Write custom NSTextField class to work with Measurement in Swift

See stackoverflow question here: [login to view URL] Looking for a custom NSTextField class (or NSTextFieldCell class) where I can read and write measurements to where the user can not edit the measurement text. I want to be able to nicely use a Measurement and MeasurementFormatter for output and input with a NSTextFieldCell. I am able to display the measurement correctly with... let areaFormatter = MeasurementFormatter() [login to view URL] = .medium [login to view URL] = .providedUnit let area = Measurement<UnitArea>( value: 123.43, unit: .squareInches) let editInput = NSTextFieldCell [login to view URL] = area [login to view URL] = areaFormatter This displays something like 123.43 in^2 The problem starts when I want to read this back in with var inputArea = [login to view URL] as! Measurement<UnitArea> I think because the get Object value of the Measurement Formatter is not defined. open func getObjectValue(_ obj: AutoreleasingUnsafeMutablePointer<AnyObject?>?, for string: String, errorDescription error: AutoreleasingUnsafeMutablePointer<NSString?>?) -> Bool Is my understanding correct? Are there any examples in Swift where this function has been defined for measurements? Right now the user can edit the entire string including the text in the units. Is there a good way to block the units in the NSTextFieldCell? I would like the user to be able to edit the number but not the units and then return the measurement with var inputArea = [login to view URL] as! Measurement<UnitArea> so this gets displayed 123.43 in^2 but only the 123.43 can be edited.