将数据作为初始值从Firestore检索到DropdownButtonFormField，以便可以更新

dependencies:
  firebase_core: ^1.0.0
  cloud_firestore: ^2.0.0

import 'package:cloud_firestore/cloud_firestore.dart';

StreamBuilder<QuerySnapshot>(
  stream: FirebaseFirestore.instance.collection('items').snapshots(),
  builder: (BuildContext context, AsyncSnapshot<QuerySnapshot> snapshot) {
    if (snapshot.hasError) {
      return Text('Error: ${snapshot.error}');
    }

    if (snapshot.connectionState == ConnectionState.waiting) {
      return Text('Loading...');
    }

    List<DropdownMenuItem<String>> dropdownItems = snapshot.data.docs.map((DocumentSnapshot document) {
      return DropdownMenuItem<String>(
        value: document.data()['value'],
        child: Text(document.data()['value']),
      );
    }).toList();

    return DropdownButtonFormField<String>(
      value: dropdownItems[0].value, // 设置初始值
      items: dropdownItems,
      onChanged: (String newValue) {
        // 更新选择的值
      },
    );
  },
);