flutter - 有没有办法帮助我修复类型 null 不是类型 Map 的子类型？

Question

我已经研究 flutter 几个月了，我开始了解如何解决 null 安全问题，但这对我来说真的很复杂，我希望得到帮助。

这是我的问题:

每当我尝试登录时，都会收到此错误:“Null”类型不是 Map 类型的子类型我尽力找出问题所在，但我无法，任何帮助都将挽救生命......

import 'package:cloud_firestore/cloud_firestore.dart';

class User {
 final String email;
 final String uid;
 final String photoUrl;
 final String username;
 final String bio;
 final List followers;
 final List following;

const User({
  required this.email,
  required this.uid,
  required this.photoUrl,
  required this.username,
  required this.bio,
  required this.followers,
  required this.following,
 });

然后我继续这里

Map<String, dynamic> toJson() => {
    "username": username,
    "uid": uid,
    "email": email,
    "photoUrl": photoUrl,
    "bio": bio,
    "followers": followers,
    "following": following,
  };

   static User fromSnap(DocumentSnapshot snap) {
   var snapshot = snap.data() as Map<String, dynamic>;

  return User(
     username: snapshot['username'],
     uid: snapshot['uid'],
     email: snapshot['email'],
     photoUrl: snapshot['photoUrl'],
     bio: snapshot['bio'],
     followers: snapshot['followers'],
     following: snapshot['following'],
      );
     }
    }

现在我在这一行收到错误:null type is not a subtype of Map

var snapshot = snap.data() as Map<String, dynamic>;

这都是我的问题，提前致谢。

Answer 1

您的snapshot值是可为空，并且您尝试将其转换为map，而不是这样:

var snapshot = snap.data() as Map<String, dynamic>;

试试这个:

var snapshot = snap.data() != null ? snap.data() as Map<String, dynamic> : {};

还将您的返回更改为:

return User(
     username: snapshot['username']??””,
     uid: snapshot['uid'] ??””,
     email: snapshot['email'] ??””,
     photoUrl: snapshot['photoUrl'] ??””,
     bio: snapshot['bio'] ??””,
     followers: snapshot['followers'] ??[],
     following: snapshot['following'] ??[],
  );