Question 231263 in Flutter Question posted in
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Flutter – ProviderNotFound Exception when using with Generics & Abstraction

I’m facing this very weird and primitive issue, I’m trying to figure it out but not able to.

I’ve designed an abstract class in following manner

abstract class BaseStatelessWidget<T extends BaseStore> extends StatelessWidget
    with RouteAware {
  late final T childStore;

  BaseStatelessWidget({super.key});

  @override
  Widget build(BuildContext context) {
    childStore = getImplementedStore();
    return Provider(
      create: <T>(_) => childStore,
      builder: (ctx, __) {
        final store = Provider.of<T>(ctx, listen: false); // <== I'm getting error here for providerNotFound excepton
        store;
        return _BaseStatefulWidget<T>(
          builder: buildScreen,
        );
      },
    );
  }

  Widget buildScreen(BuildContext context);

  T getImplementedStore();
}

This is the way I’ve implemented the Above abstract class,

class Counter extends BaseStatelessWidget<CounterStore> {
  Counter({super.key});

  @override
  Widget buildScreen(BuildContext context) {
    return Scaffold(
      body: Container(
        color: AppColors.white,
        child: Column(
          children: [
            Observer(
              builder: (ctx) {
                return Text(
                  '${getStore<CounterStore>(ctx).counter} Times Clicked',
                );
              },
            )
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          getStore<CounterStore>(context).increment();
        },
        child: const Icon(Icons.add),
      ),
    );
  }

  @override
  CounterStore getImplementedStore() {
    return getIt<CounterStore>();
  }
}

T getStore<T extends BaseStore>(BuildContext context) {
  return Provider.of<T>(context, listen: false);
}

As soon as I call the Provider.of method to get the injected object, I’m getting the exception.
Can someone point out, if I’m missing anything here??

Tags:

2

Answers


  1. Chosen as BEST ANSWER
    0

    After thorough research, I've found a solution that could be helpful to others encountering a similar issue.

    The provider package offers two methods to create a provider:

    • Using the Provider(create: (context) => TExtends())
    • Utilizing Provider.value(value: TExtends()).

    It's important to note that the first approach lacks abstraction support. Attempting to access T directly from the context will result in an exception. This is because the provided object will represent the implementation of T, rather than T itself.

    If your objective is to obtain an instance of T instead of TExtends, I recommend opting for the second method. This will allow you to resolve the issue.

    Here's an example implementation:

    abstract class BaseStatelessWidget<T extends BaseStore> extends StatelessWidget
    with RouteAware {
  late final T childStore;

  BaseStatelessWidget({super.key});

  @override
  Widget build(BuildContext context) {
    childStore = getImplementedStore();
    return Provider<T>.value( // <== This will solve the issue
      value: childStore,
      builder: (ctx, __) {
        final store = Provider.of<T>(ctx, listen: false); // You'll get instance of T, instead of childStore's instance
        store;
        return _BaseStatefulWidget<T>(
          builder: buildScreen,
        );
      },
    );
  }

  Widget buildScreen(BuildContext context);

  T getImplementedStore();
}

  2. 0

    When declaring the provider, drop the return type in the create function:

        abstract class BaseStatelessWidget<T extends BaseStore> extends StatelessWidget with RouteAware {
  late final T childStore;

  BaseStatelessWidget({super.key});

  @override
  Widget build(BuildContext context) {
    childStore = getImplementedStore();
    return Provider(
      create: (_) => childStore,//Drop the return type
      builder: (ctx, __) {
        final store = Provider.of<T>(ctx, listen: false);
        return Text(store.value());
      },
    );
  }

  T getImplementedStore();
}

    Tried the same and was able to get the provider’s value as expected.

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