检查状态构造函数中的参数

_StudentAddState(List<Student>? students)

您不需要将数据从Widget传递到其状态，您可以使用widegt.data从State类访问Widget数据

class _StudentAddState extends State<StudentAdd> with StudentValidationMixin {

  List<Student>? get students = widget.students;
  var student = Student.withoutInfo();
  var formKey = GlobalKey<FormState>();

  ...
}

如果选中此tudentAdd(List<Student>? students)，则在_StudentAddState中调用StudentAdd构造函数。

_StudentAddState(StudentAdd(List<Student>? students)) {
    this.students = students;
  }

你需要使用

_StudentAddState(List<Student>? students) {
    this.students = students;
  }

你也可以避免传递参数，而我们可以使用widget.varableName来访问类级变量。为了初始化item，我们在state中使用了initState

class StudentAdd extends StatefulWidget {
  StudentAdd({
    Key? key,
    required this.students,
  }) : super(key: key);

  List<Student>? students;
  @override
  State<StudentAdd> createState() => _StudentAddState();
}

class _StudentAddState extends State<StudentAdd> {
  @override
  void initState() {
    super.initState();

    ///getting students also, this can be done anyplace on state class
    print(widget.students?.length);
  }
 //....
}