问Flutter使用了一个‘`BuildContext`’，它是提供程序的祖先

EN

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:

consider using `builder` like so:

  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(),
                     ),
                   );
                },
              ),
            ),
          );
  }
 }

 @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),
                ),
              ],
            ),
          ),
        ),
      ),
    );

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);
  }