Flutter:如何在Firestore数据库中获取项的子项

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

class SubItemsPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Sub Items'),
      ),
      body: StreamBuilder<QuerySnapshot>(
        stream: FirebaseFirestore.instance
            .collection('collectionName')
            .doc('documentID')
            .collection('subCollectionName')
            .snapshots(),
        builder: (BuildContext context, AsyncSnapshot<QuerySnapshot> snapshot) {
          if (snapshot.hasError) {
            return Text('Error: ${snapshot.error}');
          }

          if (snapshot.connectionState == ConnectionState.waiting) {
            return CircularProgressIndicator();
          }

          return ListView(
            children: snapshot.data.docs.map((DocumentSnapshot document) {
              Map<String, dynamic> data = document.data() as Map<String, dynamic>;
              return ListTile(
                title: Text(data['title']),
                subtitle: Text(data['subtitle']),
              );
            }).toList(),
          );
        },
      ),
    );
  }
}