Jawaban

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

void main() {
  runApp(
    ChangeNotifierProvider(
      create: (context) => UserModel(),
      child: MyApp(),
    ),
  );
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: LoginScreen(),
    );
  }
}

class UserModel extends ChangeNotifier {
  String _username = '';
  String _password = '';

  String get username => _username;
  String get password => _password;

  void login(String username, String password) {
    _username = username;
    _password = password;
    notifyListeners();
  }

  void logout() {
    _username = '';
    _password = '';
    notifyListeners();
  }
}

class LoginScreen extends StatefulWidget {
  @override
  _LoginScreenState createState() => _LoginScreenState();
}

class _LoginScreenState extends State<LoginScreen> {
  final _formKey = GlobalKey<FormState>();
  String _username = '';
  String _password = '';

  void _submitForm() {
    if (_formKey.currentState!.validate()) {
      _formKey.currentState!.save();
      Provider.of<UserModel>(context, listen: false).login(_username, _password);
      Navigator.push(
        context,
        MaterialPageRoute(
          builder: (context) => HomeScreen(),
        ),
      );
    }
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Login Screen'),
      ),
      body: Padding(
        padding: const EdgeInsets.all(16.0),
        child: Form(
          key: _formKey,
          child: Column(
            children: [
              TextFormField(
                decoration: InputDecoration(labelText: 'Username'),
                validator: (value) {
                  if (value == null || value.isEmpty) {
                    return 'Please enter your username';
                  }
                  return null;
                },
                onSaved: (value) {
                  _username = value!;
                },
              ),
              TextFormField(
                decoration: InputDecoration(labelText: 'Password'),
                obscureText: true,
                validator: (value) {
                  if (value == null || value.isEmpty) {
                    return 'Please enter your password';
                  }
                  if (value.length < 6) {
                    return 'Password must be at least 6 characters';
                  }
                  return null;
                },
                onSaved: (value) {
                  _password = value!;
                },
              ),
              SizedBox(height: 20),
              ElevatedButton(
                onPressed: _submitForm,
                child: Text('Submit'),
              ),
            ],
          ),
        ),
      ),
    );
  }
}

class HomeScreen extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final user = Provider.of<UserModel>(context);

    return Scaffold(
      appBar: AppBar(
        title: Text('Home Screen'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Text(
              'Welcome, ${user.username}!',
              style: TextStyle(fontSize: 24),
            ),
            SizedBox(height: 20),
            ElevatedButton(
              onPressed: () {
                Navigator.push(
                  context,
                  MaterialPageRoute(
                    builder: (context) => ProfileScreen(),
                  ),
                );
              },
              child: Text('Go to Profile'),
            ),
          ],
        ),
      ),
    );
  }
}

class ProfileScreen extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final user = Provider.of<UserModel>(context);

    return Scaffold(
      appBar: AppBar(
        title: Text('Profile Screen'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Text(
              'Username: ${user.username}',
              style: TextStyle(fontSize: 24),
            ),
            Text(
              'Password: ${user.password}',
              style: TextStyle(fontSize: 24),
            ),
            SizedBox(height: 20),
            ElevatedButton(
              onPressed: () {
                user.logout();
                Navigator.pushAndRemoveUntil(
                  context,
                  MaterialPageRoute(
                    builder: (context) => LoginScreen(),
                  ),
                  (route) => false,
                );
              },
              child: Text('Logout'),
            ),
          ],
        ),
      ),
    );
  }
}

Penjelasan:

• UserModel: Mengelola state pengguna yang sedang login. login method menyimpan username dan password, serta logout method menghapus data pengguna.

• LoginScreen: Stateful widget yang mengelola form login. Validasi input dilakukan dan jika validasi berhasil, pengguna dinavigasi ke HomeScreen.

• HomeScreen: Stateless widget yang menampilkan pesan selamat datang menggunakan username dari UserModel. Tombol untuk navigasi ke ProfileScreen.

• ProfileScreen: Stateless widget yang menampilkan informasi username dan password dari UserModel. Tombol untuk logout yang menghapus data pengguna dari UserModel dan kembali ke LoginScreen.