_Cast Error: 在空值上使用了空检查运算符

Question

14 浏览2023年3月7日

匿名的 2023年3月7日

0 Comments

我刚开始学编程，不知道如何解决这个问题。

我使用的是Java 8版本，并使用Android Studio和Flutter制作应用程序。

我正在制作一个待办事项列表应用程序，当我：

创建一个新列表
更新一个列表
删除一个列表

时，会发生转换错误。

class MyApp extends StatelessWidget {


      @override
      Widget build(BuildContext context) {
        return MaterialApp(
          title: 'Flutter ToDo List',
          debugShowCheckedModeBanner: false,
          theme: ThemeData(
            fontFamily: "NanumSquare",
            primaryColor: Color(0xFF424874), //主要颜色
          ),
            home: HomeScreen()
        );
      }
    }

这是main.dart文件。

错误出现在HomeScreen上。

class HomeScreen extends StatefulWidget {


  @override
  _HomeScreenState createState() => _HomeScreenState();
}
class _HomeScreenState extends State {
  late Future> _noteList;
  final DateFormat _dateFormatter = DateFormat("MMM dd, yyyy");
  DatabaseHelper _databaseHelper = DatabaseHelper.instance;
  void initState() {
    super.initState();
    _updateNoteList();
  }
  _updateNoteList() {
    _noteList = DatabaseHelper.instance.getNoteList();
  }
  Widget _buildNote(Note note) {
    return Padding(
      padding: EdgeInsets.symmetric(horizontal: 25),
      child: Column(
        children: [
          Container(
            margin: EdgeInsets.all(5),
            color: Color(0xFFDCD6F7),
            child: ListTile(
              title: Text(
                note.title!,
                style: TextStyle(
                    fontSize: 20,
                    color: Color(0xFF424874),
                    decoration: note.status == 0
                        ? TextDecoration.none
                        : TextDecoration.lineThrough),
              ),
              subtitle: Text(
                "${_dateFormatter.format(note.date!)}-${note.priority}",
                style: TextStyle(
                    fontSize: 15,
                    color: Color(0xFF424874),
                    decoration: note.status == 0
                        ? TextDecoration.none
                        : TextDecoration.lineThrough),
              ),
              trailing: Checkbox(
                onChanged: (value) {
                  note.status = value! ? 1 : 0;
                  DatabaseHelper.instance.updateNote(note);
                  _updateNoteList();
                  Navigator.pushReplacement(
                      context, MaterialPageRoute(builder: (_) => HomeScreen())
                  );
                },
                activeColor: Color(0xFFA6B1E1),
                value: note.status == 1 ? true : false,
              ),
              onTap: () => Navigator.push(
                  context,
                  CupertinoPageRoute(
                      builder: (_) => AddNoteScreen(
                          updateNoteList: _updateNoteList(),
                          note: note
                      )
                  ),
              ),
            ),
          ),
          Divider(
            height: 5,
            color: Color(0xFFA6B1E1),
            thickness: 2,
          )
        ],
      ),
    );
  }
  @override
  Widget build(BuildContext context) {
    return Scaffold(
        backgroundColor: Color(0xFFF4EEFF),
        floatingActionButton: FloatingActionButton(
          backgroundColor: Color(0xFF424874),
          onPressed: () {
            Navigator.push(
                context,
                CupertinoPageRoute(
                  builder: (_) => AddNoteScreen(
                    updateNoteList: _updateNoteList(),
                  ),
                ));
          },
          child: Icon(Icons.add),
        ),
        body: FutureBuilder(
            future: _noteList,
            builder: (context, AsyncSnapshot snapshot) {
              if (!snapshot.hasData) {
                return Center(
                  child: CircularProgressIndicator(),
                );
              }
              final int completedNoteCount = snapshot.data!.where((Note note) => note.status == 1).toList().length;
              return ListView.builder(
                  padding: EdgeInsets.symmetric(vertical: 80),
                  itemCount: int.parse(snapshot.data!.length.toString()) + 1,
                  itemBuilder: (BuildContext context, int index) {
                    if (index == 0) {
                      return Padding(
                        padding:
                            EdgeInsets.symmetric(horizontal: 40, vertical: 20),
                        child: Column(
                          crossAxisAlignment: CrossAxisAlignment.start,
                          children: [
                            Text(
                              "My Notes",
                              style: TextStyle(
                                  color: Color(0xFF424874),
                                  fontSize: 40,
                                  fontWeight: FontWeight.bold),
                            ),
                            SizedBox(
                              height: 10,
                            ),
                            Text(
                              "$completedNoteCount of ${snapshot.data.length}",
                              style: TextStyle(
                                  color: Color(0xFFA6B1E1),
                                  fontSize: 20,
                                  fontWeight: FontWeight.w600),
                            ),
                          ],
                        ),
                      );
                    }
                    return _buildNote(snapshot.data![index-1]);
                  });
            }));
  }
}

