Setting locale on MeasurementFormatter voids settings of numberFormatter property

Number:rdar://48920090 Date Originated:15-Mar-2019
Status:Open Resolved:
Product:iOS+SDK Product Version:iOS 10+
Classification:Other Bug Reproducible:Always

Changing the locale on a MeasurementFormatter voids numberFormatter settings. 
One would expect to be able to change the format of the numbers even after settings a locale on the formatter. Whether or not this is intended behaviour is not documented anywhere or reported on popular development forums (e.g. StackOverflow, although I will put something there to notify others).

Steps to Reproduce:
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")

2. 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") // No longer formatted to 4 fractional digits

Expected Results:
Changing the locale on MeasurementFormatter should retain the options set in the associated numberFormatter property. If there is a reason this doesn't work, it should be documented to save developers time trying to get it to work.

Actual Results:
Setting a locale on the MeasurementFormatter voids any options set in the numberFormatter property.

iOS 10.0+

Swift 4.2

Setting the numberFormatter properties again after setting a locale to the MeasurementFormatter instance does not re-enable functionality. 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") // Still not formatted as per numberFormatter options


Its because setting the locale nulls the numberFormatter, see setNumberFormatter:0x0 below from Hopper:

/* @class NSUnitFormatter */
-(void)setLocale:(void *)arg2 {
rbx = arg2;
r14 = self;
if (arg2 == 0x0) {
        rbx = [NSLocale currentLocale];
r12 = *ivar_offset(_locale);
if ([*(r14 + r12) isEqual:rbx] == 0x0) {
        r15 = *(r14 + r12);
        *(r14 + r12) = [rbx retain];
        [r15 release];
        [r14 setNumberFormatter:0x0];
        *(int8_t *)&r14->_modified = 0x1;

Internally, NSMeasurementFormatter is a wrapper of NSUnitFormatter.

And FYI if you change the temperature format in settings, that nulls the number formatter too on the next call to formatter.string(from:

By indiekiduk at Oct. 21, 2020, 12:24 p.m. (reply...)

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