Flutter StreamProvider 使用了作為提供者祖先的`BuildContext`

Question

我正在開發 Flutter 中的應用程序（我還是有點新），我遇到了以下錯誤：

Error: Could not find the correct Provider<List<Category>> above this Exercises Widget

This likely happens because you used a `BuildContext` that does not include the provider
of your choice. There are a few common scenarios:

- The provider you are trying to read is in a different route.

  Providers are "scoped". So if you insert of provider inside a route, then
  other routes will not be able to access that provider.

- You used a `BuildContext` that is an ancestor of the provider you are trying to read.

  Make sure that Exercises is under your MultiProvider/Provider<List<Category>>.
  This usually happen when you are creating a provider and trying to read it immediately.

  For example, instead of:

  ```
  Widget build(BuildContext context) {
    return Provider<Example>(
      create: (_) => Example(),
      // Will throw a ProviderNotFoundError, because `context` is associated
      // to the widget that is the parent of `Provider<Example>`
      child: Text(context.watch<Example>()),
    ),
  }
  ```

  consider using `builder` like so:

  ```
  Widget build(BuildContext context) {
    return Provider<Example>(
      create: (_) => Example(),
  // we use `builder` to obtain a new `BuildContext` that has access to the provider
  builder: (context) {
    // No longer throws
    return Text(context.watch<Example>()),
  }
),

我一直在網上看，這顯然與我在練習_add.dart 中調用 Provider.of<List<Category>>(context) 時無法獲得正確的“上下文”有關，我不太明白為什么。 因為您可以在我的練習中看到打開我的 ExerciseAdd() 頁面，它會拋出該錯誤。

我真的很感激關於如何修復我的代碼以及為什么它不起作用的解決方案（+解釋）。

練習.dart

  Widget build(BuildContext context) {
    return _isLoading
        ? Loading()
        : MultiProvider(
            providers: [
              StreamProvider<List<Exercise>>(
                create: (context) => DatabaseService().exercises,
              ),
              StreamProvider<List<Category>>(
                create: (context) => DatabaseService().categories,
              ),
              ChangeNotifierProvider<ExerciseFilter>(
                create: (context) => ExerciseFilter(isActive: true),
              )
            ],
            child: Scaffold(
              appBar: AppBar(
                title: Text('Exercises'),
                elevation: 0.0,
                actions: _buildActions(),
              ),
              body: ExerciseList(),
              floatingActionButton: FloatingActionButton(
                backgroundColor: Colors.black,
                child: Icon(Icons.add, color: Colors.white),
                onPressed: () {
                  Navigator.push(
                     context,
                     MaterialPageRoute(
                       builder: (context) => ExercisesAdd(),
                     ),
                   );
                },
              ),
            ),
          );
  }
 }

練習_add.dart

 @override
  Widget build(BuildContext context) {
    final cats = Provider.of<List<Category>>(context);
    print(cats.length);

    return Scaffold(
      appBar: AppBar(
        title: Text('Add Exercise'),
        elevation: 0.0,
      ),
      body: SingleChildScrollView(
        child: Container(
          padding: EdgeInsets.symmetric(vertical: 20.0, horizontal: 50.0),
          child: Form(
            key: _formKey,
            child: Column(
              children: <Widget>[
                SizedBox(height: 20.0),
                TextFormField(
                  decoration:
                      textInputDecoration.copyWith(hintText: 'Exercise Name'),
                  validator: (value) {
                    if (value.isEmpty) {
                      return 'Exercise name is required';
                    }

                    return null;
                  },
                  onChanged: (value) {
                    setState(() {
                      exerciseName = value;
                    });
                  },
                ),
                SizedBox(height: 20.0),
                Theme(
                  data: Theme.of(context).copyWith(canvasColor: Colors.white),
                  child: DropdownButtonFormField<String>(
                    decoration: dropdownDecoration,
                    value: exerciseCategory,
                    onChanged: (value) {
                      setState(() {
                        exerciseCategory = value;
                      });
                    },
                    items: categories.map<DropdownMenuItem<String>>((value) {
                      return DropdownMenuItem<String>(
                        value: value.name,
                        child: Text(value.name),
                      );
                    }).toList(),
                  ),
                ),
                SizedBox(height: 20.0),
                RaisedButton(
                  elevation: 0,
                  color: Colors.black,
                  child: Text(
                    'Add Exercise',
                    style: TextStyle(color: Colors.white),
                  ),
                  onPressed: () async {
                    if (_formKey.currentState.validate()) {
                      bool failed = false;
                      String uid = await _auth.getCurrentUser();

                      if (uid != null) {
                        dynamic result = DatabaseService(uid: uid)
                            .addExercise(exerciseName, exerciseCategory);
                        if (result != null) {
                          Navigator.pop(context);
                        } else {
                          failed = true;
                        }
                      } else {
                        failed = true;
                      }

                      if (failed) {
                        setState(() {
                          error = 'Failed to add exercise. Please try again';
                        });
                      }
                    }
                  },
                ),
                SizedBox(height: 12.0),
                Text(
                  error,
                  style: TextStyle(color: Colors.red, fontSize: 14.0),
                ),
              ],
            ),
          ),
        ),
      ),
    );

DatabaseService().exercises

List<Category> _categoryListFromSnapshot(QuerySnapshot snapshot) {
    return snapshot.documents.map((doc) {
      return Category(name: doc.data['category'] ?? '');
    }).toList();
  }

  Stream<List<Category>> get categories {
    return categoryCollection
        .orderBy('category')
        .snapshots()
        .map(_categoryListFromSnapshot);
  }

注意：StreamProvider 和 MultiProvider 等都是“提供者”package 的一部分（我使用的是最新版本）

Answer 1

您收到的錯誤描述了您當前所處的場景。

- 您嘗試讀取的提供程序位於不同的路徑中。

提供者是“范圍的”。 因此，如果您在路由中插入提供程序，那么其他路由將無法訪問該提供程序。

您正在導航到不同的路線並嘗試訪問提供程序，但它不再位於小部件樹中。

您只需將MultiProvider移動到您在小部件樹中使用的任何導航器上方。 您可能正在使用MaterialApp來執行此操作，因此請移動MultiProvider並將MaterialApp設為child 。