Cannot map JSON to List in Dart using a Flutter project

Question

I was running Flutter 1.0.0 (first release) and recently upgraded to 1.2.1 and there were a lot of errors and warnings that I had to correct. Mostly specifying annotation types. After correcting all of these I ran my Android app and now the code that maps JSON to a List is not working. First I'll post my original code that worked. The JSON data is from an HTTP request.

api.dart

Future<List<Device>> getDevices() async {
  var response = await _httpNetwork.get(_devicesUrl, headers: {"Cookie": _sessionId});

  if (response.statusCode < 200 || response.statusCode > 400) {
    throw Exception("Error while fetching data");
  }

  final body = json.decode(response.body);

  return body.map<Device>((device) => Device.fromJson(device)).toList();
}

device.dart

class Device {
  Device({
    this.id,
    this.name,
    this.uniqueId,
    this.status,
    this.phone,
    this.model,
    this.disabled,
  });

  factory Device.fromJson(Map<String, dynamic> json) => Device(
        id: json['id'],
        name: json['name'],
        uniqueId: json['uniqueId'],
        status: json['status'] == 'online' ? true : false,
        phone: json['phone'],
        model: json['model'],
        disabled: json['disabled'],
      );

  // API data
  int id;
  String name;
  String uniqueId;
  bool status;
  String phone;
  String model;
  bool disabled;

  Map<String, dynamic> toJson() => <String, dynamic>{
        'id': id,
        'name': name,
        'uniqueId': uniqueId,
        'status': status == true ? 'online' : 'offline',
        'phone': phone,
        'model': model,
        'disabled': disabled,
      };
}

Now here is where the issues arises with the following change in api.dart.

return json.decode(response.body).map<Device>((Map<String, dynamic> device) => Device.fromJson(device)).toList();

While this syntax is correct according to Android Studio/Flutter/Dart it doesn't seem to work. The app doesn't crash nor do I get errors in the run console, it just hits my onError code in my Observable<bool>.fromFuture() call.

I put print calls in my code to determine the return statement in api.dart was the culprit. Anyone have any insight into this issue?

This is the JSON data from the HTTP response [{id: 1, name: Pixel, uniqueId: 439610961385665, status: online, phone: 3215551234, model: XL, disabled: false}] — Tool Man
– Tool Man, Commented Apr 18, 2019 at 19:20

Tool Man · Accepted Answer · 2019-04-25 19:20:07Z

4

This is the change that fixes this issue.

return json.decode(response.body).map<Device>((dynamic device) => Device.fromJson(device)).toList();

answered Apr 25, 2019 at 19:20

Tool Man

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Comments

Infusion Analysts · Accepted Answer · 2019-04-17 06:02:36Z

2

Try this way make pojo class for response data..

class UserData {
final int albumId;
final int id;
final String title;
final String url;
final String thumbnailUrl;

UserData({this.albumId, this.id, this.title, this.url, this.thumbnailUrl});

factory UserData.fromJson(Map<String, dynamic> json) {
return new UserData(
    albumId: json['albumId'],
    id: json['id'],
    title: json['title'],
    url: json['url'],
    thumbnailUrl: json['thumbnailUrl']);
}
}

make method for fetch data..

Future<UserData> fetchData() async {
var result = await get('https://jsonplaceholder.typicode.com/photos');

if (result.statusCode == 200) {
 return UserData.fromJson(json.decode(result.body));
} else {
 // If that response was not OK, throw an error.
 throw Exception('Failed to load post');
}
}

now make list object like this way..

 Future<UserData> userDataList;

click on button..

            userDataList = fetchData();

you can also used this below code for list of data..

List<UserData> list = List();
 var isLoading = false;

void fetchData() async {
setState(() {
  isLoading = true;
});
final response = await get("https://jsonplaceholder.typicode.com/photos");
if (response.statusCode == 200) {
  list = (json.decode(response.body) as List)
      .map((data) => UserData.fromJson(data))
      .toList();
  setState(() {
    isLoading = false;
  });
} else {
  throw Exception('Failed to load photos');
}
}

edited Apr 17, 2019 at 6:02

answered Apr 17, 2019 at 5:48

Infusion Analysts

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1 Comment

Tool Man Over a year ago

Can you modify your code so it reflects mine in that it would return Future<List<UserData>>?

primo · Accepted Answer · 2019-04-17 05:25:49Z

1

I think instead of this

final body = json.decode(response.body);
  return body.map<Device>((device) => Device.fromJson(device)).toList();

you should do this

  final body = json.decode(response.body).cast<Map<String, dynamic>>();
 return body.map<Device>((json) => Device.fromJson(json)).toList();

edited Apr 17, 2019 at 5:25

answered Apr 17, 2019 at 5:03

primo

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1 Comment

Tool Man Over a year ago

The code you referenced is from when I was running Flutter 1.0.0 and that code worked. With me moving to Flutter 1.2.1 I was forced to change my code and the return statement towards the end of the posted question is the code that isn't working. Any further insight?

Sami Kanafani · Accepted Answer · 2019-04-18 19:45:37Z

1

I tried this code and it is working, one side note, your status is of type bool but you are giving it a string variable, be aware about that. and also the json.decode(response.body) will return to you the sample response i added in the code, so you don't need to change it. Hope it helps!

class Device {
  Device({
    this.id,
    this.name,
    this.uniqueId,
    this.status,
    this.phone,
    this.model,
    this.disabled,
  });

  factory Device.fromJson(Map<String, dynamic> json) => Device(
        id: json['id'],
        name: json['name'],
        uniqueId: json['uniqueId'],
        status: json['status'] == 'online' ? true : false,
        phone: json['phone'],
        model: json['model'],
        disabled: json['disabled'],
      );

  // API data
  int id;
  String name;
  String uniqueId;
  bool status;
  String phone;
  String model;
  bool disabled;

  Map<String, dynamic> toJson() => <String, dynamic>{
        'id': id,
        'name': name,
        'uniqueId': uniqueId,
        'status': status == true ? 'online' : 'offline',
        'phone': phone,
        'model': model,
        'disabled': disabled,
      };
}




void main() {
  var resp = [{"id": 1, "name": "Pixel", "uniqueId": "439610961385665", 
               "status": "online", "phone": "3215551234", "model": "XL", "disabled": false}];

  List json = resp;
  for(var item in json){
    Device device  = Device.fromJson(item);
    print("nameDevice: ${device.status}");
  }

}

answered Apr 18, 2019 at 19:45

Sami Kanafani

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2 Comments

Tool Man Over a year ago

Can you map it to a List<Device> in your for loop?

Tool Man Over a year ago

Also status is assigned a bool status: json['status'] == 'online' ? true : false,

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