Flutter + Firebase:我如何遍历子集合？

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 userDocument = usersCollection.doc('user_id');
CollectionReference subCollection = userDocument.collection('sub_collection');

QuerySnapshot subCollectionSnapshot = await subCollection.get();

subCollectionSnapshot.docs.forEach((DocumentSnapshot document) {
  Map<String, dynamic> data = document.data();
  // 处理子集合文档数据
});