How to correctly extend a custom Provider in Flutter

Question

I have a parent ChangeNotifier ParentProvider, and 2 other providers extend the ParentProvider;

ChildProviderA, ChildProviderB.

I have a route which is called MyScreen

class MyScreen extends StatelessWidget {
  @override
  Widget build(BuildContext context) {

    if(ModalRoute.of(context).settings.arguments != null){
      return ChangeNotifierProvider(
        create: (context) => ChildProviderA(),
        child: _Screen()
      );
    }
    return ChangeNotifierProvider(
      create: (context) => ChildProviderB(),
      child: _Screen()
    );
  }
}

depending on the arguments I want to use a provider, and in either case, I want to call a function called initData() in the ParentProvider from the _Screen widget.

@override
  Widget build(BuildContext context) {
...
    Provider.of<ParentProvider>(context, listen: false).initData();
...
}

This gives me an error

Error: Could not find the correct Provider<ParentProvider> above this _Screen Widget

If I used Provider.of<ChildProviderA>(context, listen: false).initData();

or Provider.of<ChildProviderB>(context, listen: false).initData(); It works, but I want in either child Provider used, to be able to call initData.

How can this be done ?

Solution

Provider.of looks for provider with exactly given type. It doesn't include child types.

If the child screen needs to access some different data having same format, you can use the same provider in both cases passing it different data through constructor:

class MyScreen extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    if (condition) {
      return ChangeNotifierProvider(
        create: (context) => ParentProvider(data1),
        child: _Screen(),
      );
    }
    return ChangeNotifierProvider(
      create: (context) => ParentProvider(data2),
      child: _Screen(),
    );
  }
}

If you really have two different ChangeNotifier implementations sharing same interface, you can provide and access them using parent type:

class MyScreen extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    if (condition) {
      return ChangeNotifierProvider<ParentProvider>(
        create: (context) => ChildProviderA(),
        child: _Screen(),
      );
    }
    return ChangeNotifierProvider<ParentProvider>(
      create: (context) => ChildProviderB(),
      child: _Screen(),
    );
  }
}

Comments

[Ismaeel Sherif]

this is what I have done, but it doesn't work

[Huthaifa Muayyad]

You need to put all of them above your widget.

[Ismaeel Sherif]

The second example works. It still needed to declare an empty method in the ParentProvider to be able to run it from ChildA, but anyway it saved a lot of code

[Ismaeel Sherif]

do you mean I should use the 3 providers in the route ?

[Pavel]

To avoid empty methods in parent change notifier, you can make the parent class abstract and declare abstract method

[Pavel]

@HuthaifaMuayyad Provider.of uses getElementForInheritedWidgetOfExactType which doesn't look for subtypes, only exact given type

[Ismaeel Sherif]

@Huthaifa I think I understood your solution, but it will contain a lot of extra functions in both child classes which just points to parent functions