How to know if user prefers miles or kilometers? - iphone

Question

In my app I've got a certain distance in meters.
And I want to display it in kilometers if user prefers kilometers and display it in miles if user prefers miles. And in the first case I want to add to a string "kilometers" at the end and in the second one to add "miles".
What is the best way to achieve this goal?
Thanks.

Answer 1

To determine whether the user uses metric or not, NSLocale can tell you:
- (BOOL)isMetric {
return [[[NSLocale currentLocale] objectForKey:NSLocaleUsesMetricSystem] boolValue];
}

Answer 2

Swift equivalent of Chris' answer would be something like this:
func isMetric() -> Bool {
return ((Locale.current as NSLocale).object(forKey: NSLocale.Key.usesMetricSystem) as? Bool) ?? true
}
Note that it defaults to true under certain circumstances. Change as needed.

Answer 3

You could ask the user whether they prefer miles or kilometers, in a preference or something. Then whenever you display a distance you would say.
In pseudo c code
function distance(meters) {
if (userPrefersKM) {
return meters / 1000 + " kilometers";
else if (userPrefersMiles) {
return meters / METERS_IN_A_MILE + " miles";
}
Where METERS_IN_A_MILE would be about 1600, but you should look that up.

Answer 4

In Swift, Locale.current.usesMetricSystem gives what the user would expect. But you don't need that if you use Measurement which handles it for you.
let distanceInMeters: Double = 2353.45
let formatter = MeasurementFormatter()
formatter.unitStyle = .medium // adjust according to your need
let distance = Measurement(value: distanceInMeters, unit: UnitLength.meters)
formatter.string(from: distance)
The current locale dictates how it is presented to the user. To see how it works for different locales, try this in a Xcode Playground (examples are for UK and France):
let distanceInMeters: Double = 2353.45
let formatter = MeasurementFormatter()
formatter.unitStyle = .medium // adjust according to your need
let distance = Measurement(value: distanceInMeters, unit: UnitLength.meters)
formatter.locale = Locale(identifier: "en_UK")
formatter.string(from: distance) // 1.462 mi
formatter.locale = Locale(identifier: "en_FR")
formatter.string(from: distance) // 2,353 km

Answer 5

Unless the iPhone provides this information directly, you'll have to have a lookup table from locale to default unit. Then you should allow the user to override that default.