List of Objects In Change Notifier don't update

Question

I have a list of objects that inherit from ChangeNotifier. I have removed the types just to simplify stuff.

class TaskListsState with ChangeNotifier {
  List _lists = List();
  _currentList;

  List get lists => _lists;
  get currentList => _currentList;

  set lists(List newValue) {
    this._lists = newValue;
    notifyListeners();
  }

  set currentList(newValue) {
    this._currentList = newValue;
    notifyListeners();
  }

  TaskListsState() {
    this._lists = [
      ToDoListState(),
      ToCreateListState(),
      ToDecideListState(),
      BalancedMeListState()
    ];

    this._currentList = this._lists[0];
  }
}

All of the objects inherit from a base class but generally, this is how they look

class ToCreateListState with ChangeNotifier implements TaskListState {
  String title = "TO CREATE LIST";
  String alias = "tocreate";
  bool _loading = false;
  List<TaskState> _tasks;

  bool get loading => _loading;
  List<TaskState> get tasks => _tasks;

  set tasks(List<TaskState> newValue) {
    this._tasks = newValue;
    notifyListeners();
  }

  set loading(bool newValue) {
    _loading = newValue;
    notifyListeners();
  }

  ToCreateListState();

  removeTask(TaskState task) {
    this._tasks.remove(task);
    notifyListeners();
  }
}

Now, when I update one of the list objects, they don't update in the UI. How can I fix this, please?

Currently, this is how I am using it in the view:

 MultiProvider(
      providers: [
        ChangeNotifierProvider(
          create: (context) => TaskListsState(),
        ),

      ],
      child: MaterialApp(
        title: '',
        debugShowCheckedModeBanner: false,
        theme: ThemeData(
          primarySwatch: Colors.blue,
          fontFamily: 'Montserrat',
          visualDensity: VisualDensity.adaptivePlatformDensity,
        ),
        initialRoute: defaultHome,
        onGenerateRoute: onGenerateRoute,
      ),
    );

Then when I want to use one of the items in my screen I would do Provider.of<TaskListsState>(context).lists[index].whateverValue =. anotherValue;

However, it doesn't update.

Gabber235 · Accepted Answer · 2020-06-06T08:17:37.040

2

When you run the Provider.of<TaskListsState>(context).lists[index].whateverValue =. anotherValue; it won't call notifyListeners() because it doesn't update the lists itself.

You can simply create a method inside the TaskListsState that does the modification for you and afterwards calls the notifyListeners()

class TaskListsState with ChangeNotifier {
...
    void updateWhateverValue(int index, dynamic value) {
        lists[index].whateverValue = value;
        notifyListeners();
    }
...
}

You should replace the dynamic with the object you have.

edited Jun 06 '20 at 08:17

answered Jun 06 '20 at 08:16

Gabber235

168
2
8

Ok, I will try this now – NduJay Jun 06 '20 at 08:16
But what if I have deeply nested objects, this wouldn't be ideal....What would I do then? – NduJay Jun 06 '20 at 08:20
Or better still, if I have multiple methods to implement (my case). What would I do then? – NduJay Jun 06 '20 at 08:21
1

You can make those objects also `ChangeNotifier` and provide them where they are needed, probably in a list of some sorts. The flutter team had an awsome talk explaining this: https://youtu.be/d_m5csmrf7I?t=1456 – Gabber235 Jun 06 '20 at 08:23
I will check it out now. Thanks – NduJay Jun 06 '20 at 08:24
hello i tried this but my Consumer returns Emprty lists every time. help. – Sachin Kumar Rajput Apr 28 '21 at 11:50
https://stackoverflow.com/questions/67282912/unable-to-create-getter-in-changenotifier-class-error-when-using-provider-in-fl i posted my question here please check. – Sachin Kumar Rajput Apr 28 '21 at 12:10
can you please share your code step for doing this. – Sachin Kumar Rajput Apr 28 '21 at 13:11

score 0 · Answer 2 · answered Jun 06 '20 at 10:02

Here is another explanation: Imagine we have this class

class Products with ChangeNotifier
{

List<String> prods =new List() :

List<String> getProds => prods;

Void addprod() 
{
prods.add('chocolate');
prods.add('beer');
notifyListeners();
} 

Products()
{
addprod();
}


}

So we have a class products which has already two products.

Case of removing a product, if we call

Provider.of<Products>(context)().products.remove('beer')

Beer will get removed but all other Consumers will not get Notified, since remove is a predefined function and doesn't exist on Products class as a Notifier.

Make sure everytime you are calling a function, it's Actually updating something in your data model class, so you can notice changes.

Hope that clarify your vision.

This doesn't work for nested objects. Thanks for trying though but the youtube video provided in the first answer worked for me — NduJay, Jun 06 '20 at 10:17

List of Objects In Change Notifier don't update

2 Answers2