如何修复错误代码 "type 'null' is not a subtype of type 'map string dynamic '"

Question

我正在尝试为学校项目打印 API 地震数据，但出现错误：

type 'null' is not a subtype of type 'map string dynamic '.

如何解决这个问题？ API 正常工作。这是我的代码：

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

Future<Album> createAlbum(String events) async {
  final response = await http.get(Uri.parse('https://cdn.knmi.nl/knmi/map/page/seismologie/all_induced.json')); // url api

  if (response.statusCode == 200) {
    // status code moet 200 zijn om uit te printen.

    print(response.body); // in de console word alle data geprint
    final json = jsonDecode(response.body);
    return Album.fromJson(json[0]);

  } else {
    throw Exception(response.statusCode.toString()); // status code word geprint als het niet gelijk is aan 200
  }
}

class Album {
final String events;


  Album({required this.events});

  factory Album.fromJson(Map<String, dynamic> json) {
    return Album(
      events: json['events'] as String,    // variable fact word gepakt om uit te printen

      // place: json['[place]'],
      // generated: json['generated'],
      // coordinates: json['coordinates'],
    );
  }
}

void main() {
  runApp(const MaterialApp(
    title: 'Test', // class om een titel aan te maken in de appbar
    home: FirstRoute(),
  ));
}


class FirstRoute extends StatefulWidget {
  const FirstRoute({Key? key}) : super(key: key);

  @override
  _MyAppState createState() {
    return _MyAppState();
  }
}

class _MyAppState extends State<FirstRoute> {
  final TextEditingController _controller = TextEditingController();
  Future <Album>? _futureAlbum;

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Honden Feiten',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: Scaffold(
        appBar: AppBar(
          title: const Text('Honden Feiten'),
        ),
        body: Container(
          alignment: Alignment.center,
          padding: const EdgeInsets.all(9.0),
          child: (_futureAlbum== null ) ? buildColumn() : buildFutureBuilder(),

        ),
      ),
    );
  }

  Column buildColumn() {
    return Column(
      mainAxisAlignment: MainAxisAlignment.center,
      children: <Widget>[
        Image.network('https://e7.pngegg.com/pngimages/496/352/png-clipart-osaka-earthquake-computer-icons-earthquake-engineering-earthquakes-text-logo-thumbnail.png'),
        ElevatedButton(  // knop met de future album word geprint
          onPressed: () {
            setState(() {
              _futureAlbum = createAlbum(_controller.text); // de class wordt geprint waar de variable van de API in staan.
            });
          },
          child: const Text('Klik hier voor een honden feitje! (in het engels)'),

        ),
      ],
    );
  }
// error code of niet
  FutureBuilder<Album> buildFutureBuilder() {
    return FutureBuilder<Album>(
      future: _futureAlbum,
      builder: (context, snapshot) {
        if (snapshot.hasData) {
          return Text(snapshot.data!.events );


        } else if (snapshot.hasError) {
          return Text('${snapshot.error}');
        }

        return const CircularProgressIndicator();
      },
    );
  }
}

Answer 1

https://github.com/Lavkushwaha/Whosebug_Student

首先你不需要索引在json[0]它不是列表所以你可以直接使用 : 行号 14

return Album.fromJson(json[0]);

然后你会报错：行号 31

_CastError (type 'List<dynamic>' is not a subtype of type 'String' in type cast)

因为事件是列表而不是字符串，而您正在尝试将其映射为字符串。

我告诉你简单的解决方法

从 API 复制 json 响应，转到 QuickType -> select dart，创建它的飞镖模型
在成功响应中直接用你的json响应投模型；
然后您可以在您的应用程序中访问它

如果您还需要什么，请告诉我。

谢谢

为您的项目添加了工作代码

https://github.com/Lavkushwaha/Whosebug_Student

只需创建一个文件earthquake.model.dart粘贴它现在在第 14 行更改 final earthquake = earthquakeFromJson(jsonString);

// To parse this JSON data, do
//
//     final earthquake = earthquakeFromJson(jsonString);

import 'dart:convert';

Earthquake earthquakeFromJson(String str) => Earthquake.fromJson(json.decode(str));

String earthquakeToJson(Earthquake data) => json.encode(data.toJson());

class Earthquake {
    Earthquake({
        this.events,
    });

    final List<Event> events;

    factory Earthquake.fromJson(Map<String, dynamic> json) => Earthquake(
        events: json["events"] == null ? null : List<Event>.from(json["events"].map((x) => Event.fromJson(x))),
    );

    Map<String, dynamic> toJson() => {
        "events": events == null ? null : List<dynamic>.from(events.map((x) => x.toJson())),
    };
}

class Event {
    Event({
        this.date,
        this.depth,
        this.evaluationMode,
        this.lat,
        this.lon,
        this.mag,
        this.place,
        this.time,
        this.type,
    });

    final DateTime date;
    final String depth;
    final EvaluationMode evaluationMode;
    final String lat;
    final String lon;
    final String mag;
    final String place;
    final String time;
    final Type type;

    factory Event.fromJson(Map<String, dynamic> json) => Event(
        date: json["date"] == null ? null : DateTime.parse(json["date"]),
        depth: json["depth"] == null ? null : json["depth"],
        evaluationMode: json["evaluationMode"] == null ? null : evaluationModeValues.map[json["evaluationMode"]],
        lat: json["lat"] == null ? null : json["lat"],
        lon: json["lon"] == null ? null : json["lon"],
        mag: json["mag"] == null ? null : json["mag"],
        place: json["place"] == null ? null : json["place"],
        time: json["time"] == null ? null : json["time"],
        type: json["type"] == null ? null : typeValues.map[json["type"]],
    );

    Map<String, dynamic> toJson() => {
        "date": date == null ? null : "${date.year.toString().padLeft(4, '0')}-${date.month.toString().padLeft(2, '0')}-${date.day.toString().padLeft(2, '0')}",
        "depth": depth == null ? null : depth,
        "evaluationMode": evaluationMode == null ? null : evaluationModeValues.reverse[evaluationMode],
        "lat": lat == null ? null : lat,
        "lon": lon == null ? null : lon,
        "mag": mag == null ? null : mag,
        "place": place == null ? null : place,
        "time": time == null ? null : time,
        "type": type == null ? null : typeValues.reverse[type],
    };
}

enum EvaluationMode { MANUAL }

final evaluationModeValues = EnumValues({
    "manual": EvaluationMode.MANUAL
});

enum Type { INDUCED_EARTHQUAKE }

final typeValues = EnumValues({
    "induced earthquake": Type.INDUCED_EARTHQUAKE
});

class EnumValues<T> {
    Map<String, T> map;
    Map<T, String> reverseMap;

    EnumValues(this.map);

    Map<T, String> get reverse {
        if (reverseMap == null) {
            reverseMap = map.map((k, v) => new MapEntry(v, k));
        }
        return reverseMap;
    }
}

如何修复错误代码 "type 'null' is not a subtype of type 'map string dynamic '"

How to fix error code "type 'null' is not a subtype of type 'map string dynamic '"

api

dynamic

flutter