flutter 试图从 placholder api 发出 json 请求

Question

您好，我正在尝试通过此 URL 'https://jsonplaceholder.typicode.com/todos/1' 发出 JSON 请求并在 listView 构建器上显示数据，但是当我运行什么都不显示，屏幕是空白的，除了应用栏

这是发出请求并在列表中显示数据的主要class

import 'dart:convert';

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

import 'data/note.dart';

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        // This is the theme of your application.
        //
        // Try running your application with "flutter run". You'll see the
        // application has a blue toolbar. Then, without quitting the app, try
        // changing the primarySwatch below to Colors.green and then invoke
        // "hot reload" (press "r" in the console where you ran "flutter run",
        // or simply save your changes to "hot reload" in a Flutter IDE).
        // Notice that the counter didn't reset back to zero; the application
        // is not restarted.
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page3'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  // This widget is the home page of your application. It is stateful, meaning
  // that it has a State object (defined below) that contains fields that affect
  // how it looks.

  // This class is the configuration for the state. It holds the values (in this
  // case the title) provided by the parent (in this case the App widget) and
  // used by the build method of the State. Fields in a Widget subclass are
  // always marked "final".

  final String title;

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

class _MyHomePageState extends State<MyHomePage> {
  List<Note> _notes = List<Note>();

  Future<List<Note>> fetchJson() async {
    var url = ('https://jsonplaceholder.typicode.com/todos/1');
    var response = await http.get(url);

    var notes = List<Note>();

    if (response.statusCode == 200) {
      var jsons = json.decode(response.body);
      for (var jsons2 in jsons) {

        notes.add(Note.fromJson(jsons2));
      }
      return notes;
    } else {
      print('Failed to fetch data');
    }
  }

  @override
  void initState() {
    // TODO: implement initState
    fetchJson().then((value) {
      setState(() {
        _notes.addAll(value);
      });
    });
    super.initState();
  }

  @override
  Widget build(BuildContext context) {

    // This method is rerun every time setState is called, for instance as done
    // by the _incrementCounter method above.
    //
    // The Flutter framework has been optimized to make rerunning build methods
    // fast, so that you can just rebuild anything that needs updating rather
    // than having to individually change instances of widgets.
    return Scaffold(
        appBar: AppBar(
          // Here we take the value from the MyHomePage object that was created by
          // the App.build method, and use it to set our appbar title.
          title: Text(widget.title),
        ),
        body: ListView.builder(
          itemBuilder: (context, index) {
            return Card(
              child: Padding(
                padding: const EdgeInsets.all(16.0),
                child: Column(
                  crossAxisAlignment: CrossAxisAlignment.start,
                  children: <Widget>[
                    Text(
                      _notes[index].text,
                      style: TextStyle(fontSize: 20),
                    ),
                    Text(
                     _notes[index].title,
                      style: TextStyle(fontSize: 15, color: Colors.grey),
                    ),
                  ],
                ),
              ),
            );
          },
          itemCount: _notes.length,
        ));
  }
}

这是第二个 class，它有两个构造函数

class Note{

  String title;
  String text;

  Note(this.text,this.title);

  Note.fromJson(Map<String, dynamic>json){

    title = json['title'];
    text = json['text'];

  }

}

这是我运行应用

时显示的错误

E/flutter (13048): #0      _MyHomePageState.fetchJson (package:learn_json_phase2/main.dart:63:26)
E/flutter (13048): <asynchronous suspension>
E/flutter (13048): #1      _MyHomePageState.initState (package:learn_json_phase2/main.dart:76:5)
E/flutter (13048): #2      StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4355:58)
E/flutter (13048): #3      ComponentElement.mount (package:flutter/src/widgets/framework.dart:4201:5)
E/flutter (13048): #4      Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
E/flutter (13048): #5      Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
E/flutter (13048): #6      SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:5445:14)
E/flutter (13048): #7      Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
E/flutter (13048): #8      Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
E/flutter (13048): #9      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4243:16)
E/flutter (13048): #10     Element.rebuild (package:flutter/src/widgets/framework.dart:3947:5)
E/flutter (13048): #11     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4206:5)
E/flutter (13048): #12     ComponentElement.mount (package:flutter/src/widgets/framework.dart:4201:5)
E/flutter (13048): #13     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
E/flutter (13048): #14     Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
E/flutter (13048): #15     SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:5445:14)
E/flutter (13048): #16     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
E/flutter (13048): #17     Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
E/flutter (13048): #18     SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:5445:14)
E/flutter (13048): #19     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
E/flutter (13048): #20     Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
E/flutter (13048): #21     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4243:16)
E/flutter (13048): #22     Element.rebuild (package:flutter/src/widgets/framework.dart:3947:5)
E/flutter (13048): #23     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4206:5)
E/flutter (13048): #24     StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4381:11)
E/flutter (13048): #25     ComponentElement.mount (package:flutter/src/widgets/framework.dart:4201:5)
E/flutter (13048): #26     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
E/flutter (13048): #27     Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
E/flutter (13048): #28     SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:5445:14)
E/flutter (13048): #29     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
E/flutter (13048): #30     Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
E/flutter (13048): #31     SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:5445:14)
E/flutter (13048): #32     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
E/flutter (13048): #33     Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
E/flutter (13048): #34     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4243:16)
E/flutter (13048): #35     Element.rebuild (package:flutter/src/widgets/framework.dart:3947:5)
E/flutter (13048): #36     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4206:5)
E/flutter (13048): #37     StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4381:11)
E/flutter (13048): #38     ComponentElement.mount (package:flutter/src/widgets/framework.dart:4201:5)
E/flutter (13048): #39     Element.inflateWidget (package:flutter/src/widgets/framework.dart:3194:14)
E/flutter (13048): #40     Element.updateChild (package:flutter/src/widgets/framework.dart:2988:12)
E/flutter (13048): #41     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4243:16)
E/flutter (13048): #42     Element.rebuild (package:flutter/src/widgets/framework.dart:3947:5)
E/flutter (13048): #43     ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:4206:5)
E/flutter (13048): #44     ComponentElement.mount (package:flutter/src/widgets/framework.dart:4201:5)
E/flutter (13048): #45     Element.inflateWidget (packag

Answer 1

似乎响应数据不是列表。

试试这个抓取

 Future<List<Note>> fetchJson() async {
    var url = ('https://jsonplaceholder.typicode.com/todos'); //Change
    var response = await http.get(url);

    var notes = List<Note>();

    if (response.statusCode == 200) {
      var jsons = json.decode(response.body);
      for (var jsons2 in jsons) {

        notes.add(Note.fromJson(jsons2));
      }
      return notes;
    } else {
      print('Failed to fetch data');
    }
  }

Answer 2

https://jsonplaceholder.typicode.com/todos/1 return json 映射 未列出

这里你在地图上循环是不正确的

    for (var jsons2 in jsons) {
        notes.add(Note.fromJson(jsons2));
      }

尝试将 url 更改为 https://jsonplaceholder.typicode.com/todos whcih 将 return 待办事项列表

所以你的代码会变成这样

  var url = ('https://jsonplaceholder.typicode.com/todos/1');

flutter 试图从 placholder api 发出 json 请求

flutter trying to make json request from placholder api

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