Swift: NSTimer automatically ends without counting down

问题

Swift newbie here. I have two questions:

First, how do I create a timer which AUTOMATICALLY counts down when a ViewController scene is opened? My problem is that the NSTimer ENDS automatically when the scene is opened.

For instance, whenever I go to the said scene, the TimerLabel says: “Time’s Up!”

Before my second question, below is my tweaked code from: makeapppie.com/…/swift-swift-using-nstimer-to-make-a-timer…/

var timer = NSTimer()
let timeInterval:NSTimeInterval = 0.05
let timerEnd:NSTimeInterval = 120.0 //Timer should end in 120 seconds
var timeCount:NSTimeInterval = 0.0
func timerDidEnd(timer:NSTimer){
    timeCount = timeCount - timeInterval
    if timeCount <= 0 {
        TimerLabel.text = "Time's Up!"
        timer.invalidate()
    }
}

func timeString(time:NSTimeInterval) -> String {
    let minutes = Int(time) / 60
    let seconds = time - Double(minutes) * 60
    return String(format:"%02i:%02i",minutes,Int(seconds))
}

func StartTimer() { // Function called in viewDidLoad
    if !timer.valid{
        TimerLabel.text = timeString(timeCount)
        timer = NSTimer.scheduledTimerWithTimeInterval(timeInterval,
        target: self,
        selector: "timerDidEnd:",
        userInfo: nil,
        repeats: true)
    }
}

My second question is this: I do not want to use any UISwitch to choose whether to have the timer counting up, or counting down. I need a COUNTING DOWN timer only which AUTOMATICALLY counts down when view is opened. How do I do that?

Please take note that my time format is: “minutes:seconds” as described in the timeString function.

Please help.

回答1:

Welcome to SO. I think you're in over your head here.

A timer doesn't count up or count down. It fires on a regular interval. You write a method that gets called each time the timer fires.

Note that 0.05 is a pretty short interval for a timer. Timers are not super-accurate, so you might be trying to get too much out of them.

Your code is a confused mess. You start timeCount out at 0. Your timer method, that gets called repeatedly, is called timerDidEnd even though you want your timer to keep running. Your timerDidEnd method subtracts "timeInterval" (.05 seconds) from timeCount when the timer fires, which makes timeCount negative. You then check to see if timeCount is less than 0 (it will be the very first time through this code) and if it is, you invalidate the timer. You also never do anything to display the value of timeCount.

If you want code that counts down, you need to start out with a value that's larger than 0, and subtract a small amount from it each time the timer fires. Then you need to do something with that timeCount value. If you're not displaying it somewhere, it really isn't doing anything useful.

P.S. On an unrelated subject, the avatar pictures on SO are square. You should post a square avatar picture (or create a square crop to post.) When you post a rectangular picture it gets really stretched-out and looks bad.

回答2:

My problem is that the NSTimer ENDS automatically when the scene is opened.

It sounds like your putting your NSTimer and the code that manages it in a view controller. In that case, it's not surprising that the timer goes away when that view controller does. If the timer should persist from one view controller to the next, then you should manage it from an entirely separate object from the view controller. Since you don't want the timer object to have to keep track of all the view controllers that could possibly be interested in it, have it post notifications that interested view controllers can listen to.

来源：https://stackoverflow.com/questions/34867661/swift-nstimer-automatically-ends-without-counting-down

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nstimer