无法在初始化程序中访问实例成员“...”

Question

我正在尝试创建一个简单的 Flutter 项目以供学习。我需要删除选定的学生 -Photo- 但一开始我分配了 StudentList 私有所以当我尝试删除时 setState() 函数不起作用。如何将 StudentList 指定为全局。

这是我一开始写的代码：

class _MyAppState extends State<MyApp> {

  StudentManager ChoosedStudent = new StudentManager.WithId(0, "", "", 0, "");
  StudentManager student1 = new StudentManager.WithId(1, "Name1", "LastName1",
      22, "https://uifaces.co/our-content/donated/hvaUVob5.jpg");
  StudentManager student2 = new StudentManager.WithId(2, "Name2", "LastName2",
      70, "https://uifaces.co/our-content/donated/eqUZLcBO.jpg");
  StudentManager student3 = new StudentManager.WithId(3, "Name3", "LastName3",
      55, "https://uifaces.co/our-content/donated/-oe25tWA.png");
  StudentManager student4 = new StudentManager.WithId(
      4,
      "Name4",
      "LastName4",
      99,
      "https://images.generated.photos/ukvWCGJJF8xoZ_rvVSFDLnQ-WDkGw2WsZ53uPPm63M8/rs:fit:512:512/Z3M6Ly9nZW5lcmF0/ZWQtcGhvdG9zLzA3/OTI0MTAuanBn.jpg");
  StudentManager student5 = new StudentManager.WithId(
      5, "Name5", "LastName5", 45, "https://thispersondoesnotexist.com/image");

  @override
  Widget build(BuildContext context) {
    // TODO: implement build
    return Scaffold(
      appBar: AppBar(
        title: Text("Student Exam Result System"),
      ),
      body: bodyBuilder(context),

    );

  }

  bodyBuilder(BuildContext context) {
    List<StudentManager> StudentList = [
      student1,
      student2,
      student3,
      student4,
      student5
    ];
    return Column(
      children: [
        Expanded(
            child: ListView.builder(
                itemCount: StudentList.length,
                itemBuilder: (BuildContext context, int index) {
                  if (StudentList[index].Grade >= 50) {
                    StudentList[index].IsPassed = true;
                  } else if (StudentList[index].Grade < 50) {
                    StudentList[index].IsPassed = false;
                  }
                  return ListTile(
                    leading: CircleAvatar(
                      backgroundImage:
                          NetworkImage(StudentList[index].PhotoURL),
                    ),
                    title: Text(StudentList[index].Name +
                        " " +
                        StudentList[index].Surname),
                    subtitle: Text(
                        "${StudentList[index].Name} named students grade is "
                        "${StudentList[index].Grade}, ${StudentList[index].AlphebaticalGrade}"),
                    isThreeLine: true,
                    trailing: buildStatusIcon(StudentList[index].IsPassed),
                    onTap: () {
                      setState(() {
                        ChoosedStudent = StudentList[index];
                      });
                    },
                  );
                })),

我想这样分配 StudentList：

    class _MyAppState extends State<MyApp> {
    
      StudentManager ChoosedStudent = new StudentManager.WithId(0, "", "", 0, "");
      StudentManager student1 = new StudentManager.WithId(1, "Name1", "LastName1",
          22, "https://uifaces.co/our-content/donated/hvaUVob5.jpg");
      StudentManager student2 = new StudentManager.WithId(2, "Name2", "LastName2",
          70, "https://uifaces.co/our-content/donated/eqUZLcBO.jpg");
      StudentManager student3 = new StudentManager.WithId(3, "Name3", "LastName3",
          55, "https://uifaces.co/our-content/donated/-oe25tWA.png");
      StudentManager student4 = new StudentManager.WithId(
          4,
          "Name4",
          "LastName4",
          99,
          "https://images.generated.photos/ukvWCGJJF8xoZ_rvVSFDLnQ-WDkGw2WsZ53uPPm63M8/rs:fit:512:512/Z3M6Ly9nZW5lcmF0/ZWQtcGhvdG9zLzA3/OTI0MTAuanBn.jpg");
      StudentManager student5 = new StudentManager.WithId(
          5, "Name5", "LastName5", 45, "https://thispersondoesnotexist.com/image");
      List<StudentManager> StudentList = [
        student1,
        student2,
        student3,
        student4,
        student5
    ];

