Question

所以我正在将一个项目切换到颤振空安全，但我在空值错误上使用了空检查运算符我尝试查看其他类似的 issue 尝试但它对我不起作用完整的错误如下

export const getMessageTemplates = (): Promise<
  ReadonlyArray<MessageTemplate>
> =>
  fetch(REACT_APP_BACKEND_LOCATION + "/api/template", {
      method: "get",
      credentials: REACT_APP_BACKEND_LOCATION === "." ? "same-origin" : "include",
      headers: {
        Accept: "application/json",
      },
    })
    .then(res => res.json())
    .then(data => [{ templateId: " ", category: "", title: "blank message", defaultTemplate: false }, ...data])
    .then((data) => /* your logic here */);

等等，这会出现在多个地方，我不知道如何解决这个问题。

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

提到的另一行

  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()

Answer 1

在第 30 行的 home_banner 中，您使用的是 initData 方法，该方法返回一个不可为空的值。

因此创建一个可为空的 initData 返回类型并从 ! 中删除 _initData。

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

如果您在代码中使用 get selectedCity 而不检查 _initData 的空值，这可能会导致您的代码出错，所以也要在这个方法中添加一个空检查。

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颤振的空值

1 个答案: