Text string interpolation formatter bug with MeasurementFormatter when changing a locale setting

Number:rdar://FB8817280 Date Originated:21/10/20
Status:Open Resolved:
Product:SwiftUI Product Version:Xcode Version 12.2 beta 3 (12B5035g)
Classification:Bug Reproducible:Always
Using the Text View with string interpolation and a formatter has a bug when supplying a MeasurementFormatter. If I configure a MeasurementFormatter's numberFormatter, when I pass as the formatter param to a Text view, if I change a locale setting, e.g. the Temperature format in the Settings app, then my measurement formatter loses its number formatting.  I've attached a sample project that demonstrates the problem.

 1. Launch application in iPhone SE Simulator.
 2. The number is output with 6 fractional digits as the formatter specified.
 3. Keep app running, go to homescreen, open Settings app, General, Language & Region, Temperature Unit, change it from C to F or vice versa.
 4. Switch back to the running app.
 What expect to happen:
 Number should still be 6 decimals.
 What actually happens:
 Number is shown with only 3 decimals.

Additional info
I've tested the behaviour of the MeasurementFormatter in ObjC and Swift apps and I have only experienced the problem in SwiftUI. Also in SwiftUI if I do not pass the formatter into a Text then the issue does not happen, e.g. Text(degreesFormatter.string(from:degrees))


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