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I have a UIDatePicker which is restricted to time only. The app saves the selected time from the datePicker in a variable and when this time is reached it fires a UILocalNotification.

So there is a label which shows the remaining time until the Local Notification fires. So it basically works a bit like a countdown Timer. How can I achieve this?

Date to String: (for displaying the fireDate)

func formatTimeForDisplay(date:NSDate) -> String {
        let formatter = NSDateFormatter()
        formatter.locale = NSLocale.currentLocale()
        formatter.timeStyle = NSDateFormatterStyle.ShortStyle
        return formatter.stringFromDate(date)
        }

Extension for NSLocalNotification: (for converting the NSDate from UIDatePicker to fireDate for the LocalNotification)

extension NSDate {
    var minute: Int {
        return NSCalendar.currentCalendar().component(.Minute, fromDate: self)
    }
    var hour: Int {
        return NSCalendar.currentCalendar().component(.Hour, fromDate: self)
    }
    var day: Int {
        return NSCalendar.currentCalendar().component(.Day, fromDate: self)
    }
    var month: Int {
        return NSCalendar.currentCalendar().component(.Month, fromDate: self)
    }
    var year: Int {
        return NSCalendar.currentCalendar().component(.Year, fromDate: self)
    }
    var fireDate: NSDate {
        let today = NSDate()
        return NSCalendar.currentCalendar().dateWithEra(1,
                                                        year: today.year,
                                                        month: today.month,
                                                        day: { hour > today.hour || (hour  == today.hour
                                                            &&  minute > today.minute) ? today.day : today.day+1 }(),
                                                        hour: hour,
                                                        minute: minute,
                                                        second: 0,
                                                        nanosecond: 0
            )!
    }
R3ptor
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2 Answers2

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To display user friendly string you may refer to NSDateComponentsFormatter if your app is targeted for iOS 8 and higher.

Also, you may encounter one problem. UIDatePicker gives back a NSDate object, which encapsulates amount of seconds from Jan 1, 2001. Even if you see only time component in date picker. This means, you should pay attention to the date value of UIDatePicker and maybe "fix" it with NSCalendar API.

Andriy
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  • @R3ptor Why do you need to write an extension? You can simply call `NSDateComponentsFormatter.stringFromDate(startDate: NSDate(), toDate endDate: yourFireDate)` and get desired string for your UI. – Andriy May 31 '16 at 19:35
  • Yeah you are probably right. But could help me with my original question? I understand what you mean but I'm still quite new to swift. So how exactly would I do that with the NSCalendar API? And how would that look like in code? – R3ptor May 31 '16 at 19:43
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    @R3ptor You'll need `NSCalendar` API only if you'll encounter the issue when user can switch to next or previous day when playing with `UIDatePicker`. We had this situation in our practice. In this case you have to make sure the resulting `NSDate` is in certain range (in your case it can be between start of current day and start of next day). – Andriy May 31 '16 at 19:48
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To do this, you'll need to use an NSTimer to get update the count down label.

In your ViewController, create a variable for the timer.

var timer = NSTimer()

I also set up a temp variable to hold the future date - you didn't post code, so I'm not sure what yours is called. In my code, I refer to it as futureDate

let futureDate = NSDate().dateByAddingTimeInterval(60*60*2.4)

When your use selects a date and you want the countdown to begin, you'll want to initiate the timer firing to call a method that will update the label. In thie example, i call it updateTimeLeft.

timer = NSTimer.scheduledTimerWithTimeInterval(0.5, target: self, selector: "updateTimeLeft", userInfo: nil, repeats: true)

Then, we simply need to calculate the difference between now and the future date when the local notification will fire. We do that in the updateTimeLeft method. In my code, I have an IBOutlet to the UILabel called timeLeft.

func updateTimeLeft()
{
    let elapsedTime = futureDate.timeIntervalSinceDate(NSDate())
    self.timeLeft.text = NSDateComponentsFormatter().stringFromTimeInterval(elapsedTime)
}
wottle
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  • This will work, but there's at least two basic problems with this solution: 1. Localization(you can't rely that in all countries and languages time components are formatted the same way). 2. There's situations, when you can't rely that hour has 3600 seconds and day has 24 hours (see UTC for more information). So, it is preferable to let system handle it properly. – Andriy May 31 '16 at 19:51
  • Yeah, I already updated it to use the NSDateComponentsFormatter. – wottle May 31 '16 at 19:55
  • Just a side note: `NSTimer` is not a reliable way to detect the time left, as it can be off sometimes... – Pranav Wadhwa May 31 '16 at 21:45
  • @penatheboss what would you suggest? – R3ptor May 31 '16 at 21:53
  • Take a look at this: http://stackoverflow.com/a/35496756/5143847 I know it's a lot of work, but it will help your code run better – Pranav Wadhwa May 31 '16 at 23:19
  • I'm not using the NSTimer to do any time calculations. I only use the timer to update the label occasionally. All time calculations are using date comparison between the time the user selected and the current time. – wottle Jun 01 '16 at 02:19
  • Also, unless the user needs precision for the countdown timer in the tenth or hundredth of a second range, the NSTimer will suffice for this use. The link to the other question is tackling a whole different problem, but also the poster was trying to use NSTimer as the actual time counting mechanism. I am not in my example. – wottle Jun 01 '16 at 02:22