空值检查运算符用于 flutter 的空值

Question

所以我正在将一个项目切换到 flutter null safety 但我得到了 Null check operator used on a null value error 我尝试查看其他类似的尝试过但它对我不起作用完整的错误是如下

The following _CastError was thrown building HomeBanner(dirty, dependencies: [MediaQuery, _InheritedProviderScope<HomeViewModel>], state: _HomeBannerState#2de56):
Null check operator used on a null value

The relevant error-causing widget was: 
HomeBanner file:///D:/fabricoMAt/lib/ui/views/home_screen.dart:26:17
When the exception was thrown, this was the stack: 
#0      HomeViewModel.initData (package:knoty/business_logic/view_models/home_viewmodel.dart:25:33)
#1      _HomeBannerState._getImageSliders (package:knoty/ui/widgets/home_banner.dart:30:18)
#2      _HomeBannerState.build (package:knoty/ui/widgets/home_banner.dart:20:23)
#3      StatefulElement.build (package:flutter/src/widgets/framework.dart:4612:27)
#4      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4495:15)
...
====================================================================================================

======== Exception caught by widgets library =======================================================
Null check operator used on a null value
The relevant error-causing widget was: 
WhomeServices file:///D:/fabricoMAt/lib/ui/views/home_screen.dart:43:17

等等，这是在多个地方发生的，我不知道如何解决这个问题。

  HomeViewModel model = Provider.of<HomeViewModel>(context);
Size screenSize = MediaQuery.of(context).size;
return model.initData == null   //home_banner line 30
    ? [1, 2, 3, 4]
        .map(
          (item) => Shimmer.fromColors(
            child: Container(
              margin: EdgeInsets.only(right: 8),
              width: screenSize.width * .7,
              height: screenSize.width * .35,
              decoration: BoxDecoration(
                borderRadius: BorderRadius.all(
                  Radius.circular(8.0),
                ),
                color: Colors.black,
              ),
            ),
            baseColor: Colors.grey.shade300,
            highlightColor: Colors.grey.shade100,
          ),
        )
        .toList()

提到的另一行

 Init? _initData;

 City get selectedCity {
  return _initData!.cities
    .firstWhere((element) => element.id == _initData!.selectedCityId);
 }

 Init get initData => _initData!;// home_viewmodel.dart 25

Answer 1

在第 30 行的 home_banner 中，您正在使用 initData 方法，该方法 return 是一个不可为 null 的值。

因此创建一个可为空的 return 类型的 initData 并从 _initData 中删除 !。

Init? get initData => _initData;// home_viewmodel.dart 25

并且如果您在代码中使用 get selectedCity 而没有检查 _initData 的空值，这可能会导致您的代码出错，所以在此方法中也添加一个空检查。

City get selectedCity {
   City? city = _initData?.cities
  .firstWhere((element) => element.id == _initData?.selectedCityId);

  if(city==null){
     return City();// return default city
  }else{
     return city;
  }
}

空值检查运算符用于 flutter 的空值

Null check operator used on a null value for flutter

android

dart

flutter

dart-null-safety