Flutter List null safety The default 'List' constructor isn't available when null safety is enabled

Question

I try to make flutter null safety migration, I have this code working before and now I have this error on List(10)

The default 'List' constructor isn't available when null safety is enabled. Try using a list literal, 'List.filled' or 'List.generate'

Here is my code

 List<CardModel> createCards() {
        List<String> asset = [];
        List(10).forEach((f) => asset.add('0${(asset.length + 1)}.png'));
        List(10).forEach((f) => asset.add('0${(asset.length - 10 + 1)}.png'));
        return List(20).map((f) {
          int index = Random().nextInt(1000) % asset.length;
          String _image =
              'assets/' + asset[index].substring(asset[index].length - 6);
          asset.removeAt(index);
          return CardModel(
              id: 20 - asset.length - 1, image: _image, key: UniqueKey());
        }).toList();
      }

score 1 · Accepted Answer · answered Jun 12 '22 at 20:35

Try:

 List<CardModel> createCards() {
        List<String> asset = [];
        List.filled(10, null).forEach((f) => asset.add('0${(asset.length + 1)}.png'));
        List.filled(10, null).forEach((f) => asset.add('0${(asset.length - 10 + 1)}.png'));
        return List.filled(10, null).map((f) {
          int index = Random().nextInt(1000) % asset.length;
          String _image =
              'assets/' + asset[index].substring(asset[index].length - 6);
          asset.removeAt(index);
          return CardModel(
              id: 20 - asset.length - 1, image: _image, key: UniqueKey());
        }).cast<CardModel>().toList();
      }

Note that when you ommit the type param List(...) the default is dynamic, so: List<dynamic>() equals to List()

If List(N) create a list of type dynamic of size N with all values equals to null, then after null-safety you can replicate this by using List.filled(10, null).

And use cast<T>() to convert the list type to be able to use it as return value which expects the type CardModel.

Flutter List null safety The default 'List' constructor isn't available when null safety is enabled

1 Answers1