List.contains not working with initialized List

Question

• I have a fixed List i.e fixGuestList that is being initialized in init(). On a button tap, a list of Guests appears. On tapping the guest, the selected guest is being added to the fixed list i.e fixGuestList. So basically I'm adding elements from one list to the fixed list.

• If the fixGuestList already contains the element present in the guestList. I want to pop up a snackBar() guest already added else add the element to fix list.

• It is only working when the list is not been initialized. If I persist the list and on init() I assign the elements to the fixed list. And try to add duplicate guests the snackbar doesn't pop up it adds the duplicate guest to the list. For eg; I added guest 'A' from the guestList. And then I restarted the app and then again add the guest 'A' the guest 'A' is added even tho the fixedGuestList contains 'A' (I checked it in print statement 'A' exists) If I add 'A' and do not restart and try again try to add 'A' the snackbar pops up.

• Getx is being used as StateManagement and GetStorage is used for Persisit data. The values of both list are maps.

Code are as follow:

  class GuestController extends GetxController {

  final _userDataController = Get.find<UserDataController>();


  List fixGuestList = List.filled(3, null, growable: false);
  List? guestList = _userDataController.userDataModel.value.totalGuest! // This list is filled via API

  // returns and empty index for fixGuestList
  int _returnIndex() {
    if (fixGuestList[0] == null) {
      return 0;
    } else if (fixGuestList[1] == null) {
      return 1;
    } else if (fixGuestList[2] == null) {
      return 2;
    }
    return 3;
  }

  // Add function
  void addToLane({required int index, required BuildContext context}) {
    if (fixGuestList.contains(guestList![index])) {
      ScaffoldMessenger.of(context).showSnackBar(_snackBar);
    } else {
      fixGuestList[_returnIndex()] = guestList![index];
      storageBox.write('FixGuestList', fixGuestList);
    }
    update();
  }

  //
  void onInitAssign() async {
    final _boxValue = storageBox.read('FixGuestList');

    if (_boxValue == null) {
      List fixGuestList = List.filled(3, null, growable: false);
    } else {
      fixGuestList = _boxValue;
    }
    update();
  }

  // OnInit Function
  @override
  void onInit() {
    onInitAssign();
    super.onInit();
  }
}

API Class:

class FirbaseAPI extends GetxController {
  
  
  
  @override
  void onInit() {
   userDataModel.bindStream(stream());
    super.onInit();
  }
  
   // Stream User Model
  Rx<UserDataModel> userDataModel = UserDataModel().obs;

  // Stream
  Stream<UserDataModel> stream() {
    return FirebaseFirestore.instance
        .collection('users')
        .doc('userA')
        .snapshots()
        .map((ds) {
      var mapData = ds.data();

      UserDataModel extractedModel = UserDataModel.fromMap(mapData);
      return extractedModel;
    });
  }
}

Model Class:

class UserDataModel {
  String? clubName;
  List? totalGuest;

  UserDataModel({
    this.clubName,
    this.totalGuest,
  });

  factory UserDataModel.fromMap(dynamic fieldData) {
    return UserDataModel(
      totalGuest: fieldData['totalGuest'],
    );
  }
}

Answer 1

contains uses operator equals (see here), commonly known as ==, to determine if something is equal.

To quote the documentation linked above:

The default behavior for all Objects is to return true if and only if this and other are the same object.

Override this method to specify a different equality relation on a class.

So whatever class it is you have there, I suggest you override the == operator, to make sure that two different instances of the same data still match. Otherwise you will see what you see now: only actual identical instances match.