Flutter注册成功（数据库已存数据）但页面无法跳转问题求助

注册数据已成功存入数据库，接口响应状态码为200，但页面无法跳转至首页，控制台报错type 'Null' is not a subtype of type 'String'，相关代码及日志如下：

register_page.dart 核心代码 #

Widget build(BuildContext context) {
  AuthProvider authProvider = Provider.of<AuthProvider>(context);

  handleRegister() async {
    print('handleRegister() called');
    if (await authProvider.register(
        name: nameController.text,
        username: usernameController.text,
        email: emailController.text,
        password: passwordController.text)) {
      print('register() returned true');
      Navigator.pushNamed(context, '/home');
    } else {
      print('register() returned false');
    }
  }    

  Widget button(){
    return Container(
      height: 50,
      width: double.infinity,
      margin: const EdgeInsets.only(top: 30),
      child: TextButton(
        onPressed: handleRegister,
        style: TextButton.styleFrom(
            backgroundColor: primaryColor,
            shape: RoundedRectangleBorder(
                borderRadius: BorderRadius.circular(12)
            )
        ),
        child: Text("Sign Up", style: primaryTextStyle.copyWith(
            fontSize: 16,
            fontWeight: medium
          ),
        ),
      ),
    );
  }
  // ...其他代码
}

authservice.dart 核心代码 #

class AuthService {
  String baseUrl = "http://192.168.100.229:8000/api";

  Future<UserModel> register(
      {required String name,
      required String username,
      required String email,
      required String password}) async {
    var url = '$baseUrl/register';
    var headers = {'Content-Type': 'application/json'};
    var body = jsonEncode({
      'name': name,
      'username': username,
      'email': email,
      'password': password
    });

    var response = await http.post(Uri.parse(url), headers: headers, body: body);

    log('code ${response.statusCode} ');
    log('status : ${response.statusCode == 200? true : false}');

    if (response.statusCode == 200) {
      var data = jsonDecode(response.body)['data'];
      UserModel user = UserModel.fromJson(data['user']);
      user.token = data['token_type'] + " " + data['access_token'];
      log("register success");
      return user;
    } else {
      log("register fail");
      throw Exception("register failed");
    }
  }
}

authprovider.dart 核心代码 #

class AuthProvider with ChangeNotifier {
  late UserModel _user;

  UserModel get user => _user;

  set user(UserModel user) {
    _user = user;
    notifyListeners();
  }

  Future<bool> register(
      {required String name,
      required String username,
      required String email,
      required String password}) async {
    try {
      UserModel user = await AuthService().register(
          name: name, username: username, email: email, password: password);

      _user = user;
      log("auth provider true");
      return true;
    } catch (e) {
      log("auth provider false", error: e);
      return false;
    }
  }
}

控制台日志 #

I/flutter ( 6788): handleRegister() called
I/flutter ( 6788): register() returned false
[log] auth provider false
      type 'Null' is not a subtype of type 'String'

解决方案 #

1. 定位空值来源 #

报错type 'Null' is not a subtype of type 'String'说明在字符串拼接或JSON解析时，某个预期为String的字段实际是Null，结合代码分析：

• 拼接token时，data['token_type']或data['access_token']可能为Null
• UserModel.fromJson(data['user'])解析时，UserModel中定义的非空String字段在接口返回的user数据中为Null

2. 修复Token拼接的空值问题 #

在authservice.dart中修改token赋值逻辑，增加空值兜底：

// 原代码
user.token = data['token_type'] + " " + data['access_token'];

// 修改后
user.token = (data['token_type'] ?? '') + " " + (data['access_token'] ?? '');

3. 修复UserModel的JSON解析逻辑 #

检查UserModel的fromJson方法，确保所有非空字段都处理Null情况：
比如如果UserModel定义如下：

class UserModel {
  final String id;
  final String name;
  final String username;
  String? token;

  UserModel({required this.id, required this.name, required this.username, this.token});

  factory UserModel.fromJson(Map<String, dynamic> json) {
    return UserModel(
      id: json['id'] ?? '', // 增加空值兜底
      name: json['name'] ?? '',
      username: json['username'] ?? '',
    );
  }
}

如果接口返回的user字段可能为Null，必须用??运算符设置默认值，或者将字段改为可空类型（String?）。

4. 调试接口返回的完整数据 #

在authservice.dart中打印完整的响应体，确认返回的字段结构：

log('response body: ${response.body}');

通过日志确认data['token_type']、data['access_token']以及data['user']下的所有字段是否存在且不为Null。

5. 验证Provider逻辑 #

修复空值问题后，AuthService.register将正常返回UserModel，不会抛出异常，AuthProvider.register会返回true，触发Navigator.pushNamed(context, '/home')完成页面跳转。

内容的提问来源于stack exchange，提问作者leo