//print(numOfCorrectAnswers)
}
}
return numOfCorrectAnswers
}
func nextQuestion(){
showResultView(isCorrect: (question?.isAnswerCorrect)!)
questionCounter = questionCounter + 1
question = fetchQuestion()
setQuizView(question: question!)
}
func fetchQuestion() -> Question{
return allQuestions.list[questionCounter]
}
func fetchCheckBoxList() -> [CheckBox]{
let arr : [CheckBox] = [checkBox1,checkBox2,checkBox3,checkBox4]
return arr
}
}

Answer 1

Timers are not particularly accurate. They can suffer from significant jitter.
A better approach is to create a Date that represents the expiration time (ie Date(timeIntervalSinceNow:10) and then run a Timer with a much shorter interval (I would suggest around 0.1 second). You can then calculate the time remaining based on the target Date and check if the target date is in the past.