Get value of localized Unit from Measurement

Question

I need to get the internationalized value of a Measurement.

let speed = Measurement(value: 15, unit: UnitSpeed.kilometersPerHour)
let formatter = MeasurementFormatter()
formatter.string(from: speed) // is "9.32 mph"
speed.converted(to: .milesPerHour).value // this is what I want, but I have to hardcode the unit I am converting to

Can I get the value (9.32) without manually splitting strings and casting it? For temperatures you can use formatter.unitOptions = .temperatureWithoutUnit but this will stop the conversion and I would still end up with a string.

Is there anything like speed.converted(to: default).value to get the value of the localized Measurement?

Answer 1

What I think you are after is the measurement system for the locale. The Mac currently understands three systems: metric, US and UK. The UK uses metric for lengths and weights but still uses miles/mph for road distances and speeds, hence the third option here – the usesMetricSystem property only supports two options and returns true for the UK.

To get the measurement system in Swift requires using NSLocale, this extension will add it to Locale:

extension Locale
{
   var measurementSystem : String?
   {
      return (self as NSLocale).object(forKey: NSLocale.Key.measurementSystem) as? String
   }
}

For added "fun" Apple doesn't actually specify the values of this property, they give examples but not a full definition. You can get the known three values querying the property on three locales known to use them, e.g. you could add the following to the extension:

   static let metricMeasurementSystem = Locale(identifier: "fr_FR").measurementSystem!
   static let usMeasurementSystem = Locale(identifier: "en_US").measurementSystem!
   static let ukMeasurementSystem = Locale(identifier: "en_UK").measurementSystem!

Of course if Swift did support them it would probably define an enum for the possibilities, you could do that as well.

HTH

BTW: For those who think the UK is being awkward, the "US" system is only used in three countries: US, Myanmar and Liberia.