Flutter新版Firebase通过snapshot取数据报[]运算符未定义错误

cloud_firestore 2.0及以上的空安全版本调整了类型校验规则，DocumentSnapshot的data()方法返回值默认类型为Object?，不再是旧版本默认的Map<String, dynamic>，直接对返回值调用[]操作符读取字段，就会抛出The operator '[]' isn't defined for the type 'Object'错误，和字段名写法、是否给data加括号无关，本质是缺少显式类型转换。

无需降级依赖，显式将snapshot.data()的返回值转换为Map<String, dynamic>类型后再读取字段即可正常运行，修正后的完整代码如下：

import 'package:cloud_firestore/cloud_firestore.dart';

class UserModel {
  final String UID = "uid";
  final String USERNAME = "username";
  final String EMAIL = "email";
  final String FIRSTNAME = "first name";
  final String LASTNAME = "last name";
  final String BIRTHDAY = "birthday";
  final String GENDER = "gender";


  String _uid;
  String _username;
  String _email;
  String _firstName;
  String _lastName;
  String _birthday;
  String _gender;

//  getters
  String get uid => _uid;
  String get username => _username;
  String get email => _email;
  String get firstName => _firstName;
  String get lastName => _lastName;
  String get birthday => _birthday;
  String get gender => _gender;


  UserModel.fromSnapshot(DocumentSnapshot snapshot) {
    // 显式转换data()返回值为Map类型
    final data = snapshot.data() as Map<String, dynamic>;
    _uid = data[UID];
    _username = data[USERNAME];
    _email = data[EMAIL];
    _firstName = data[FIRSTNAME];
    _lastName = data[LASTNAME];
    _birthday = data[BIRTHDAY];
    _gender = data[GENDER];
  }
}

• 不要使用snapshot.data['uid']写法，data()是DocumentSnapshot的实例方法而非属性，必须带括号调用，旧版本可运行是因为当时做了隐式getter兼容，新版已移除该兼容逻辑。
• 若读取的文档可能不存在，建议在转换前先判断snapshot.exists属性，避免空指针异常。

内容的提问来源于stack exchange，提问作者albaum