未定义函数“notifyListeners”。尝试导入定义“notifyListeners”的库，答案

【问题标题】：The function 'notifyListeners' isn't defined. Try importing the library that defines 'notifyListeners',未定义函数“notifyListeners”。尝试导入定义“notifyListeners”的库，
【发布时间】：2021-12-25 05:00:44
【问题描述】：

由于某种原因，虽然我已经导入了库，但我收到了这个错误。我试过 flutter clean 但没有任何效果。

    import 'package:flutter/cupertino.dart';
import 'package:provider/provider.dart';

import 'package:flutter/foundation.dart';
import 'dart:convert';

class Product with ChangeNotifier {
  final String id;
  final String title;
  final String description;
  final double price;
  final String imageUrl;
  bool isFavourite; //This is not final as it will change again again

  Product(
      {required this.description,
      required this.id,
      required this.imageUrl,
      this.isFavourite = false,
      required this.price,
      required this.title});
}

void toggleFavouriteStatus(bool isFavourite) {
  isFavourite = !isFavourite;
  notifyListeners();
}

【问题讨论】：

标签： flutter flutter-layout flutter-dependencies flutter-web

【解决方案1】：

您将方法放在类的外面，而它应该放在里面：

import 'package:flutter/cupertino.dart';
import 'package:provider/provider.dart';
import 'package:flutter/foundation.dart';
import 'dart:convert';

class Product with ChangeNotifier {
  // ...

  void toggleFavouriteStatus(bool isFavourite) {
    isFavourite = !isFavourite; 
    notifyListeners();
  }
}

此外，您可能不需要此方法中的参数，因为它只是切换值：

  void toggleFavouriteStatus() {
    isFavourite = !isFavourite; 
    notifyListeners();
  }

【讨论】：