错误信息如下所示。

======== 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      _AddNoteScreenState._delete (package:crud_sqlite_app/screens/add_note_screen.dart:80:26)
#1      _InkResponseState._handleTap (package:flutter/src/material/ink_well.dart:989:21)
#2      GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:182:24)
#3      TapGestureRecognizer.handleTapUp (package:flutter/src/gestures/tap.dart:607:11)
#4      BaseTapGestureRecognizer._checkUp (package:flutter/src/gestures/tap.dart:296:5)
...
Handler: "onTap"
Recognizer: TapGestureRecognizer#e7613
  debugOwner: GestureDetector
  state: ready
  won arena
  finalPosition: Offset(150.8, 655.2)
  finalLocalPosition: Offset(110.8, 46.2)
  button: 1
  sent tap down
====================================================================================================

这是add_note_screen.dart文件。

class AddNoteScreen extends StatefulWidget {


  final Note? note;
  final Function? updateNoteList;
  AddNoteScreen({this.note, this.updateNoteList});
  @override
  _AddNoteScreenState createState() => _AddNoteScreenState();
}
class _AddNoteScreenState extends State {
  final _formKey = GlobalKey();
  String _title = "";
  String _priority = "Low";
  String btnText = "Add Note";
  String titleText = "Add Note";
  DateTime _date = DateTime.now();
  TextEditingController _dateController = TextEditingController();
  final DateFormat _dateFormatter = DateFormat("MMM dd, yyyy");
  final List _priorities = ["Low", "Medium", "High"];
  @override
  void initState(){
    super.initState();
    if(widget.note != null) {
      _title = widget.note!.title!;
      _date = widget.note!.date!;
      _priority = widget.note!.priority!;
      setState(() {
        btnText = "Update Note";
        titleText = "Update Note";
      });
    }
    else {
      setState(() {
        btnText = "Add Note";
        titleText = "Add Note";
      });
    }
    _dateController.text = _dateFormatter.format(_date);
  }
  @override
  void dispose() {
    _dateController.dispose();
    super.dispose();
  }
  _handleDatePicker() async {
    final DateTime? date = await showDatePicker(
        context: context,
        initialDate: _date,
        firstDate: DateTime(2000),
        lastDate: DateTime(2100));
    if (date != null && date != _date) {
      setState(() {
        _date = date;
      });
      _dateController.text = _dateFormatter.format(date);
    }
  }
  _delete() {
    DatabaseHelper.instance.deleteNote(widget.note!.id!);
    Navigator.pushReplacement(
        context,
        MaterialPageRoute(
            builder: (_)=> HomeScreen()
        )
    );
    widget.updateNoteList!();
  }
  _submit() {
    if (_formKey.currentState!.validate()) {
      _formKey.currentState!.save();
      print("$_title, $_date, $_priority");
      Note note = Note(
        title: _title,
        date: _date,
        priority: _priority
      );
      if (widget.note == null){
        note.status = 0;
        DatabaseHelper.instance.insertNote(note);
        Navigator.pushReplacement(context, MaterialPageRoute(builder: (_)=> HomeScreen())
        );
      }
      else{
        note.id = widget.note!.id;
        note.status = widget.note!.status;
        DatabaseHelper.instance.updateNote(note);
        Navigator.pushReplacement(context, MaterialPageRoute(builder: (_)=> HomeScreen())
        );
      }
      widget.updateNoteList!();
    }
  }
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Color(0xFFF4EEFF),
      body: GestureDetector(
        onTap: () => FocusScope.of(context).unfocus(),
        child: SingleChildScrollView(
          child: Container(
            padding: EdgeInsets.symmetric(horizontal: 40, vertical: 80),
            child: Column(
              crossAxisAlignment: CrossAxisAlignment.start,
              children: [
                GestureDetector(
                  onTap: () => Navigator.pushReplacement(
                      context,
                      MaterialPageRoute(
                        builder: (_) => HomeScreen(),
                      )
                  ),
                  child: Icon(
                    Icons.arrow_back,
                    size: 30,
                    color: Color(0xFFA6B1E1),
                  ),
                ),
                SizedBox(
                  height: 20.0,
                ),
                Text(
                  titleText,
                  style: TextStyle(
                      color: Color(0xFF424874),
                      fontSize: 40,
                      fontWeight: FontWeight.bold),
                ),
                SizedBox(
                  height: 10,
                ),
                Form(
                  key: _formKey,
                  child: Column(
                    children: [
                      Padding(
                        padding: EdgeInsets.symmetric(vertical: 20),
                        child: TextFormField(
                          style: TextStyle(fontSize: 18),
                          decoration: InputDecoration(
                              labelText: "Title",
                              labelStyle: TextStyle(fontSize: 18),
                              border: OutlineInputBorder(
                                  borderRadius: BorderRadius.circular(10))),
                          validator: (input) => input!.trim().isEmpty
                              ? "Please enter a note title"
                              : null,
                          onSaved: (input) => _title = input!,
                          initialValue: _title,
                        ),
                      ),
                      Padding(
                        padding: EdgeInsets.symmetric(vertical: 20),
                        child: TextFormField(
                          readOnly: true, //hide keyboard
                          controller: _dateController,
                          style: TextStyle(fontSize: 18),
                          onTap: _handleDatePicker,
                          decoration: InputDecoration(
                              labelText: "Date",
                              labelStyle: TextStyle(fontSize: 18),
                              border: OutlineInputBorder(
                                  borderRadius: BorderRadius.circular(10))),
                        ),
                      ),
                      Padding(
                        padding: EdgeInsets.symmetric(vertical: 20),
                        child: DropdownButtonFormField(
                          isDense: true,
                          icon: Icon(Icons.arrow_drop_down_circle),
                          iconSize: 22,
                          iconEnabledColor: Color(0xFF424874),
                          items: _priorities.map((String priority) {
                            return DropdownMenuItem(
                                value: priority,
                                child: Text(
                                  priority,
                                  style: TextStyle(
                                    color: Colors.black,
                                    fontSize: 18,
                                  ),
                                ));
                          }).toList(),
                          style: TextStyle(fontSize: 18),
                          decoration: InputDecoration(
                              labelText: "Priority",
                              labelStyle: TextStyle(fontSize: 18),
                              border: OutlineInputBorder(
                                  borderRadius: BorderRadius.circular(10)
                              )
                          ),
                          // validator: (input) => _priority == null ? "Please",
                          onChanged: (value) {
                            setState(() {
                              _priority = value.toString();
                            });
                          },
                          value: _priority,
                        ),
                      ),
                      Container(
                        margin: EdgeInsets.symmetric(vertical: 20),
                        height: 60.0,
                        width: double.infinity,
                        decoration: BoxDecoration(
                            borderRadius: BorderRadius.circular(30)),
                        child: ElevatedButton(
                          style: ButtonStyle(
                            backgroundColor: MaterialStateProperty.all(Theme.of(context).primaryColor)
                          ),
                          child: Text(
                            btnText,
                            style: TextStyle(color: Colors.white, fontSize: 20),
                          ),
                          onPressed: _submit,
                        ),
                      ),
                      widget.note != null ? Container(
                        margin: EdgeInsets.symmetric(vertical: 20),
                        height: 60,
                        width: double.infinity,
                        decoration: BoxDecoration(
                          color: Color(0xFF424874),
                          borderRadius: BorderRadius.circular(30)
                        ),
                        child: ElevatedButton(
                          style: ButtonStyle(
                              backgroundColor: MaterialStateProperty.all(Theme.of(context).primaryColor)
                          ),
                          child: Text("Delete Note",
                          style: TextStyle(
                            color: Colors.white,
                            fontSize: 20
                          ),
                          ),
                          onPressed: _delete,
                        ),
                      ): SizedBox.shrink()
                    ],
                  ),
                )
              ],
            ),
          ),
        ),
      ),
    );
  }
}

