如何在Flutter FutureBuilder中引用Firestore documentID

dependencies:
  flutter:
    sdk: flutter
  firebase_core: ^1.0.0
  cloud_firestore: ^2.0.0

import 'package:firebase_core/firebase_core.dart';
import 'package:cloud_firestore/cloud_firestore.dart';

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

CollectionReference usersCollection = FirebaseFirestore.instance.collection('users');
DocumentReference userDoc = FirebaseFirestore.instance.collection('users').doc('user1');

FutureBuilder<DocumentSnapshot>(
  future: userDoc.get(),
  builder: (BuildContext context, AsyncSnapshot<DocumentSnapshot> snapshot) {
    if (snapshot.connectionState == ConnectionState.waiting) {
      return CircularProgressIndicator();
    }
    if (snapshot.hasError) {
      return Text('Error: ${snapshot.error}');
    }
    if (snapshot.hasData && !snapshot.data!.exists) {
      return Text('Document does not exist');
    }
    if (snapshot.hasData) {
      String documentId = snapshot.data!.id;
      return Text('Document ID: $documentId');
    }
    return Text('Loading');
  },
);