Create a timer that runs each second, on the second? - ios

Question

I have a countdown timer to a specific date. It looks pretty good, and updates each second to show the countdown. Here is the code I'm using to create the timer:
func scheduleTimeLabelsUpdateTimer() {
var components = Calendar.current.dateComponents([.day, .hour, .minute, .second], from: Date())
components.second! += 1
let nextSecondDate = Calendar.current.date(from: components)!
let timer = Timer(fireAt: nextSecondDate, interval: 1, target: self, selector: #selector(updateTimeLabels), userInfo: nil, repeats: true)
RunLoop.main.add(timer, forMode: .commonModes)
}
However, I'd like it to update each second, on the second, so that the clock updates at the same time as each second passes by. Right now it updates each second from when this method is called, which is in viewDidLoad().
For example if the countdown is set for midnight, I want it to hit zero exactly at midnight. Right now it may hit zero slightly after midnight, depending on how far into the second it was when the user opened this screen.
EDIT: This is how the countdown is displayed to the user. updateTimeLabels() just sets the text for each of those labels based on the amount of time left until that date. I would like each of the labels to be updated exactly on each second. This way the countdown will "hit zero" exactly on time. Notice how right now, the number of seconds hits zero, and then the system clock on the status bar updates. I would like these to happen at the same time.
This code, which I found somewhere on Stack Overflow many months ago, is being called in updateTimeLabels() to calculate the remaining time:
public func timeOffset(from date: Date) -> (days: Int, hours: Int, minutes: Int, seconds: Int) {
// Number of seconds between times
var delta = Double(self.seconds(from: date))
// Calculate and subtract whole days
let days = floor(delta / 86400)
delta -= days * 86400
// Caluclate and subtract whole hours
let hours = floor(delta / 3600).truncatingRemainder(dividingBy: 24)
delta -= hours * 3600
// Calculate and subtract whole minutes
let minutes = floor(delta / 60.0).truncatingRemainder(dividingBy: 60)
delta -= minutes * 60
// What's left is seconds
let seconds = delta.truncatingRemainder(dividingBy: 60)
return (Int(days), Int(hours), Int(minutes), Int(seconds))
}

Answer 1

In your code, you never use nanosecond when you init your component
So the timer will always hit at the round second number.
I test your code below print the Date() in the selector
I'm not sure if this is what you are talking about "hit zero", hope this would help

Answer 2

Since a Timer is scheduled on the run loop and other things are happening in the run loop, it isn't particularly accurate; it will fire after the specified interval has passed, but not necessarily exactly when the interval has passed.
Rather than trying to schedule the timer for the desired time, you should run your timer faster than 1 second, say at 0.5 seconds and update your time remaining label each time the timer fires. This will give a smoother update to the display:
var timer: Timer?
func scheduleTimeLabelsUpdateTimer() {
self.timer = Timer.scheduledTimer(withTimeInterval: 0.5, repeats: true) { (timer) in
self.updateTimeLabels()
}
}
UPDATE
Don't do all of that math; iOS has nice libraries built in to do all of this for you; Simply create your target date and use DateComponents to work it out for you.
import UIKit
class ViewController: UIViewController {
#IBOutlet weak var label: UILabel!
#IBOutlet weak var countDownLabel: UILabel!
let dateFormatter = DateFormatter()
var targetTime: Date!
var calendar = Calendar.current
var timer: Timer?
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view, typically from a nib.
var components = DateComponents()
components.setValue(3, for: .month)
components.setValue(3, for: .day)
components.setValue(2017, for: .year)
components.setValue(0, for: .hour)
components.setValue(0, for: .minute)
components.setValue(1, for: .second)
self.targetTime = calendar.date(from: components)
self.timer = Timer.scheduledTimer(withTimeInterval: 0.5, repeats: true, block: { (timer) in
self.updateLabel()
})
self.dateFormatter.timeStyle = .long
self.dateFormatter.dateStyle = .short
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
// Dispose of any resources that can be recreated.
}
func updateLabel() {
let now = Date()
self.label.text = self.dateFormatter.string(from: now)
let components = calendar.dateComponents([.day,.hour,.minute,.second], from: now, to: self.targetTime)
self.countDownLabel.text = String(format: "%d days %d hours, %d minutes %d seconds", components.day!, components.hour!, components.minute!,components.second!)
}
}