groupValues method
Implementation
List<String> groupValues(String Function(UTrackRecord record) selector) {
final Set<String> values = <String>{};
for (final UTrackRecord record in _tracks) {
final String value = selector(record);
if (value.isNotEmpty) values.add(value);
}
final List<String> list = values.toList()..sort();
return list;
}