Dart/Flutter: Strings of element inside a List becomes empty when passing as an argument (Why??)

Question

Strings of element inside a List becomes empty when passing as an argument. It was working before. I don't know what happened that it stopped working, and started passing empty.

I have a model called SubjectiveList, it is the list I am talking about.

class SubjectiveList {
  String id;
  String name;
  List<Item> items;
  SubjectiveList({this.id, this.name, this.items});
}

This list has the property items. What becomes empty is the properties inside the Item object.

class Item {
  String id;
  String name;
  Content content;

  Item({this.id, this.name, this.content});
}

On the debugger, The newList instance appears fine, with the object names (ps: the ID is okay to be null at this point because it will come from Firestore Database later)

Here is the code with the screenshots:

Future<dynamic> showListInfoDialog() {
    final userData = Provider.of<UserData>(context, listen: false);
    GlobalKey<FormState> _addListInfoFormKey = GlobalKey<FormState>();
    final ValueNotifier<int> tabIndex =
        Provider.of<ValueNotifier<int>>(context, listen: false);
    TempListViewModel tempList =
        Provider.of<TempListViewModel>(context, listen: false);

    return showDialog(
      context: context,
      child: SimpleDialog(
        title: Text("List Info"),
        children: <Widget>[
          Padding(
            padding: const EdgeInsets.all(defaultSpacing),
            child: Form(
              key: _addListInfoFormKey,
              child: Column(
                children: <Widget>[
                  TextFormField(
                    onChanged: (val) => tempList.setListName(val),
                    validator: (val) => val.isEmpty ? 'Write a name' : null,
                    decoration: InputDecoration(
                      prefixIcon: Icon(Icons.featured_play_list),
                      labelText: "List Name",
                    ),
                  ),
                  SizedBox(height: defaultSpacing),
                  SizedBox(
                    width: double.infinity,
                    child: RaisedButton(
                      child: Text("Create List"),
                      color: successColor,
                      onPressed: () {
                        if (_addListInfoFormKey.currentState.validate()) {
                          final newList = SubjectiveList(
                              name: tempList.list.name,
                              items: tempList.list.items);
                          DatabaseService(uid: userData.uid)
                              .addListToDatabase(newList); // <-- HERE
                          tempList.init();
                          tabIndex.value = 0;
                          Navigator.of(context).pop();
                        }
                      },
                    ),
                  )
                ],
              ),
            ),
          ),
        ],
      ),
    );
  }

And then it appears empty when coming to the function!!

Future addListToDatabase(SubjectiveList list) async { <-- HERE
    DocumentReference listDocument =
        await userDocument.collection('lists').add({'name': list.name});
    [...]
  }

Try using await >> await DatabaseService(uid: userData.uid) .addListToDatabase(newList); — edenar, Jun 08 '20 at 06:29
It worked with async await! Thank you so much! Now I understand what happened. In Flutter the line "final newList = SubjectiveList(name: tempList.list.name, items: tempList.list.items);" makes a pointer reference, and not an declaration of the current value. So, when it goes to the next line and executes tempList.init() it is clearing the list before getting the argument in the function. What I really don't understand is that it was working before... I just worked changing the design in the past 2 days, and somehow it changed there. Probably I've changed the line order but I don't remember. — Hyung Tae Carapeto Figur, Jun 08 '20 at 14:39
Same happens to me. When the `list.clear()` is called, the list that you pass also cleared. Thank you for that tip @HyungTaeCarapetoFigur — AJ Seraspi, May 11 '21 at 12:14
You can also check the answer here https://stackoverflow.com/questions/58389591/how-to-copy-list-values-to-another-list-in-flutter — AJ Seraspi, May 11 '21 at 12:35

score 1 · Accepted Answer · answered Jun 08 '20 at 14:51

Thanks @edenar Now I understand what happened. In Flutter the line "final newList = SubjectiveList(name: tempList.list.name, items: tempList.list.items);" makes a pointer reference, and not an declaration of the current value. So, when it goes to the next line and executes tempList.init() it is clearing the list before getting the argument in the function.

So it worked putting await in that line.

Dart/Flutter: Strings of element inside a List becomes empty when passing as an argument (Why??)

1 Answers1