Dart中无法访问对象属性问题求助

问题现象 #

• 开发中无法访问对象属性，调试打印tempo.id无输出
• 直接调用其他文件的对象实例时，打印对象仅显示Instance of Object$，无法看到属性值
• 当前使用GetIt测试数据库功能，本地数据存储在项目其他文件中

相关代码 #

1. 对象实例创建代码 #

studentProfilecreation tempo = studentProfilecreation("Firebaseid1", "A", "B", '', "tdd");

2. Widget文件调用代码 #

Widget build(BuildContext context) {
    studentProfilecreation temp = tempo;
    String id = tempo.id;
    print(id);
    MediaQueryData queryData;
}

3. studentProfilecreation类定义 #

class studentProfilecreation {
  String id = "";
  String firstName = "";
  String lastName = "";
  String profileImage = '';
  String major = "";
  String bio = "";
  Map<String, File> coursesAndnotes = Map();
//Constructor
  studentProfilecreation(
    String id,
    String firstName,
    String lastName,
    String profileImage,
    String major,
  ) {
    id = id;
    firstName = firstName;
    lastName = lastName;
    profileImage = profileImage;
    major = major;
  }

  studentProfilecreation.studentProfilecreationempty() {
    id = "";
    firstName = "";
    lastName = "";
    profileImage = '';
    major = "";
  }

  studentProfilecreation.createcopy(studentProfilecreation original) {
    id = original.id;
    firstName = original.firstName;
    lastName = original.lastName;
    profileImage = original.profileImage;
    major = original.major;
  }
}

4. 目标Widget代码 #

class studentProfile extends StatelessWidget {
  String id = "";
  studentProfile(String id) {
    this.id = id;
  }

  @override
  Widget build(BuildContext context) {
    studentProfilecreation temp = tempo;
    String id = tempo.id;
    print(id);
}

问题根源 #

核心问题出在studentProfilecreation类的构造函数中：构造函数里的赋值语句（如id = id）只是给参数变量赋值，并没有修改类的成员变量。因为参数名和成员变量名相同，没有用this.区分，导致赋值操作完全无效，类的成员变量始终保持初始的空字符串值。

另外，打印对象显示Instance of Object$是因为类没有重写toString()方法，默认的toString只能输出实例类型，无法展示属性内容。

修复方案 #

1. 修正构造函数赋值 #

三种可选方式：

• 方式一：用this.明确指向成员变量

studentProfilecreation(
    String id,
    String firstName,
    String lastName,
    String profileImage,
    String major,
  ) {
    this.id = id;
    this.firstName = firstName;
    this.lastName = lastName;
    this.profileImage = profileImage;
    this.major = major;
  }

• 方式二：使用初始化列表（推荐写法）

studentProfilecreation(
    String id,
    String firstName,
    String lastName,
    String profileImage,
    String major,
  ) : id = id,
       firstName = firstName,
       lastName = lastName,
       profileImage = profileImage,
       major = major;

• 方式三：使用构造函数语法糖（最简洁）

studentProfilecreation(
    this.id,
    this.firstName,
    this.lastName,
    this.profileImage,
    this.major,
  );

2. 规范命名（可选但推荐） #

遵循Dart命名规范：类名用大驼峰，构造函数别名用小驼峰：

class StudentProfileCreation {
  // ...成员变量...

  StudentProfileCreation(this.id, this.firstName, this.lastName, this.profileImage, this.major);

  StudentProfileCreation.empty() {
    id = "";
    firstName = "";
    lastName = "";
    profileImage = '';
    major = "";
  }

  StudentProfileCreation.copy(StudentProfileCreation original) {
    id = original.id;
    firstName = original.firstName;
    lastName = original.lastName;
    profileImage = original.profileImage;
    major = original.major;
  }
}

3. 重写toString()方便调试 #

添加后打印对象可直接查看属性值：

@override
String toString() {
  return 'StudentProfileCreation{id: $id, firstName: $firstName, lastName: $lastName, profileImage: $profileImage, major: $major}';
}

内容的提问来源于stack exchange，提问作者Edgar Torres