Parsing JSON that has a nested array of objects in Dart? - json

Question

I am making a Flutter app and I am using The MovieDB api to get data. When I call the api and ask for a specific movie, this is the general format that I get back:
{
"adult": false,
"backdrop_path": "/wrqUiMXttHE4UBFMhLHlN601MZh.jpg",
"belongs_to_collection": null,
"budget": 120000000,
"genres": [
{
"id": 28,
"name": "Action"
},
{
"id": 12,
"name": "Adventure"
},
{
"id": 878,
"name": "Science Fiction"
}
],
"homepage": "http://www.rampagethemovie.com",
"id": 427641,
"imdb_id": "tt2231461",
"original_language": "en",
"original_title": "Rampage",
...
}
I have setup a model class for to parse this and the class is defined as such:
import 'dart:async';
class MovieDetail {
final String title;
final double rating;
final String posterArtUrl;
final backgroundArtUrl;
final List<Genre> genres;
final String overview;
final String tagline;
final int id;
const MovieDetail(
{this.title, this.rating, this.posterArtUrl, this.backgroundArtUrl, this.genres, this.overview, this.tagline, this.id});
MovieDetail.fromJson(Map jsonMap)
: title = jsonMap['title'],
rating = jsonMap['vote_average'].toDouble(),
posterArtUrl = "http://image.tmdb.org/t/p/w342" + jsonMap['backdrop_path'],
backgroundArtUrl = "http://image.tmdb.org/t/p/w500" + jsonMap['poster_path'],
genres = (jsonMap['genres']).map((i) => Genre.fromJson(i)).toList(),
overview = jsonMap['overview'],
tagline = jsonMap['tagline'],
id = jsonMap['id'];
}
class Genre {
final int id;
final String genre;
const Genre(this.id, this.genre);
Genre.fromJson(Map jsonMap)
: id = jsonMap['id'],
genre = jsonMap['name'];
}
My issue is that I can't get the genre to parse properly from the JSON. When I get the JSON and pass it through my model class, I get the following error:
I/flutter (10874): type 'List<dynamic>' is not a subtype of type 'List<Genre>' where
I/flutter (10874): List is from dart:core
I/flutter (10874): List is from dart:core
I/flutter (10874): Genre is from package:flutter_app_first/models/movieDetail.dart
I thought this would work because I have made a different class for the Genre object and passed in the JSON array as a list. I don't understand how List<dynamic> isn't a child of List<Genre> because doesn't the keyword dynamic imply any object? Does anyone know how to parse a nested JSON array into custom objects?

Answer 1

Try genres = (jsonMap['genres'] as List).map((i) => Genre.fromJson(i)).toList()
The issue: calling map without the cast makes it a dynamic call, which means the return type from Genre.fromJson is also dynamic (not Genre).
Take a look at https://flutter.io/json/ for some hints.
There are solutions, like https://pub.dartlang.org/packages/json_serializable, that makes this much easier

Answer 2

I think JSONtoDart Converter is very useful, Must try...

Answer 3

After receiving the response, first of all, you need to extract the arrays separately. Then you can easily map. This is the way I do it.
List<Attempts> attempts;
attempts=(jsonDecode(res.body)['message1'] as List).map((i) => Attempts.fromJson(i)).toList();
List<Posts> posts;
attempts=(jsonDecode(res.body)['message2'] as List).map((i) => Post.fromJson(i)).toList();
Refer below example.
Future<List<Attempts>> getStatisticData() async {
String uri = global.serverDNS + "PaperAttemptsManager.php";
var res = await http.post(
uri,
headers: <String, String>{
'Content-Type': 'application/json; charset=UTF-8',
},
body: jsonEncode(<String, String>{
'userName': widget.userId,
'subject': widget.subjectName,
'method': "GETPTEN",
}),
);
if (res.statusCode == 200) {
List<Attempts> attempts;
attempts=(jsonDecode(res.body)['message'] as List).map((i) => Attempts.fromJson(i)).toList();
return attempts;
} else {
throw "Can't get subjects.";
}
}
Model Class
class Attempts {
String message, userName, date, year, time;
int status, id, marks, correctAnswers, wrongAnswers, emptyAnswers;
Attempts({
this.status,
this.message,
this.id,
this.userName,
this.date,
this.year,
this.marks,
this.time,
this.correctAnswers,
this.wrongAnswers,
this.emptyAnswers,
});
factory Attempts.fromJson(Map<String, dynamic> json) {
return Attempts(
status: json['status'],
message: json['message'],
id: json['ID'],
userName: json['USERNAME'],
date: json['DATE'],
year: json['YEAR'],
marks: json['MARKS'],
time: json['TIME'],
correctAnswers: json['CORRECT_ANSWERS'],
wrongAnswers: json['WRONG_ANSWERS'],
emptyAnswers: json['EMPTY_ANSWERS'],
);
}
}