如何在Flutter Firebase中检查用户是否已登录

import 'package:flutter/material.dart';
import 'package:firebase_core/firebase_core.dart';
import 'package:firebase_auth/firebase_auth.dart';

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp();
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Firebase Demo',
      home: HomePage(),
    );
  }
}

class HomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    User? user = FirebaseAuth.instance.currentUser;
    if (user == null) {
      // 用户未登录
      return LoginPage();
    } else {
      // 用户已登录
      return DashboardPage();
    }
  }
}

class LoginPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Login'),
      ),
      body: Center(
        child: ElevatedButton(
          child: Text('Login with Firebase'),
          onPressed: () {
            // 在此处实现登录逻辑
          },
        ),
      ),
    );
  }
}

class DashboardPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Dashboard'),
      ),
      body: Center(
        child: Text('Welcome!'),
      ),
    );
  }
}