LateInitializationError:字段'avatar‘尚未初始化

Question

下面是我的json的样子

{
    "data": {
        "id": 2,
        "email": "floki@mail.com",
        "first_name": "Valjakudze",
        "last_name": "shipment",
        "avatar": "https://upload.wikimedia.org/wikipedia/commons/thumb/7/7e/Circle-icons-profile.svg/1024px-Circle-icons-profile.svg.png"
    }
}

我正在使用Dio和以下两个类来进行回复。

响应类

import 'package:json_annotation/json_annotation.dart';
import 'package:json_crud/classes/user.dart';
part 'user_response.g.dart';

@JsonSerializable()
class UserResponse{

  @JsonKey(name: "data")
  User? user;

  UserResponse();

  factory UserResponse.fromJson(Map<String, dynamic> json) => _$UserResponseFromJson(json);
  Map<String, dynamic> toJson() => _$UserResponseToJson(this);

}

下面是我的UserClass

import 'package:json_annotation/json_annotation.dart';

part 'user.g.dart';

@JsonSerializable()
class User{

  @JsonKey(name: "id")
  late int id;

  @JsonKey(name: "email")
  late String email;

  @JsonKey(name: "first_name")
  late String firstName;

  @JsonKey(name: "last_name")
  late String lastName;

  @JsonKey(name: "avatar")
  late String avatar;

  User();

  factory User.fromJson(Map<String, dynamic> json) => _$UserFromJson(json);
  Map<String, dynamic> toJson() => _$UserToJson(this);

}

下面是我的UserProfileScreen，我应该在这里设置文本

import 'dart:developer';

import 'package:dio/dio.dart';
import 'package:flutter/material.dart';
import 'package:json_crud/classes/user.dart';
import 'package:json_crud/classes/user_response.dart';
import 'package:json_crud/http_service.dart';

class UserProfile extends StatefulWidget {

  const UserProfile({Key? key}) : super(key: key);

  @override
  _UserProfileState createState() => _UserProfileState();
}

class _UserProfileState extends State<UserProfile> {

  HttpService? httpService;

  UserResponse? userResponse;

  User? user;

  bool isLoading = false;

  Future getUser () async {

    Response response;

    try {
      log("TryCatchCalled");

      response = await httpService!.getRequest("/b/BGT5");

      if(response.statusCode == 200){

        setState(() {

          userResponse = UserResponse.fromJson(response.data);

          user = userResponse!.user;

          log("ResponseStatusCode200 ${response.statusCode}");

        });

      }else{
        log("ResponseStatusCode ${response.statusCode}");
      }
    } on Exception catch (e) {
      isLoading = false;
      log("GetUserResponseException ${e.toString()}");
    }
  }

  @override
  void initState() {

    httpService = HttpService();
    userResponse = UserResponse();
    user = User();

    getUser();

    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text("User Profile"),
      ),
      body: isLoading
          ? const Center(child: CircularProgressIndicator())
          :  Container(
        width: double.infinity,
        child: Column(
          children: [
            Image.network(user!.avatar),
            Container(height: 16,),
            Text("Hello, ${user!.firstName} ${user!.lastName}")
          ],
        ),
      ),
    );
  }
}

我得到了LateInitializationError:字段'avatar‘还没有初始化错误

下面是我通过更改我的用户类尝试过的内容

import 'package:json_annotation/json_annotation.dart';


part 'user.g.dart';

@JsonSerializable()
class User{

  @JsonKey(name: "id")
  int? id;

  @JsonKey(name: "email")
  String? email;

  @JsonKey(name: "first_name")
  String? firstName;

  @JsonKey(name: "last_name")
  String? lastName;

  @JsonKey(name: "avatar")
  String? avatar;

  User();

  factory User.fromJson(Map<String, dynamic> json) => _$UserFromJson(json);
  Map<String, dynamic> toJson() => _$UserToJson(this);

}

但是，当我在后面更改类变量时，会出现一个新的错误:参数类型“String？”不能分配参数类型'String‘，因为'String?’是可空的，而“字符串”不是。Image.network(用户！.avatar)

下面是完整的错误日志

======== Exception caught by widgets library =======================================================
The following LateError was thrown building UserProfile(dirty, state: _UserProfileState#c6357):
LateInitializationError: Field 'avatar' has not been initialized.

The relevant error-causing widget was: 
  UserProfile UserProfile:file:///C:/Users/Emman/AndroidStudioProjects/json_crud/lib/main.dart:28:19
When the exception was thrown, this was the stack: 
#0      User.avatar (package:json_crud/classes/user.dart)
#1      _UserProfileState.build (package:json_crud/screens/user_profile.dart:81:19)
#2      StatefulElement.build (package:flutter/src/widgets/framework.dart:4705:27)
#3      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4588:15)
#4      StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4763:11)
#5      Element.rebuild (package:flutter/src/widgets/framework.dart:4311:5)
#6      ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4566:5)
#7      StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4754:11)
#8      ComponentElement.mount (package:flutter/src/widgets/framework.dart:4561:5)
...     Normal element mounting (169 frames)
#177    Element.inflateWidget (package:flutter/src/widgets/framework.dart:3631:14)
#178    MultiChildRenderObjectElement.inflateWidget (package:flutter/src/widgets/framework.dart:6261:36)
#179    MultiChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:6272:32)
...     Normal element mounting (391 frames)
#570    Element.inflateWidget (package:flutter/src/widgets/framework.dart:3631:14)
#571    Element.updateChild (package:flutter/src/widgets/framework.dart:3383:18)
#572    RenderObjectToWidgetElement._rebuild (package:flutter/src/widgets/binding.dart:1198:16)
#573    RenderObjectToWidgetElement.mount (package:flutter/src/widgets/binding.dart:1167:5)
#574    RenderObjectToWidgetAdapter.attachToRenderTree.<anonymous closure> (package:flutter/src/widgets/binding.dart:1112:18)
#575    BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2531:19)
#576    RenderObjectToWidgetAdapter.attachToRenderTree (package:flutter/src/widgets/binding.dart:1111:13)
#577    WidgetsBinding.attachRootWidget (package:flutter/src/widgets/binding.dart:944:7)
#578    WidgetsBinding.scheduleAttachRootWidget.<anonymous closure> (package:flutter/src/widgets/binding.dart:924:7)
(elided 11 frames from class _RawReceivePortImpl, class _Timer, dart:async, and dart:async-patch)
====================================================================================================

我怎么解决这件事

Answer 1

它显示了更改后的错误来自何处：

类型'String?‘是可空的，而“String”不是。

所以，当您像这样键入变量firstName：String? firstName;时，编译器对firstName = null非常满意。

当您像String firstName;一样键入它时，那么它的类型"String" _prevents就不会变成null。

也许您在某个地方输入了应该是String? (或由if获取)的内容，如String，f.e。在……里面

factory User.fromJson(Map<String, dynamic> json) => _$UserFromJson(json);

Answer 2

在init方法中调用getUser，但是由于它是异步的并返回一个未来，所以需要等待。否则，当您的构建方法被调用时，您的代码将无法完成，并且您的变量仍然被设置为它们的默认值(这意味着您的late变量从未在这些实例上被初始化过，并且得到了所看到的错误)。

由于不能在本身不是await的方法中使用async关键字(而initState不是)，所以需要找到一种解决方法。最常见的是使用FutureBuilder小部件。

您可以在这里找到一个通用设置的好例子：What is a Future and how do I use it?