Flutter The argument type 'String' can't be assigned to the parameter type 'Map<String, dynamic>'

user_15604147 0
Viewed 2166

I got the problem when I want to fetch Data from PONS API.

import 'package:eng_it_translator/model/model.dart';
import 'package:http/http.dart';
import 'dart:convert';

import 'package:http/http.dart' as http;

class Network {
  Future<Vocabulary> fetchData({String keyword}) async {
    var headers = {
      'X-Secret':
          'XXXXXXXXXXXXX',
    };

    var params = {
      'q': 'house',
      'l': 'deen',
    };
    var query = params.entries.map((p) => '${p.key}=${p.value}').join('&');
    var url = Uri.parse("https://api.pons.com/v1/dictionary?$query");

    var res = await http.get(url, headers: headers);
    if (res.statusCode == 200) {
      var data = json.decode(res.body);
      var realdata =
          List<Vocabulary>.from(data).map((x) => Vocabulary.fromJson(x));
      print(realdata);
    } else {
      throw Exception('http.get error: statusCode= ${res.statusCode}');
    }
  }
}

The argument type 'Vocabulary' can't be assigned to the parameter type 'Map<String, dynamic>'.

Body of DATA is in Map structure, but I can't transform it to List.

class Vocabulary {
  String lang;
  List<Hits> hits;

  Vocabulary({this.lang, this.hits});

  Vocabulary.fromJson(Map<String, dynamic> json) {
    lang = json['lang'];
    if (json['hits'] != null) {
      hits = new List<Hits>();
      json['hits'].forEach((v) {
        hits.add(new Hits.fromJson(v));
      });
    }
  }
flutter fetch api
3 Answers

I think you must convert json to Map try this

       hits: json['hits'] != null
      ? json['hits'].map<Hits>((v) => new Hits.fromJson(v)).toList()
user_135367860

You can convert your Vocabulary data to List by adding toList()

List.from(data)
              .map((x) => Vocabulary.fromJson(x))
              .toList());
user_83023600

There is a reason for the error. If you print res.body you will find that it is a JSON object. JSON and JSON object is different in a way that in JSON there needs to be "" around key also. for e.g.

name:"Pramod" // valid JSON object. Invalid JSON
"name":"Pramod" // valid JSON. valid json object.

To solve your problem, first convert res.body to valid JSON.

String s = json.encode(res.body);

Then covert it to JSON object you would like

Student student = Student.fromJson(jsonDecode(s)); // there is no typo here

Note: jsonDecode comes from dart:convert

user_31397790
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