public TextSpinner<T> setDisplayMapperPure(@Nullable final Func1<T, String> displayMapper) {
        return setDisplayMapper(displayMapper == null ? null : v -> TextParam.text(displayMapper.call(v)));
    }
for textview: returns display value currently used for showing (note: used for unit testing)
main/src/main/java/cgeo/geocaching/ui/TextSpinner.java