Setting a locale of MeasurementFormatter instance voids any numberFormatter property options set on the instance

Solved Thanks to Martin R this has been solved by having to put the locale first. So:

import Foundation
let formatter = MeasurementFormatter()
formatter.locale = Locale(identifier: "en_GB") // Set locale first
formatter.numberFormatter.minimumFractionDigits = 4 // Now set numberFormatter properties
formatter.numberFormatter.maximumFractionDigits = 4
let distanceInKm = 16.093 // ~10 mi
var measurement: Measurement<UnitLength> = Measurement(value: distanceInKm, unit: .kilometers)
let string = formatter.string(from: measurement) // 9.998 mi
assert(string == "9.9998 mi") // Passes

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After several hours of confusion, I have found that setting the locale on a MeasurementFormatter instance voids any numberFormatter settings also set on it.

For example: 1. Run this:

import Foundation
let formatter = MeasurementFormatter()
formatter.numberFormatter.minimumFractionDigits = 4
formatter.numberFormatter.maximumFractionDigits = 4
let distanceInKm = 16.093 // ~10 mi
var measurement: Measurement<UnitLength> = Measurement(value: distanceInKm, unit: .kilometers)
let string = formatter.string(from: measurement) // 9.998 mi
assert(string == "9.9998 mi") // Passes

Now add a locale to the MeasurementFormatter instance and create a string from the same distance

formatter.locale = Locale(identifier: "en_GB") // Set the locale
let string2 = formatter.string(from: measurement) // 10 mi
assert(string2 == "9.9998 mi") // FAILS: No longer formatted to 4 fractional digits

I would expect in the second case for the assertion still to work, getting a string formatted to 4 decimal places as set by the numberFormatter property. Instead it seems to be ignored.

I have tried setting the numberFormatter properties again after setting a locale, but that doesn't change anything, e.g.,

formatter.locale = Locale(identifier: "en_GB") // Set the locale
formatter.numberFormatter.minimumFractionDigits = 4 // set numberFormatter properties again
formatter.numberFormatter.maximumFractionDigits = 4
let string3 = formatter.string(from: measurement) // still 10 mi
assert(string3 == "9.9998 mi") // FAILS: Still not formatted as per numberFormatter options

I couldn't find this being reported on SO, and thought I'd put it here so that other people can find it rather than wasting time. I have opened a radar with apple (and noticed someone else did a similar one for units of time in 2017!!!). I'm still hoping I am missing something, and/or if anyone has any suggestions on how to still format the number using a locale (in my actual source I'm actually working with metric and imperial distances based on user's locale), that would be much appreciated.

Solution

The only reliable way to use MeasurementFormatter with a custom locale seems to be:

Create a new formatter,
assign formatter.locale first,
then assign the other properties, such as the desired number of fraction digits,
use the new formatter.

A similar behavior is mentioned in Technical Q&A QA1480 for (NS)DateFormatter:

... you should first set the locale of the date formatter ...

Example:

let formatter = MeasurementFormatter()
formatter.locale = Locale(identifier: "de_DE")
formatter.numberFormatter.minimumFractionDigits = 4
formatter.numberFormatter.maximumFractionDigits = 4

let distanceInKm = 16.093
let measurement = Measurement(value: distanceInKm, unit: UnitLength.kilometers)
let string = formatter.string(from: measurement)

print(string) // 16,0930 km