对空值使用空检查答案

【问题标题】：Null check used on null value对空值使用空检查
【发布时间】：2022-07-13 22:30:27
【问题描述】：

当我尝试将用户信息写入 firestore 时，我不断收到“用于空值的空检查运算符”。我该如何解决这个错误？

这是我尝试实现的代码的 sn-p-

    Widget build(BuildContext context) {
    final UserProvider userProvider = Provider.of<UserProvider>(context);
    return Scaffold(
      appBar: AppBar(
        elevation: 1.0,
        iconTheme: const IconThemeData(
          color: Colors.black,
        ),
        title: const Text(
          "Inventory",
          style: TextStyle(color: Colors.black, fontWeight: FontWeight.bold),
        ),
        backgroundColor: Colors.white,
      ),
      body: SingleChildScrollView(
        child: Container(
          padding: const EdgeInsets.symmetric(vertical: 60.0, horizontal: 16.0),
          child: Column(
            children: <Widget>[
              GestureDetector(
                onTap: () => _selectImage(context),
                child: Container(
                  margin: const EdgeInsets.only(),
                  width: MediaQuery.of(context).size.width,
                  height: 210.0,
                  decoration: BoxDecoration(
                    image: DecorationImage(
                      image: _file == null
                          ? const NetworkImage(
                                  'https://i.stack.imgur.com/l60Hf.png')
                              as ImageProvider
                          : MemoryImage(_file!),
                    ),
                  ),
                ),
              ),
              const SizedBox(
                height: 10,
              ),
              Container(
                padding: const EdgeInsets.only(top: 10),
                width: MediaQuery.of(context).size.width,
                margin: const EdgeInsets.symmetric(horizontal: 20),
                child: TextFieldInput(
                  maxLines: 1,
                  maxLength: TextField.noMaxLength,
                  controller: _mealNameController,
                  labelText: 'Meal Name',
                ),
              ),
              Container(
                padding: const EdgeInsets.only(top: 10),
                width: MediaQuery.of(context).size.width,
                margin: const EdgeInsets.symmetric(horizontal: 20),
                child: TextFieldInput(
                  maxLines: 1,
                  maxLength: TextField.noMaxLength,
                  controller: _priceController,
                  labelText: 'price',
                ),
              ),
              Container(
                padding: const EdgeInsets.only(top: 10),
                width: MediaQuery.of(context).size.width,
                margin: const EdgeInsets.symmetric(horizontal: 20),
                child: TextFieldInput(
                  maxLines: 5,
                  controller: _mealDescriptionController,
                  labelText: 'Meal description',
                  maxLength: 150,
                ),
              ),
              const SizedBox(height: 28.0),
              Column(
                mainAxisAlignment: MainAxisAlignment.spaceBetween,
                children: [
                  mealSize(context),
                  const SizedBox(height: 9.0),
                  mealSize(context),
                  const SizedBox(height: 9.0),
                  mealSize(context),
                ],
              ),
              const SizedBox(height: 40.0),
              GestureDetector(
                  onTap: () => addMenu(userProvider.getUser.name),
                  child: const Button(btnText: "Add Food Item")),
            ],
          ),
        ),
      ),
    );
  }

userprovider.dart：

    class UserProvider with ChangeNotifier {
  User? _user;

  final AuthController _authMethods = AuthController();

  User get getUser => _user!;

  Future<void> refreshUser() async {
    User user = await _authMethods.getUserDetails();
    _user = user;
    notifyListeners();
  }
}

这是我调用函数时遇到的错误：

══════ Exception caught by gesture ═══════════════════════════════════════════
The following _CastError was thrown while handling a gesture:
Null check operator used on a null value

When the exception was thrown, this was the stack
#0      `UserProvider.getUse`r
package:webite/provider/user_provider.dart:11
#1      _AddMealState.build.<anonymous closure>

【问题讨论】：

标签： flutter dart flutter-provider dart-null-safety

【解决方案1】：

您是否尝试过查看这篇文章的答案？长得和我莫名的相似。 Null check operator used on a null value

尤其是在两个地点：


  User get getUser => _user!; <---- right here

                        image: _file == null
                          ? const NetworkImage(
                                  'https://i.stack.imgur.com/l60Hf.png')
                              as ImageProvider
                          : MemoryImage(_file!), <--- and here
    ```

【讨论】：

所以如果检查 _user == null 我应该或多或少？
听起来是个好主意，从技术上讲，您不应该让用户在 Firestore 为空时向其写入信息