TypeConverter error.

[mytc]

I have a simple list of string that want to save into a table column, here is the converter I wrote, but it has many errors on the override method. (Please read the comment from the codes)

class ListConverter extends TypeConverter<List<String>, String>{
  const ListConverter();
@override
  List<String> mapToDart(String fromDb) { 

//'ListConverter.mapToDart' ('List<String> Function(String)') isn't a valid override of 'TypeConverter.mapToDart' ('List<String>? Function(String?)')

    if (fromDb == '') {

//A value of type 'Null' can't be returned from the method 'mapToDart' because it has a return type of 'List<String>'

      return null;
    }
    return json.decode(fromDb);
  }

  @override
  String mapToSql(List<String> value) {

//'ListConverter.mapToSql' ('String Function(List<String>)') isn't a valid override of 'TypeConverter.mapToSql' ('String? Function(List<String>?)')

    if (value.length == 0) {
      return '';
    }

    return json.encode(value);
  }
}

class Todos extends Table{
TextColumn get tags => text().map(const ListConverter()).nullable()();
 TextColumn get categories => text().map(const ListConverter()).nullable()();
}

I don't understand why the invalid override means. I can't find the documentation that explain this.
And why return null also become an error, but the example in here suggested this.

Since this is a simple string list, do i need a @j.JsonSerializable() class for this? And yes, how to do it?

[simolus3]

Thank you for starting this discussion. The example from the docs has not been migrated to null safety, which I think is causing the confusion here.

Something like this should work, the docs should update soon

class ListConverter extends TypeConverter<List<String>, String>{
  const ListConverter();
  @override
  List<String>? mapToDart(String? fromDb) { 
    if (fromDb == null || fromDb == '') {
      return null;
    }
    return json.decode(fromDb);
  }

  @override
  String? mapToSql(List<String>? value) {
    if (value == null || value.length == 0) {
      return null;
    }

    return json.encode(value);
  }
}

[mytc]

yes, the errors gone now.