Eroor : Type '_InternalLinkedHashMap<String, dynamic>' is not a subtype of type 'List<dynamic>' in type cast

Question

I am new to Flutter, I try to fetch data from my api on "10.0.2.2:8000/api/membres" but got error like type List dynamic is not a subtype of type 'List. I'm following the example of flutter: https://flutter.dev/docs/cookbook/networking/fetch-data#complete-example

Please help me to follow a helpful tutorial and tell me how to fix this code.

MyHomePage.dart

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

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

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

class _MyHomePageState extends State<MyHomePage> {

  static const url = "10.0.2.2:8000/api/membres";

  Future<List<Map<String, dynamic>>> _future;
  @override
  void initState() {
    super.initState();
    _future = fetch();
  }

  Future<List<Map<String, dynamic>>> fetch() {
    return http
        .get("http://10.0.2.2:8000/api/membres")
        .then((response) {
          return response.statusCode == 200
              ? response.body
              : throw 'Error when getting data';
        })
        .then((body) => json.decode(body)) //
        .then((list) => (list as List).cast<Map<String, dynamic>>());
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Home'),
      ),
      body: RefreshIndicator(
        onRefresh: () async {
          _future = fetch();
          setState(() {});
          return _future;
          },
        child: FutureBuilder<List<Map<String, dynamic>>>(
          future: _future,
          builder: (context, snapshot) {
            if (snapshot.hasError) {
              return Center(
                child: Container(
                  constraints: BoxConstraints.expand(),
                  child: SingleChildScrollView(
                    physics: AlwaysScrollableScrollPhysics(),
                    child: Text(snapshot.error.toString()),),),);}
            if (!snapshot.hasData) {
              return Center(
                child: CircularProgressIndicator(),
              );}
            return ListView.builder(
              itemCount: snapshot.data.length,
              itemBuilder: (BuildContext context, int index) {
                final item = snapshot.data[index];
                return ListTile(
                  title: Text(item['nom']),
                  subtitle: Text(item['prenom']),
                );
              },
            );
          },
        ),
      ),
    );
  }
}

I tested the API with Postman and it works :

And this is the error :

In the emulator :

Solution

You have a couple of issues...

You should probably tidy up fetch by making it async:

Future<Map<String, dynamic>> fetch() async {
  http.Response response = await http.get('http://10.0.2.2:8000/api/membres');
  if (response.statusCode != 200) return null;
  return json.decode(response.body);
}

If you look at the json you get in postman, you see that the top item is a map - with at least one key membres. That key seems to contain a list of other maps. It looks from your type that you expect to get that list.

If that assumption is correct you could adapt it as follows:

Future<List<Map<String, dynamic>>> fetch() async {
  http.Response response = await http.get('http://10.0.2.2:8000/api/membres');
  if (response.statusCode != 200) return null;
  return List<Map<String, dynamic>>.from(json.decode(response.body)['membre']);
}

Comments

[Yoss]

OMG, finally it works thank you alooooot, But can you explain to me this part of code above and what was the issue please ?

[Richard Heap]

Basically, anything in JSON is either a map (object in json spec), a list (array in json spec) or a value. When you call json.decode you get back something that might be a List or it might be a Map depending on the contents of the json. As you walk through/down the decoded map/list you find more such dynamic things that might be one or the other. Your json definitely had a map at the top, which had a list as its value. You get its value by indexing by the key ['keyName'] but that gives you a List<dynamic> which you can convert with List<T>.from

[Richard Heap]

You were trying to jamb the top level map coming from your json into a list, but it's not a list, it's a map. It's member membre is a list. (No pun intended: member/membre !)

[Yoss]

Okey that's clear ,you mean when there is a key like " membre" i need to use map and then i use the list ?

[Richard Heap]

Yes, if your json text says something like: {"membre":[{"id":"56"},{"id":"57"}]} as it does in this case

[Nirmala Sudhir]

@RichardHeap: I was wondering around how to solve this error "Type '_InternalLinkedHashMap<String, dynamic>' is not a subtype of type 'List<dynamic>' in type cast" for a long time. You saved my day . Thank you Richard :)