Firebase规则和flutter :如何检查用户名的可用性

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    // 检查用户名是否可用
    function isUsernameAvailable(username) {
      // 检查用户名是否已存在于users集合中
      return !exists(/databases/$(database)/documents/users/$(username));
    }

    // 允许读取用户文档
    match /users/{username} {
      allow read: if true;
    }

    // 允许创建新用户
    match /users/{username} {
      allow create: if isUsernameAvailable(username);
    }
  }
}

import 'package:cloud_firestore/cloud_firestore.dart';

Future<bool> isUsernameAvailable(String username) async {
  final snapshot = await FirebaseFirestore.instance
      .collection('users')
      .doc(username)
      .get();
  return !snapshot.exists;
}

void main() async {
  final username = 'example_username';
  final isAvailable = await isUsernameAvailable(username);
  if (isAvailable) {
    print('Username is available');
  } else {
    print('Username is not available');
  }
}