flutter - 为共享首选项用户名定义变量时出现问题

Question

我目前正在测试并尝试 Flutter 应用程序的新设置页面。我是 Flutter 新手，所以我一直在学习教程，但是我收到了一个错误，该错误是教程视频中没有通过 VS Code 实现的，其中显示:

The named parameter 'username' isn't defined. Try correcting the name to an existing named parameter's name, or defining a named parameter with the name 'username'.dartundefined_named_parameter

和

The parameter 'username' can't have a value of 'null' because of its type, but the implicit default value is 'null'. Try adding either an explicit non-'null' default value or the 'required' modifier.dartmissing_default_value_for_parameter

哪个 VS Code 在两个文件中都用红色下划线了“用户名”变量(分别为第 217 行和第 8 行)。我正在使用变量进行测试并在线查找解决方案，但我找不到我理解的非常简单的解决方案。如果这是一个非常简单的问题，我很抱歉，但我是 Flutter 新手，不明白出了什么问题。感谢您抽出时间。

代码的总体目标是保存用户名并将其显示在其他页面中。

ma​​in.dart 文件(第 148 - 222 行)

// Settings Page & Account Information
class Settings extends StatelessWidget {
  Settings({Key? key}) : super(key: key);

  final _usernameController = TextEditingController();
  
  @override
  Widget build(BuildContext context) {
    final theme = MediaQuery.of(context).platformBrightness == Brightness.dark
        ? 'DarkTheme'
        : 'LightTheme';

    return Scaffold(
        appBar: AppBar(title: const Text('Settings'), actions: <Widget>[
          IconButton(
              onPressed: () async {
                _saveSettings;
              },
              icon: const Icon(Icons.save),
              tooltip: 'Save Settings')
        ]),
        body: SingleChildScrollView(
            child: Padding(
          padding: const EdgeInsets.fromLTRB(8, 8, 8, 8),
          child: Column(
            children: [
              Column(
                // Account
                children: [
                  const Padding(
                      padding: EdgeInsets.fromLTRB(0, 12, 0, 0),
                      child: Text('Account Information',
                          style: TextStyle(
                            fontSize: 17.0,
                          ))),
                  Padding(
                    padding: const EdgeInsets.fromLTRB(12, 0, 12, 12),
                    child: TextField(
                      controller: _usernameController,
                      inputFormatters: [LengthLimitingTextInputFormatter(25)],
                      decoration: InputDecoration(
                        hintText: 'Username',
                        labelText: 'Username',
                        border: OutlineInputBorder(
                            borderRadius: BorderRadius.circular(10.0)),
                      ),
                    ),
                  ),
                ],
              ),
              Container(
                child: Column(
                  // App Settings
                  children: [
                    // SwitchListTile(value: DarkMode, onChanged: Light => Dark => Light)
                    // ChangeThemeButtonWidget(),
                    TextButton(
                        onPressed: _saveSettings,
                        child: const Text('Save Settings'))
                  ],
                ),
              ),
            ],
          ),
        )));
  }

  void _saveSettings() {
    final newSettings = Settings(
      username: _usernameController.text,
    );

    print(newSettings);
  }
}

saved_data.dart 文件

import 'package:shared_preferences/shared_preferences.dart';
import 'package:bit/main.dart';

class Settings {
  final String username;

  Settings({
    this.username,
  });
}

Answer 1

您应该考虑为这两个类使用不同的名称。另外，对于第二个设置类，消息很明确:

Try adding either an explicit non-'null' default value or the 'required' modifier.dartmissing_default_value_for_parameter

所以，设置变成:

class SettingsModel {
  final String username;

  SettingsModel({
    required this.username,
  });
}

并且，设置应该是:

void _saveSettings() {
    final newSettings = SettingsModel(
      username: _usernameController.text,
    );
  }