如何在Flutter中从firebase实时数据库中检索当前用户名

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

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

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

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  DatabaseReference _databaseRef;

  @override
  void initState() {
    super.initState();
    _databaseRef = FirebaseDatabase.instance.reference().child('users/currentUser/username');
  }

  Future<String> getCurrentUsername() async {
    DataSnapshot snapshot = await _databaseRef.once();
    return snapshot.value.toString();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Firebase Database Demo'),
      ),
      body: Center(
        child: FutureBuilder<String>(
          future: getCurrentUsername(),
          builder: (BuildContext context, AsyncSnapshot<String> snapshot) {
            if (snapshot.connectionState == ConnectionState.waiting) {
              return CircularProgressIndicator();
            } else if (snapshot.hasError) {
              return Text('Error: ${snapshot.error}');
            } else {
              return Text('Current Username: ${snapshot.data}');
            }
          },
        ),
      ),
    );
  }
}