但这会抛出“无法在初始化程序中访问实例成员“...”。”错误。有没有其他方法可以全局分配StudentList，错误原因是什么？

如有必要，请提供完整版本的代码：

import 'dart:ffi';

import 'package:by_myself/models/students.dart';
import 'package:flutter/material.dart';

void main() {
  runApp(MaterialApp(home: MyApp()));
}

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {

  StudentManager ChoosedStudent = new StudentManager.WithId(0, "", "", 0, "");
  StudentManager student1 = new StudentManager.WithId(1, "Name1", "LastName1",
      22, "https://uifaces.co/our-content/donated/hvaUVob5.jpg");
  StudentManager student2 = new StudentManager.WithId(2, "Name2", "LastName2",
      70, "https://uifaces.co/our-content/donated/eqUZLcBO.jpg");
  StudentManager student3 = new StudentManager.WithId(3, "Name3", "LastName3",
      55, "https://uifaces.co/our-content/donated/-oe25tWA.png");
  StudentManager student4 = new StudentManager.WithId(
      4,
      "Name4",
      "LastName4",
      99,
      "https://images.generated.photos/ukvWCGJJF8xoZ_rvVSFDLnQ-WDkGw2WsZ53uPPm63M8/rs:fit:512:512/Z3M6Ly9nZW5lcmF0/ZWQtcGhvdG9zLzA3/OTI0MTAuanBn.jpg");
  StudentManager student5 = new StudentManager.WithId(
      5, "Name5", "LastName5", 45, "https://thispersondoesnotexist.com/image");

  @override
  Widget build(BuildContext context) {
    // TODO: implement build
    return Scaffold(
      appBar: AppBar(
        title: Text("Student Exam Result System"),
      ),
      body: bodyBuilder(context),

    );

  }

  bodyBuilder(BuildContext context) {
    List<StudentManager> StudentList = [
      student1,
      student2,
      student3,
      student4,
      student5
    ];
    return Column(
      children: [
        Expanded(
            child: ListView.builder(
                itemCount: StudentList.length,
                itemBuilder: (BuildContext context, int index) {
                  if (StudentList[index].Grade >= 50) {
                    StudentList[index].IsPassed = true;
                  } else if (StudentList[index].Grade < 50) {
                    StudentList[index].IsPassed = false;
                  }
                  return ListTile(
                    leading: CircleAvatar(
                      backgroundImage:
                          NetworkImage(StudentList[index].PhotoURL),
                    ),
                    title: Text(StudentList[index].Name +
                        " " +
                        StudentList[index].Surname),
                    subtitle: Text(
                        "${StudentList[index].Name} named students grade is "
                        "${StudentList[index].Grade}, ${StudentList[index].AlphebaticalGrade}"),
                    isThreeLine: true,
                    trailing: buildStatusIcon(StudentList[index].IsPassed),
                    onTap: () {
                      setState(() {
                        ChoosedStudent = StudentList[index];
                      });
                    },
                  );
                })),
        Text("Student: " + ChoosedStudent.Name),
        Center(
          child: Row(
            children: [
              Flexible(
                fit: FlexFit.tight,
                flex: 1,
                child: RaisedButton(
                  onPressed: () {},
                  color: Colors.blueAccent,
                  textColor: Colors.white,
                  child: Row(
                    children: [
                      SizedBox(
                        width: 16.0,
                      ),
                      Icon(Icons.add),
                      Text("Add"),
                    ],
                  ),
                ),
              ),
              Flexible(
                fit: FlexFit.tight,
                flex: 1,
                child: RaisedButton(
                  onPressed: () {},
                  color: Colors.blueAccent,
                  textColor: Colors.white,
                  child: Row(
                    children: [
                      SizedBox(
                        width: 16.0,
                      ),
                      Icon(Icons.update),
                      Text("Update"),
                    ],
                  ),
                ),
              ), //KAY
              Flexible(
                fit: FlexFit.tight,
                flex: 1,
                child: RaisedButton(
                  onPressed: () {
                    setState(() {
                      StudentList.remove(ChoosedStudent);
                      print(StudentList);
                    });
                  },
                  color: Colors.blueAccent,
                  textColor: Colors.white,
                  child: Row(
                    children: [
                      SizedBox(
                        width: 16.0,
                      ),
                      Icon(Icons.delete),
                      Text("Delete"),
                    ],
                  ),
                ),
              ),
            ],
          ),
        ),
      ],
    );
  }
}

Widget buildStatusIcon(bool IsPassed) {
  if (IsPassed == true) {
    return Icon(Icons.done);
  } else if (IsPassed == false) {
    return Icon(Icons.warning);
  } else {
    return Icon(Icons.error);
  }
}