How to fetch an array data from API and map it to the dart object?

Question

I'm trying to use Flutter documentation to map an array data (comes from API) to the dart object. The documentation uses a single Json object, not an array. I have the following codes:

Json data:

[
    {
        "channelId" :   1
        "channelTitle"  :   "Photos"
        "channelImage"  :   pr01.jpg
        "channelLastPost"   :   null
        "lastUpdate"    :   null
        "userRef"   :   1
    },
    {
        "channelId" :   2
        "channelTitle"  :   "Science"
        "channelImage"  :   pr02.jpg
        "channelLastPost"   :   "For test ...."
        "lastUpdate"    :   "2023-01-03"
        "userRef"   :   1
    }

]

ChannelListModel.dart:

class ChannelListModel {
  String creator;
  String? image;
  String title;
  String lastPost;
  String lastUpdate;

  ChannelListModel(
      {required this.creator,
      required this.image,
      required this.title,
      required this.lastPost,
      required this.lastUpdate});

  factory ChannelListModel.fromJson(Map<String, dynamic> json) {
    return ChannelListModel(
        creator: json['userRef'],
        image: json['channelImage'],
        title: json['channelTitle'],
        lastPost: json['channelLastPost'],
        lastUpdate: json['lastUpdate']);
  }
  Map<String, dynamic> toJson() {
    return {
      "userRef" : creator,
      "channelImage" : image,
      "channelTitle" : title,
      "channelLastPost" : lastPost,
      "lastUpdate" : lastUpdate
    };
  }
}

HttpRequest.dart:

class HttpServices {
  Future<List<ChannelListModel>> getChannelList() async {
    var url = base.BaseURL.channelListUrl;
    final response = await http.get(Uri.parse(url));
    if (response.statusCode == 200) {
      // If the server did return a 200 OK response,
      // then parse the JSON.
      return List<ChannelListModel>.fromJson(jsonDecode(response.body)); //I have problem in this line
    } else {
      // If the server did not return a 200 OK response,
      // then throw an exception.
      throw Exception('Failed to load album');
    }
  }
}

ChannelPage.dart:

class _ChannelsState extends State<Channels> {

  List<ChannelListModel> channels = [];

  @override
  void initState() {
    super.initState();
    channels  = getChannelsFromHttp(); // A valid array object needs to be provided here.
  }

  getChannelsFromHttp()async{
    var httpService = HttpServices();
    var result = await httpService.getChannelList();
    return result;
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: ListView.builder(
        itemCount: channels.length,
        itemBuilder: (context, index) =>
            ChannelCard(channelModel: channels[index]),
      ),
    );
  }
}

I want to classify my codes so I decided to provided different dart files for each part. How can I fix my code?

Reminds me a bit of swift when you try to do that: List<hannelListModel>.fromJson(jsonDecode(response.body)); where elements conform to Codable, so that they can automatically get decoded. Anyway the code in ChannelList is correct (yet it should be renamed Channel). you should cast the result of jsonDecode(response.body) as a List<Map<String, dynamic>> data. below that, you make a for loop, for ( Map<String, dynamic> chunk in data) and there you can decode each chunk individually with this ChannelListModel.fromJson method of yours — Crocobag
– Crocobag, Commented Jan 13, 2023 at 8:08

rrttrr · Accepted Answer · 2023-01-13 08:44:45Z

1

Instead of

returnList<ChannelListModel>.fromJson(jsonDecode(response.body));

Try this code,

List<ChannelListModel> channels = [];
final res = jsonDecode(response.body);
channels.addAll(List<ChannelListModel>.from(
  (res).map((x) => ChannelListModel.fromJson(x))));
return channels;

Added based on comments

@override
  void initState() {
    super.initState();
    getChannelsFromHttp();
  }

  getChannelsFromHttp()async{
    var httpService = HttpServices();
    var result = await httpService.getChannelList();
    setState((){
     channels = result;
    });
  }

edited Jan 13, 2023 at 8:44

answered Jan 13, 2023 at 8:07

rrttrr

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2 Comments

Alex Wright Over a year ago

I tested your solution. In ChannelPage.dart, When I use channels = getChannelsFromHttp() the emulator screen gets red and says: The Future<dynamic> is not a subtype of List<ChannelListModel> . How can I fix it?

rrttrr Over a year ago

Added that to the answer.

Soliev · Accepted Answer · 2023-01-13 08:45:54Z

Your fromJson factory returns single ChannelListModel. You can't use List<ChannelListModel>.fromJson. Instead iterate through List and convert each json to ChannelListModel

class HttpServices {
  Future<List<ChannelListModel>> getChannelList() async {
    var url = base.BaseURL.channelListUrl;
    final response = await http.get(Uri.parse(url));
    if (response.statusCode == 200) {
      // If the server did return a 200 OK response,
      // then parse the JSON.
      //return List<ChannelListModel>.fromJson(jsonDecode(response.body));
      final data = jsonDecode(response.body) as List<dynamic>;
      return data.map((e) => ChannelListModel.fromJson(e as Map<String, dynamic>))
        .toList();

    } else {
      // If the server did not return a 200 OK response,
      // then throw an exception.
      throw Exception('Failed to load album');
    }
  }
}

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