Flutter:从Firestore下拉菜单中检索数据，并将选定的值上传回Firestore

dependencies:
  flutter:
    sdk: flutter
  firebase_core: latest_version
  cloud_firestore: latest_version

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

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: Text('Firestore Example')),
        body: FirestoreDropdownExample(),
      ),
    );
  }
}

class FirestoreDropdownExample extends StatefulWidget {
  @override
  _FirestoreDropdownExampleState createState() => _FirestoreDropdownExampleState();
}

class _FirestoreDropdownExampleState extends State<FirestoreDropdownExample> {
  final TextEditingController _controller = TextEditingController();
  List<String> _options = [];
  String _selectedOption;

  @override
  void initState() {
    super.initState();
    fetchOptions();
  }

  Future<void> fetchOptions() async {
    QuerySnapshot snapshot = await FirebaseFirestore.instance.collection('options').get();
    setState(() {
      _options = snapshot.docs.map((doc) => doc['name']).toList();
    });
  }

  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        DropdownButtonFormField(
          value: _selectedOption,
          decoration: InputDecoration(labelText: 'Select an option'),
          items: _options.map((option) {
            return DropdownMenuItem(value: option, child: Text(option));
          }).toList(),
          onChanged: (value) {
            setState(() {
              _selectedOption = value;
            });
          },
        ),
        ElevatedButton(
          onPressed: () {
            uploadSelectedOption();
          },
          child: Text('Upload'),
        ),
      ],
    );
  }

  Future<void> uploadSelectedOption() async {
    if (_selectedOption != null) {
      await FirebaseFirestore.instance.collection('user_selections').add({
        'selected_option': _selectedOption,
        'timestamp': DateTime.now(),
      });
      ScaffoldMessenger.of(context).showSnackBar(SnackBar(content: Text('Uploaded successfully')));
    }
  }
}