flutter - 当我调用距离时，方法“toDouble”在 null 上被调用

Question

我从模型类中获取数据，当我从任何其他选项卡屏幕切换到此屏幕时，需要一两秒才能摆脱此错误：-

    The method 'toDouble' was called on null.
    Receiver: null
    Tried calling: toDouble().

这是什么原因，我该如何摆脱这种情况？这是我显示它的地方，并评论了控制台中提到的链接在单击错误时将我带到的位置旁边的行：-

           Padding(
                    padding: const EdgeInsets.all(15.0),
                    child: Card(
                      color: themeProvider.isDarkMode? black :white,
                      child: Padding(
                        padding: const EdgeInsets.all(8.0),
                        child: ListTile(
                          title: Text(
                            "Maximum distance",
                            style: TextStyle(
                                fontSize: 18,
                                color: mRed,
                                fontWeight: FontWeight.w500),
                          ),
                          trailing: Text(
                            "$distance Km.",
                            style: TextStyle(fontSize: 16),
                          ),
                          subtitle: Slider(
                              value: distance.toDouble(),/// Error takes me to this line.
                              inactiveColor: Colors.blueGrey,
                              min: 1.0,
                              max: 500,
                              activeColor: mRed,
                              onChanged: (val) {
                                changeValues
                                    .addAll({'maximum_distance': val.round()});
                                setState(() {
                                  distance = val.round();
                                });
                              }),
                        ),
                      ),
                    ),
                  ),

这是同一屏幕中的另一个错误：-

     The getter 'start' was called on null.
     Receiver: null
     Tried calling: start


     Padding(
                    padding: const EdgeInsets.all(15.0),
                    child: Card(
                      color: themeProvider.isDarkMode? black :white,
                      child: Padding(
                        padding: const EdgeInsets.all(8.0),
                        child: ListTile(
                          title: Text(
                            "Age range",
                            style: TextStyle(
                                fontSize: 18,
                                color: mRed,
                                fontWeight: FontWeight.w500),
                          ),
                          trailing: Text(
            "${ageRange.start.round()}- // Error take me here ${ageRange.end.round()}",
                            style: TextStyle(fontSize: 16),
                          ),
                          subtitle: RangeSlider(
                              inactiveColor: Colors.blueGrey,
                              values: ageRange,
                              min: 18.0,
                              max: 100.0,
                              divisions: 25,
                              activeColor: mRed,
                              labels: RangeLabels('${ageRange.start.round()}',
                                  '${ageRange.end.round()}'),
                              onChanged: (val) {
                                changeValues.addAll({
                                  'age_range': {
                                    'min': '${val.start.truncate()}',
                                    'max': '${val.end.truncate()}'
                                  }
                                });
                                setState(() {
                                  ageRange = val;
                                });
                              }),
                        ),
                      ),
                    ),
                  ),

标签： flutterdart

Answer 1

如果您distance为 null，您将无法使用它的属性，例如distance.toDouble().

避免错误的一种方法是有条件地分配这样的值，

value: distance != null ? distance.toDouble() : 1.0,

现在，如果有的话distance，null您将1.0改为分配默认值。

Text(ageRange != null ? "${ageRange.start.round()} - ${ageRange.end.round()}" : "invalid",
  style: TextStyle(fontSize: 16),
),