Custom NSNumberFormatter

less than 1 minute read

When using an API/Framework you may need to provide a NSNumberFormatter.

iosChart.valueFormatter = MyCustomNSNumberFormatter

But what happens if one of the options NSNumberFormatter has doesn’t have the options you want and you need something highly customized. And when I say custom, I don’t mean deciding if you need a currency style.

NSNumberFormatter().numberStyle = NSNumberFormatterStyle.CurrencyStyle

I mean if you need something highly customized then you need to create a subclass and override the stringFromNumber method. Here is an example of how I needed to turn the number of seconds into the format mm:ss but using a custom NSNumberFormatter (‘ios-charts’ required this).

class FeedDurationFormatter: NSNumberFormatter {
  required init(coder aDecoder: NSCoder) {
    super.init(coder: aDecoder)
  override init() {
    self.locale = NSLocale.currentLocale()
  override func stringFromNumber(duration: NSNumber) -> String? {
    let duration = duration.floatValue
    let minutes = floor(duration / 60)
    let seconds = duration % 60.0
    // time string, we don't want the decimals
    let timeString = String(format: "%01dm %01ds", Int(minutes), Int(seconds))
    return timeString
  // Swift 1.2 or above
  static let sharedInstance = FeedDurationFormatter()