How to correctly extend a custom Provider in Flutter

Issue

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(),
    );
  }
}

Answered By – Pavel

Answer Checked By – David Goodson (FlutterFixes Volunteer)