我做了一个提供程序来获取食物类的多个示例的求和值，并在按钮上显示求和值，但由于某种原因，除非我离开并返回页面或热加载，否则它不会显示。
我有一个列表userOrders，其中包含添加到购物车的所有项目，fold函数应该总结价格和数量的乘积，并在按钮上显示它。

• • 食物**

class Food {
  String imgUrl;
  String desc;
  String name;
  String waitTime;
  num score;
  int price;
  int pq;
  int quantity;
  bool favourited;
  List<Map<String, String>> ingredients;
  String about;
  bool highlight;
  Food(this.imgUrl, this.desc, this.name, this.pq, this.waitTime, this.score,
      this.price, this.quantity, this.ingredients, this.about, this.favourited,
      {this.highlight = false});

  int pqProduct(Food food) {
    pq = food.quantity * food.price;
    return pq;
  }

• • 供应商**

class FavFood with ChangeNotifier {
  int peeQee() {
    int pqe =
        userOrders.fold(0, (a, element) => a + element.pqProduct(element));
    return pqe;
  }

  void update() {
    notifyListeners();
  }
}

• • 按钮**

class NextButton extends StatefulWidget {
  @override
  State<NextButton> createState() => _NextButtonState();
}

class _NextButtonState extends State<NextButton> {
  @override
  Widget build(BuildContext context) {
    return userOrders.isNotEmpty
        ? Container(
            color: Colors.transparent,
            padding: const EdgeInsets.fromLTRB(10.0, 10.0, 5.0, 5.0),
            height: 60,
            width: double.infinity,
            child: ElevatedButton(
              style: ButtonStyle(
                shape: MaterialStateProperty.all<RoundedRectangleBorder>(
                  RoundedRectangleBorder(
                    borderRadius: BorderRadius.circular(18.0),
                    side: const BorderSide(color: Colors.transparent),
                  ),
                ),
              ),
              child: Text(
                '${context.read<FavFood>().peeQee}',
                style: const TextStyle(
                  fontWeight: FontWeight.bold,
                  fontSize: 18.0,
                ),
              ),
              onPressed: () {},
            ),
          )
        : const Text('');
  }
}

该按钮应该更新价格时，一个项目被添加，但它没有，直到我重新加载
Food的示例

Food(
        'assets/images/sha-modified.png',
        'Most Recommended',
        'Shawarma',
        0,
        '15 min',
        4.5,
        1500,
        0,
        [
          {
            'Flatbread': 'assets/images/flatbread-modified.png',
          },
          {'Cabbage': 'assets/images/cabbb-modified.png'},
          {'Chicken': 'assets/images/chicken-modified.png'},
          {'Sauce': 'assets/images/sauce-modified.png'},
        ],
        'A sandwich of sliced lamb or chicken, vegetables, and often tahini wrapped in pita bread',
        highlight: true,
        false,
      ),