无法使用Flutter项目将DSON映射到Dart中的List

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?

Answer 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();

Answer 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');
}
}

Answer 3

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}");
  }

}

Answer 4

这是修复此问题的更改。

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