How to pass data from one provider model to another?

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
)

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:

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,
  });
}

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!

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),