使用NSTimer以秒为单位的倒数计时（从00:20到0）

问题描述

我需要将标签倒计时从20秒降低到0，然后重新开始。这是我第一次在Swift中进行项目，并且尝试使用 NSTimer.scheduledTimerWithTimeInterval 。此倒计时应在给定的时间内循环运行。

I need to have a label countdown from 20secs to 0 and start over again. This is my first time doing a project in Swift and I am trying to use NSTimer.scheduledTimerWithTimeInterval. This countdown should run in a loop for a given amount of times.

我很难实施开始和开始方法（循环）。我基本上找不到20秒开始计时的方法，当结束时，请重新开始计时。

I am having a hard time implementing a Start and Start again method (loop). I basically am not finding a way to start the clock for 20s and when it's over, start it again.

我很欣赏如何操作
Wagner的任何想法

I'd appreciate any idea on how to do thatWagner

 @IBAction func startWorkout(sender: AnyObject) {

    timer = NSTimer.scheduledTimerWithTimeInterval(0.01, target: self, selector: Selector("countDownTime"), userInfo: nil, repeats: true)
    startTime = NSDate.timeIntervalSinceReferenceDate()

}

func countDownTime() {

    var currentTime = NSDate.timeIntervalSinceReferenceDate()

    //Find the difference between current time and start time.
    var elapsedTime: NSTimeInterval = currentTime - startTime

    //calculate the seconds in elapsed time.
    let seconds = UInt8(elapsedTime)
    elapsedTime -= NSTimeInterval(seconds)

    //find out the fraction of milliseconds to be displayed.
    let fraction = UInt8(elapsedTime * 100)

    //add the leading zero for minutes, seconds and millseconds and store them as string constants
    let strSeconds = seconds > 9 ? String(seconds):"0" + String(seconds)
    let strFraction = fraction > 9 ? String(fraction):"0" + String(fraction)

    //concatenate minuets, seconds and milliseconds as assign it to the UILabel
    timeLabel.text = "\(strSeconds):\(strFraction)"
}

推荐答案

您应该从现在开始将日期endTime设置为20s，然后只需检查日期timeIntervalSinceNow。一旦timeInterval达到0，则将其设置为从现在起20秒

You should set your date endTime 20s from now and just check the date timeIntervalSinceNow. Once the timeInterval reaches 0 you set it 20 seconds from now again

import UIKit

class ViewController: UIViewController {

    @IBOutlet weak var strTimer: UILabel!

    var endTime = NSDate().dateByAddingTimeInterval(20)
    var timer = NSTimer()

    func updateTimer() {
        let remaining = endTime.timeIntervalSinceNow
        strTimer.text = remaining.time
        if remaining <= 0 {
            endTime = NSDate().dateByAddingTimeInterval(20)
        }
    }
    override func viewDidLoad() {
        super.viewDidLoad()
        strTimer.text =  "20:00"
        timer = NSTimer.scheduledTimerWithTimeInterval(0.01, target: self, selector: "updateTimer", userInfo: nil, repeats: true)
        NSRunLoop.mainRunLoop().addTimer(timer, forMode: NSRunLoopCommonModes)
    }
    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
    }
}

extension NSTimeInterval {
    var time:String {
        return String(format:"%02d:%02d", Int((self) % 60 ),Int(self*100 % 100 )  )
    }
}

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