我的无状态小部件触发了一条我无法分配的错误消息

Question

我经常在这里找到非常快速和非常称职的帮助。希望你能再次帮助我。 我有三天的错误消息，我无法解决。 我在一个小视频中向您解释了这个问题。 视频的 link 是：“https://drive.google.com/file/d/1T9uOnEaNp5W6_kcO6eV9o64Fp3sRkB3f/view?usp=sharing”

我真的希望你看到我没有看到的错误！

这是Foodcard的代码，根据Flutter应该会导致错误：

The method '-' was called on null.
Receiver: null
Tried calling: -(30.0)
The relevant error-causing widget was: 
  FoodCard file:///C:/Users/stefa/AndroidStudioProjects/cronum_app_web%20-%20Kopie/lib/Components/ProviderComponents.dart:298:9

美食卡：

class FoodCard extends StatelessWidget {
  FoodCard({
    @required this.description,
    @required this.imagePath,
    @required this.price,
    @required this.foodName,
    @required this.foodItem,
    @required this.increaseCallback,
    @required this.decreaseCallback,
    this.count = 0,
  });

  final double radius = 40;
  static double listViewHeight;
  final double margin = 15;
  final double containerHeight = 130;
  final int count;
  final String imagePath;
  final String foodName;
  final String price;
  final String description;
  final FoodItem foodItem;
  final Function increaseCallback;
  final Function decreaseCallback;

