如何在 Dart/Flutter 中初始化类型 ValueChanged？

Question

我是 Flutter 的新手，特别是 Flutter for Web。我正在尝试实现一些可能简单且基本的目标，但我面临着困难。

这是我的 main.dart

Widget build(BuildContext context) {
bool loggedIn = false;
return MaterialApp(
  home: loggedIn ? Navigator(
    pages: [
      MaterialPage(child: DashboardPage())
    ],
    onPopPage: (route, result) => route.didPop(result),
  ) : LoginPage(didLoggedIn: (user) => print('Hello, ' + user) )
);
}
}

我想要存档的内容：如果未登录，请转到登录屏幕。用户成功登录后，我想执行一个回调来打印 hello （实际上我会将状态设置为已登录，但没关系）。

然而，我在实现这个回调时遇到了困难，也许我采取了错误的方法。这是登录页面代码：

import 'package:flutter/material.dart';
import 'package:email_validator/email_validator.dart';

class LoginPage extends StatefulWidget {
  @override
  LoginPageState createState() {
    return LoginPageState();
  }
}
class LoginPageState extends State<LoginPage> {
  final username = TextEditingController();
  final password = TextEditingController();
  final _formKey = GlobalKey<FormState>();
  bool rememberMe = true;
  //final ValueChanged didLoggedIn;
  Widget _buildUsernameField() {
    return TextFormField(
        controller: username,
        decoration: InputDecoration(labelText: 'Your E-Mail'),
        validator: (value) {
          if (value == null ||
              value.isEmpty ||
              !EmailValidator.validate(value)) {
            return 'Invalid E-Mail';
          }
          return null;
        });
  }

  Widget _buildPasswordField() {
    return TextFormField(
        controller: password,
        obscureText: true,
        decoration: InputDecoration(labelText: 'Password'),
        validator: (value) {
          if (value == null || value.isEmpty) {
            return 'Please enter your password';
          }
          return null;
        });
  }

  @override
  Widget build(BuildContext context) {
    return Card(
      child: Container(
        color: Colors.white,
        alignment: Alignment.center,
        child: Container(
          //color: Colors.green,
          width: 600,
          height: 300,
          child: Column(
            children: [
              Form(
                key: _formKey,
                child: Column(
                  mainAxisAlignment: MainAxisAlignment.center,
                  children: <Widget>[
                    _buildUsernameField(),
                    _buildPasswordField(),
                  ],
                ),
              ),
              Column(
                children: [
                  CheckboxListTile(title: Text('Remember me') ,
                      controlAffinity: ListTileControlAffinity.leading,
                      value: rememberMe, onChanged: (bool? value) {
                        setState(() {
                          rememberMe = value!;
                        });
                      }),
                  ElevatedButton(
                    onPressed: () {
                      if (_formKey.currentState!.validate()) {
                        print(username.text + "/" + password.text);
                        //didLoggedIn(username.text);
                      }
                    },
                    child: Text('Log me in'),
                  ),
                  Text('Forgot your password ?')
                ],
              )
            ],
          ),
        ),
      ),
    );
  }
}

问题是这一行：final ValueChanged didLoggedIn;

它说我需要初始化它。怎么做 ？

顺便说一句，正如我所说，我是新手，所以也许这不是实现我的目标的最佳方式，所以如果有人想给我一个更好的解决方案，这将非常受欢迎。

谢谢 ！

Answer 1

您需要如下更改“LoginPage”。

• 将“didLoggedIn”移动到“LoginPage”而不是“LoginPageState”。
  
• 创建“LoginPage”的构造函数来接收“didLoggedIn”
• 使用“widget”访问“didLoggedIn”。字首。

import 'package:flutter/material.dart';
import 'package:email_validator/email_validator.dart';

class LoginPage extends StatefulWidget {
  final ValueChanged didLoggedIn;

  LoginPage({required this.didLoggedIn});

  @override
  LoginPageState createState() {
    return LoginPageState();
  }
}
class LoginPageState extends State<LoginPage> {
  final username = TextEditingController();
  final password = TextEditingController();
  final _formKey = GlobalKey<FormState>();
  bool rememberMe = true;

  Widget _buildUsernameField() {
    return TextFormField(
        controller: username,
        decoration: InputDecoration(labelText: 'Your E-Mail'),
        validator: (value) {
          if (value == null ||
              value.isEmpty ||
              !EmailValidator.validate(value)) {
            return 'Invalid E-Mail';
          }
          return null;
        });
  }

  Widget _buildPasswordField() {
    return TextFormField(
        controller: password,
        obscureText: true,
        decoration: InputDecoration(labelText: 'Password'),
        validator: (value) {
          if (value == null || value.isEmpty) {
            return 'Please enter your password';
          }
          return null;
        });
  }

  @override
  Widget build(BuildContext context) {
    return Card(
      child: Container(
        color: Colors.white,
        alignment: Alignment.center,
        child: Container(
          //color: Colors.green,
          width: 600,
          height: 300,
          child: Column(
            children: [
              Form(
                key: _formKey,
                child: Column(
                  mainAxisAlignment: MainAxisAlignment.center,
                  children: <Widget>[
                    _buildUsernameField(),
                    _buildPasswordField(),
                  ],
                ),
              ),
              Column(
                children: [
                  CheckboxListTile(title: Text('Remember me') ,
                      controlAffinity: ListTileControlAffinity.leading,
                      value: rememberMe, onChanged: (bool? value) {
                        setState(() {
                          rememberMe = value!;
                        });
                      }),
                  ElevatedButton(
                    onPressed: () {
                      if (_formKey.currentState!.validate()) {
                        print(username.text + "/" + password.text);
                        //didLoggedIn(username.text);
                        widget.didLoggedIn(username.text);
                      }
                    },
                    child: Text('Log me in'),
                  ),
                  Text('Forgot your password ?')
                ],
              )
            ],
          ),
        ),
      ),
    );
  }
}