使用 StreamProvider 的颤振提供者 MultiProvider

Question

I am developing a Flutter app using Provider (provider: 3.0.0+1). I am using MultiProvider using StreamProvider with controller. But I am always getting an error.

Below is my code

main.dart

Widget build(BuildContext context) {
return MultiProvider(
  providers: [
    StreamProvider<RecipeStreamService>.value(value: RecipeStreamService().controllerOut)
  ],
  child: MaterialApp(
        debugShowCheckedModeBanner: false,
        title: 'Home Food',
        routes: {
          '/register': (BuildContext context) => RegisterPage(),
          '/login': (BuildContext context) => LoginPage()
        },
        theme: ThemeData(
          primaryColor: Colors.lightBlue[300],
          accentColor: Colors.green[300],
          textTheme: TextTheme(
            headline: TextStyle(fontSize: 42.0, fontWeight: FontWeight.bold, color: Colors.black54),
            title: TextStyle(fontSize: 22.0, fontStyle: FontStyle.italic),
            body1: TextStyle(fontSize: 18.0),
            button: TextStyle(fontSize: 20.0,fontWeight: FontWeight.normal, color: Colors.white)
          )
        ),
        home: HomePage(title: 'Home'),
      ),
  );
}

RecipeStreamService.dart

class RecipeStreamService {
   List<Recipe> _recipes = <Recipe>[];

   final _controller = StreamController<List<Recipe>>.broadcast();
   get controllerOut => _controller.stream;
   get controllerIn => _controller.sink;

   RecipeStreamService() {
      getRecipes();
   }

   addNewRecipe(Recipe recipe) {
     _recipes.add(recipe);
    controllerIn.add(_recipes);
  }

  getRecipes() async{
     List<Map<String, dynamic>> result = await ApiService().getRecipes();
     List<Recipe> data = result.map((data) => Recipe.fromMap(data)).toList();
     data.map((f) => addNewRecipe(f));
 }

 void dispose() {
   _controller.close();
 }
}

But I Am always getting this error:

type '_BroadcastStream<List<Recipe>>' is not a subtype of type 'Stream<RecipeStreamService>'
I/flutter (16880): When the exception was thrown, this was the stack:
I/flutter (16880): #0      MyApp.build (package:app_recipe/main.dart:20:80)

Line: 20:80 in main.dart is (RecipeStreamService().controllerOut)

****** UPDATE ******

Changed Multiprovider to below code

 providers: [
    StreamProvider<List<Recipe>>.value(value: RecipeStreamService().controllerOut)
  ],

Also in HomePage.dart, where I use it, I have

final recipeService = Provider.of<List<Recipe>>(context);

Now, recipeService is always coming as null

Thanks

Answer 1

The reason you are seeing your error is because the type parameter you are passing to StreamProvider<T> as T is not matching what you are returning to create .

In this case, you want to stream values of type List<Recipe> .
Therefore, you should also pass that for the generic type:

StreamProvider<List<Recipe>>.value(value: stream)

Where the stream is your List<Recipe> stream.

---

If Provider.of<List<Recipe>>(context) is returning null for you, it means that you have not added a value to the stream.

If you want to have something other than null before you stream emits a value, you can pass initialValue :

StreamProvider<List<Recipe>>.value(
  value: stream,
  initialValue: initialRecipes,
)

Answer 2

according to the answer of this question StreamProvider not updating List

RecipeStreamService() will start a new instance,

but i still can't make the streamprovider work well