非常抱歉代码太长了。

如果你能帮助我，我会非常感激。

谢谢！

0

1 答案

匿名的 · Answer 1 · 2023-08-24T06:01:43+00:00

当你在一个未初始化的可空实例上使用感叹号操作符(!)时，就会出现这个错误。这个操作符只应该在你确定变量不会为null时使用，我不建议使用它。请查看这个答案。

在Dart和Flutter中，可空类型是非常重要的概念。它们允许我们在变量中存储可能为null的值，并且在使用这些值之前进行空值检查。然而，在使用可空类型时，有时候可能会发生错误。

一个常见的错误是在一个未初始化的可空实例上使用感叹号操作符(!)。这个操作符告诉编译器，你确定这个变量不会为null，因此可以安全地使用它。但是，如果这个变量实际上是null，就会抛出一个_Cast Error : Null check operator used on a null value的错误。

解决这个问题的方法是避免使用感叹号操作符(!)。而是使用安全调用操作符(?.)和空值合并操作符(??)。安全调用操作符允许我们在访问可能为null的值之前进行空值检查，如果变量为null，就会返回null而不是抛出错误。空值合并操作符则允许我们在变量为null时提供一个备选值。

下面是一个示例代码，演示了如何使用安全调用操作符和空值合并操作符来避免_Cast Error : Null check operator used on a null value错误：

String? nullableString;


// 使用安全调用操作符和空值合并操作符
String nonNullString = nullableString?.toUpperCase() ?? "Default Value";
print(nonNullString);

在上面的代码中，我们首先声明了一个可空字符串变量nullableString。然后，我们使用安全调用操作符(?.)和空值合并操作符(??)来将nullableString的值转换为大写。如果nullableString为null，那么nonNullString将被赋值为"Default Value"。

通过使用安全调用操作符和空值合并操作符，我们可以避免_Cast Error : Null check operator used on a null value错误，并且在处理可能为null的值时更加安全和可靠。

总结起来，当你在一个未初始化的可空实例上使用感叹号操作符(!)时，就会出现_Cast Error : Null check operator used on a null value错误。为了避免这个错误，我们应该使用安全调用操作符(?.)和空值合并操作符(??)来进行空值检查和提供备选值。这样可以使我们的代码更加健壮和可靠。