An improved Swift NumberFormatter supporting Units
UnitFormatter
The ‘better’ NumberFormatter for Apple platforms supporting values with appended units and allowing the User to enter values (format string to number) even without or with a wrongly formatted unit string.
Motivation
The NumberFormatter belonging to the Foundation framework is specifically designed to format numerical values into strings and vice versa. It was not intended to add a unit to such strings.
There is a possible workaround by setting a positiveSuffix and negativeSuffix to
the name of the unit (including a separating space), but these were never intended
to serve this purpose. They shall rather be different for positive and negative numbers
and shall be used in such circumstances where a negative / positive sign is rather
appended than prepended (e. g. in bookkeeping).
When formatting a string to a number with a NumberFormatter with defined suffixes, the User is forced to provide the exact unit string, which is quite easy for “mm”, but with “°C” or “mv/V / kN” things may become tricky for the user trying to figure out the correct suffix.
Solution
UnitFormatter jumps in here. It’s basic feature is, that string-to-number conversion
is much more tolerant. Independently of the suffix entered by the user, only the remainder
of the string will be interpreted as a number. So, in scenarios, where the user enters the
wrong unit. This will simply be ignored by the UnitFormatter and the correct numerical
value should be returned.
In the other direction, where a number is formatted into a string, the unit is just added as a suffix.
Known Issues
Currently, all non-numerical characters (decimal numbers, decimal point characters) are kept for numerical interpretation. This results in the fact that negative numbers can’t be entered. Also all suffixes and prefixes and thousands-separators (e.g ‘)are ignored during formatting.
This shall be fixed by referencing the NumberFormatters behviour. But for the time beeing this is considered being “good enough”.
Use of the UnitFormatter
The UnitFormatter is a subclass of the NumberFormatter and hence, can be used in
every place, where the NumberFormatter is used. Simply set the unit and the
paddingString being used between value and unit and you’re done.