How to pass data from one provider model to another?

Question

I want use provider (ChangeNotifierProvider) and ChangeNotifier for manage app state. But how I can access state from one model in another model?

Use case: In chat app, one model for store user information. Other model use user information (for example user id) to make call to database (Firestore) and get stream of chat data.

For example:

class Model1 extends ChangeNotifier {
  final List<Item> items = [];

class Model2 extends ChangeNotifier {
//Access items from Model1 here
items;

Is this possible? I not like have very big Model because is hard to maintain.



Thanks!

Answer 1

Using provider, one model doesn't access another model.

Instead, you should use ProxyProvider, to combine a model from others.

Your models would look like:

class Foo with ChangeNotifier {
  int count = 0;

  void increment() {
    count++;
    notifyListeners();
  }
}

class Bar with ChangeNotifier {
  int _count;
  int get count => _count;
  set count(int value) {
    if (value != count) {
      _count = value;
      notifyListeners();
    }
  }
}

And then you can use ChangeNotifierProxyProvider this way (assuming there's a `ChangeNotifierProvider higher in your widget tree):

ChangeNotifierProxyProvider<Foo, Bar>(
  initialBuilder: (_) => Bar(),
  builder: (_, foo, bar) => bar
    ..count = foo.count, // Don't pass `Foo` directly but `foo.count` instead
)

Answer 2

From v4 the ChangeNotifierProxyProvider the provider can be constructed like so:

ChangeNotifierProxyProvider<Foo, Bar>(
  create: (_) => Bar(),
  update: (_, foo, bar) => bar
    ..count = foo.count,
)

Notice that initialBuilder: has changed to create: and builder: to update:

Answer 3

I have tried to implement this simply.

Provider version ^4.3.3

Here i given a simple example

main.dart

ChangeNotifierProxyProvider<AuthProvider, ProductsProvider>(
       create: (context) => ProductsProvider(),
       update: (context, auth, previousProducts) => previousProducts
            ..update(auth.token, previousProducts.items == null ? [] : previousProducts.items),
),

authProvider.dart

class AuthProvider with ChangeNotifier {
  String _token;
  // _token is a response token.
  String get token {
    return _token;
}

productsProvider.dart

Product is a class of single product

class ProductsProvider with ChangeNotifier {
  List<Product> _items = [];
  String _authToken;
  void update(authToken, items) {
    _items = items;
    _authToken = authToken;
    notifyListeners();
  }
}

Product.dart

class Product with ChangeNotifier {
  final String id;
  final String title;
  final String description;
  final double price;
  final String imageUrl;
  bool isFavorite;

  Product({
    @required this.id,
    @required this.title,
    @required this.description,
    @required this.price,
    @required this.imageUrl,
    this.isFavorite = false,
  });
}

Answer 4

I'm trying to implement the same scenario, here is my code: Main file:

        ChangeNotifierProvider<SharedPreferencesData>(
      create: (context) => SharedPreferencesData(),
    ),
    ChangeNotifierProxyProvider<SharedPreferencesData, DatabaseService>(
      create: (_) => DatabaseService(),
      update: (_, sharedPreferencesData, databaseService) {
        print('ChangeNotifierProxyProvider RAN');
        databaseService..setSpCompanyId = sharedPreferencesData.sPCompanyId;
        return null;
      },
    ),

SharedPreferencesData File:

    class SharedPreferencesData with ChangeNotifier {
    String sPCompanyId = '';
    void setCompanyId(String cpID) {
     sPCompanyId = cpID;
     print('setCompanyId sPCompanyId:$sPCompanyId');
     notifyListeners();
    }
  }

DataBaseService file:

class DatabaseService with ChangeNotifier {
  String sPCompanyId = 'empty';
  String get getSpCompanyId => sPCompanyId;

  set setSpCompanyId(String value) {
    print('value in setter database before if: $value');
    if (value != getSpCompanyId) {
      print('value in setter database: $value');
      sPCompanyId = value;
      notifyListeners();
    }
  }

The DatabaseService Class doesn't update. I've added the prints to figure out what is running and what is not. When I run my code, the prints in Shared_Preferences / setCompanyId method RUNS correctly. Yet the print function in main.dart file print('ChangeNotifierProxyProvider RAN'); AND in DataBaseService file print('value in setter database: $value'); does not run.

What am I missing? Why the notify sPCompanyId setter notifyListeners() in SharedPreferencesData is not updating DatabaseService String sPCompanyId through the ChangeNotifierProxyProvider? Thanks!

Answer 5

I did Something like this. We are passing "items".

>class Model1 extends ChangeNotifier {
  final List<Item> items = [];
}

>class Model2 extends ChangeNotifier {
//Access items from Model1 here
 Model1 items;
Model2(this.items);
// Now I can access items in the entire class. 
}

>ChangeNotifierProxyProvider<Model1, Model2>(
//create the class to pass the value, which is M2. 
  create: (_) => Model2(Provider.of<Model1>(context)),
  update: (_, model1, model2) => model2..items
)



>Model2 model2 = Provider.of<Model2>(context, listen: false);
>>Text(model2.items),