  @override
  Widget build(BuildContext context) {
    return Container(
      margin: EdgeInsets.symmetric(horizontal: 10, vertical: margin),
      width: 220,
      decoration: BoxDecoration(
        borderRadius: BorderRadius.circular(radius),
        gradient: LinearGradient(
          begin: Alignment.centerLeft,
          end: Alignment.topRight,
          colors: ([
            gradientColor1.withOpacity(opacityOfHeader),
            primaryColor.withOpacity(opacityOfHeader),
            gradientColor2.withOpacity(opacityOfHeader),
          ]),
        ),
        boxShadow: [
          BoxShadow(
            blurRadius: 8,
            color: Colors.black.withOpacity(0.3),
            offset: Offset(5, 5),
          ),
        ],
        color: Colors.white,
      ),
      child: Column(
        children: <Widget>[
          Stack(
            alignment: Alignment.center,
            children: <Widget>[
              Column(
                children: <Widget>[
                  SizedBox(
                    height: listViewHeight - margin * 2 - containerHeight,
                  ),
                  Container(
                    height: containerHeight,
                    decoration: BoxDecoration(
                        borderRadius: BorderRadius.only(
                          topLeft: Radius.circular(radius),
                          topRight: Radius.circular(radius),
                          bottomLeft: Radius.circular(radius),
                          bottomRight: Radius.circular(radius),
                        ),
                        boxShadow: [
                          BoxShadow(
                              color: Colors.black.withOpacity(0.2),
                              offset: Offset(0, -5),
                              blurRadius: 8)
                        ],
                        color: Colors.white),
                    child: Column(
                      children: <Widget>[
                        Padding(
                          padding: EdgeInsets.only(top: 40),
                          child: Center(
                            child: Text(
                              foodName,
                              style: TextStyle(
                                color: Colors.black,
                                fontSize: 17,
                                fontWeight: FontWeight.w900,
                              ),
                            ),
                          ),
                        ),
                        Padding(
                          padding: EdgeInsets.symmetric(
                              horizontal: 20, vertical: 15),
                          child: Row(
                            mainAxisAlignment: MainAxisAlignment.spaceBetween,
                            children: <Widget>[
                              InkWell(
                                onTap: () {
                                  Navigator.push(
                                    context,
                                    MaterialPageRoute(
                                      builder: (context) => DetailPage(
                                        foodItem: foodItem,
                                        increaseCount: increaseCallback,
                                        decreaseCount: decreaseCallback,
                                      ),
                                    ),
                                  );
                                },
                                child: Container(
                                  height: 30,
                                  width: 80,
                                  decoration: BoxDecoration(
                                    boxShadow: [
                                      BoxShadow(
                                          offset: Offset(5, 5),
                                          color: Colors.black.withOpacity(0.3),
                                          blurRadius: 8)
                                    ],
                                    color: buttonColor,
                                    borderRadius: BorderRadius.circular(25),
                                  ),
                                  child: Padding(
                                    padding: EdgeInsets.symmetric(
                                        horizontal: 7, vertical: 5),
                                    child: Center(
                                      child: Text(
                                        'Details',
                                        style: TextStyle(
                                          color: Colors.white,
                                          fontSize: 16,
                                          fontWeight: FontWeight.bold,
                                        ),
                                      ),
                                    ),
                                  ),
                                ),
                              ),
                              Container(
                                color: Colors.grey.withOpacity(0.5),
                                height: 25,
                                width: 1,
                              ),
                              Container(
                                height: 30,
                                width: 80,
                                decoration: BoxDecoration(
                                  borderRadius: BorderRadius.circular(25),
                                  color: buttonColor,
                                  boxShadow: [
                                    BoxShadow(
                                      blurRadius: 8,
                                      color: Colors.black.withOpacity(0.3),
                                      offset: Offset(5, 5),
                                    ),
                                  ],
                                ),
                                child: Row(
                                  mainAxisAlignment:
                                      MainAxisAlignment.spaceAround,
                                  children: <Widget>[
                                    InkWell(
                                      onTap: decreaseCallback,
                                      child: Container(
                                        height: 22,
                                        width: 22,
                                        decoration: BoxDecoration(
                                          borderRadius:
                                              BorderRadius.circular(5),
                                          color: buttonColor,
                                        ),
                                        child: Center(
                                          child: Icon(
                                            Icons.remove,
                                            color: Colors.white,
                                            size: 22,
                                          ),
                                        ),
                                      ),
                                    ),
                                    Text(
                                      count.toString(),
                                      style: TextStyle(
                                        color: Colors.white,
                                        fontSize: 16,
                                        fontWeight: FontWeight.w900,
                                      ),
                                    ),
                                    InkWell(
                                      onTap: increaseCallback,
                                      child: Container(
                                        height: 22,
                                        width: 22,
                                        decoration: BoxDecoration(
                                          borderRadius:
                                              BorderRadius.circular(15),
                                          color: Colors.white,
                                        ),
                                        child: Center(
                                          child: Icon(
                                            Icons.add,
                                            color: buttonColor,
                                            size: 22,
                                          ),
                                        ),
                                      ),
                                    ),
                                  ],
                                ),
                              ),
                            ],
                          ),
                        ),
                      ],
                    ),
                  ),
                ],
              ),
              Positioned(
                top: 30,
                child: InkWell(
                  onTap: () {
                    Navigator.push(
                      context,
                      MaterialPageRoute(
                        builder: (context) => DetailPage(
                          foodItem: foodItem,
                          increaseCount: increaseCallback,
                          decreaseCount: decreaseCallback,
                        ),
                      ),
                    );
                  },
                  child: Image(
                    height: listViewHeight / 2,
                    width: listViewHeight / 2,
                    image: AssetImage(imagePath),
                  ),
                ),
              ),
            ],
          )
        ],
      ),
    );
  }

如果不是因为 FoodCard，我创建 Foodcards 列表的功能也可能是问题所在。 我对错误消息感到惊讶，因为它非常不具体。在我自动创建列表之前，一切都很好。

  List<Widget> buildEntdeckenCards() {
    List<Widget> foodCardList = [];
    for (FoodItem foodItem in top10) {
      print(foodItem.foodName);
      foodCardList.add(
        FoodCard(
          description: foodItem.description,
          foodName: foodItem.foodName,
          foodItem: foodItem,
          price: foodItem.price,
          imagePath: foodItem.imagePath,
          decreaseCallback: () {
            decreaseCount(foodItem);
          },
          increaseCallback: () {
            increaseCount(foodItem);
          },
        ),
      );
    }
    return foodCardList;
  }

Answer 1

我认为您的 listViewHeight 变量为空，您正试图在表达式

中使用它

  SizedBox(height: listViewHeight - margin * 2 - containerHeight,),

我认为这就是您看到以下错误的原因 The method '-' was called on null。在使用之前给 listViewHeight 赋值。