Flutter 断言失败：布尔表达式不能为空

Question

我在 Vipin Vijayan 的 tutorial on parsing JSON in ListView 上关注 Flutter 的 fetch data from the internet cookbook 并收到以下错误：

═══════════════════════════════════小部件库捕获的异常 ═══════════════════════════════════抛出以下断言 构建 MyApp（脏，状态：_MyAppState#a2d98）：断言失败： 布尔表达式不能为空

相关的导致错误的小部件是 MyApp package:le_moineau/main.dart:35 抛出异常时，这个 是堆栈 #0 _MyAppState.build 包：le_moineau/main.dart:70 #1 StatefulElement.build 包：flutter/…/widgets/framework.dart:4802 #2 ComponentElement.performRebuild package:flutter/…/widgets/framework.dart:4685 #3 StatefulElement.performRebuild 包：flutter/…/widgets/framework.dart:4857 #4 Element.rebuild package:flutter/.../widgets/framework.dart:4379 ...

这是我的 Users.dart 使用 https://app.quicktype.io/ 解析的代码

// To parse this JSON data, do
//
//     final users = usersFromJson(jsonString);

import 'dart:convert';

List<User> usersFromJson(String str) =>
    List<User>.from(json.decode(str).map((x) => User.fromJson(x)));

String usersToJson(List<User> data) =>
    json.encode(List<dynamic>.from(data.map((x) => x.toJson())));

class User {
  User({
    this.userId,
    this.id,
    this.title,
    this.completed,
  });

  int userId;
  int id;
  String title;
  bool completed;

  factory User.fromJson(Map<String, dynamic> json) => User(
        userId: json["userId"],
        id: json["id"],
        title: json["title"],
        completed: json["completed"],
      );

  Map<String, dynamic> toJson() => {
        "userId": userId,
        "id": id,
        "title": title,
        "completed": completed,
      };
}

这是我的 main.dart 代码：

import 'dart:async';
//import 'dart:convert';
import 'dart:io';

import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;
import 'others/Users.dart';

class MyHttpOverrides extends HttpOverrides {
  @override
  HttpClient createHttpClient(SecurityContext context) {
    return super.createHttpClient(context)
      ..badCertificateCallback =
          (X509Certificate cert, String host, int port) => true;
  }
}

Future<List<User>> getUsers() async {
  final response =
      await http.get(Uri.https('jsonplaceholder.typicode.com', 'todos'));

  //await http.get("https://192.168.1.4:8081/json/todos.json");
  if (response.statusCode == 200) {
    // If the server did return a 200 OK response,
    // then parse the JSON.
    final List<User> users = usersFromJson(response.body);
    return users;
  } else {
    // If the server did not return a 200 OK response,
    // then throw an exception.
    throw Exception('Failed to load users');
  }
}

void main() => runApp(MyApp());

class MyApp extends StatefulWidget {
  MyApp({Key key}) : super(key: key);

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

class _MyAppState extends State<MyApp> {
  List<User> _users;
  bool _loading;

  @override
  void initState() {
    HttpOverrides.global = new MyHttpOverrides();
    super.initState();
    _loading = true;
    getUsers().then((users) {
      setState(() {
        _users = users;
        _loading = false;
      });
    });
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Fetch Data Example',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: Scaffold(
        appBar: AppBar(
          title: Text(_loading ? 'Loading...' : 'Users'),
        ),
        body: Container(
          color: Colors.white,
          child: ListView.builder(
              itemCount: null == _users ? 0 : _users.length,
              itemBuilder: (context, index) {
                User user = _users[index];
                return ListTile(
                  title: Text('This is a test'),
                  subtitle: Text(user.title),
                );
              }),
        ),
      ),
    );
  }
}

即使我使用 Vipin 的股票 Widget Build 代码，我也会遇到同样的错误。有人可以看看并给我一些建议吗？

Answer 1

尝试在bool变量中初始化真值

bool _loading = true; 

并删除initState()中的_loading = true;

Answer 2

由于 Flutter 2.0.x 附带 Null Safety，因此不允许使用 bool _loading;，因此您可以使用 bool? _loading; 或 bool _loading = true;。 ? 表示您的变量接受空值。

更新：插入代码

网络服务

class NetService {
...
  static Future<T?> getJson<T>(String url, {int okCode = 200}) {
    return http.get(Uri.parse(url))
      .then((response) {
        if (response.statusCode == okCode) {
          return jsonDecode(response.body) as T;
        }
        _printDataNotOK(response);
        return null;
      })
      .catchError((err) => _printError(err));
  }
....
}

json_placeholder_service.dart

import 'dart:async';

import 'package:jsonplaceholder/services/networking.dart';


class JsonPlaceholderService {
  static const _baseUrl = 'https://jsonplaceholder.typicode.com';
  /* ---------------------------------------------------------------------------- */
  static Future<List?> _doGet(String path) async {
    return await NetService.getJson<List>(_baseUrl + path)
      .whenComplete(() => print('\nFetching done!'));
  }
  /* ---------------------------------------------------------------------------- */
  static Future<List?> fetchTodos() => _doGet('/todos');
}

home_page.dart

import 'package:flutter/material.dart';
import 'package:jsonplaceholder/services/json_placeholder_service.dart';


class HomePage extends StatelessWidget {
  /* ---------------------------------------------------------------------------- */
  const HomePage({Key? key}) : super(key: key);
  /* ---------------------------------------------------------------------------- */
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Hi!'),
        centerTitle: true,
      ),
      body: SingleChildScrollView(
        child: FutureBuilder<List?>(
          future: JsonPlaceholderService.fetchTodos(),
          builder: (context, snapshot) {
            if (snapshot.hasError) return Text(snapshot.error.toString());
            if (snapshot.connectionState == ConnectionState.done) {
              var data = snapshot.data;
              if (data != null && data.isNotEmpty) {
                return Column(
                  crossAxisAlignment: CrossAxisAlignment.start,
                  children: List<Widget>.generate(data.length, (i) {
                    var item = data[i];
                    return Text('${item['userId']} - ${item['id']} - ${item['title']}');
                  }),
                );
              }
              return Text('No data received!');
            }
            return CircularProgressIndicator();
          },
        ),
      ),
    );
  }
}

结果：