Parse JSON to Dart with nested List objects - json

Question

How to parse from JSON to Dart object including 2 nested classes inside of properties oof the class, Lists?
A JSON object Video that contains a list of Question objects.
Question objects contains a list of Ans objects.
{
"id": 1,
"videoUrl": "https://assets.mixkit.co/videos/preview/mixkit-machinery-of-a-very-close-watch-3673-large.mp4",
"questions": [
{
"id": "0-903669-72-2",
"type": "mc",
"body": "Understand reality truth food agency artist.",
"correctAnswerId": "0-259-85157-4",
"ans": [
{
"id": "0-259-85157-4",
"body": "Play."
},
{
"id": "0-694-71578-6",
"body": "Whose trip."
},
{
"id": "0-13-124278-4",
"body": "Of figure why."
},
{
"id": "0-8169-6726-1",
"body": "Station culture green."
}
]
},
{
"id": "1-872297-31-5",
"type": "mc",
"body": "Especially resource benefit beautiful world six.",
"correctAnswerId": "1-61799-113-9",
"ans": [
{
"id": "0-384-69655-4",
"body": "Form."
},
{
"id": "0-89336-975-6",
"body": "Call."
},
{
"id": "1-61799-113-9",
"body": "Money three young."
},
{
"id": "1-60950-585-9",
"body": "Three threat back."
}
]
},
{
"id": "0-297-13339-X",
"type": "mc",
"body": "Of smile coach second firm ahead.",
"correctAnswerId": "1-916803-19-9",
"ans": [
{
"id": "0-15-955520-5",
"body": "Add old catch."
},
{
"id": "0-606-65499-2",
"body": "Well great task."
},
{
"id": "0-7364-1942-X",
"body": "Arrive resource speech kid."
},
{
"id": "1-916803-19-9",
"body": "Reach brother book."
}
]
},
{
"id": "0-254-52906-2",
"type": "ms",
"correctAnswers": [
"1-146-90255-7",
"0-17-470673-1"
],
"body": "Particularly affect but necessary.",
"ans": [
{
"id": "0-17-278557-X",
"body": "Response bill."
},
{
"id": "0-17-470673-1",
"body": "Attack sister interview."
},
{
"id": "0-16-027096-0",
"body": "Design garden."
},
{
"id": "1-146-90255-7",
"body": "Short break."
}
]
},
{
"id": "0-926285-49-1",
"type": "ms",
"correctAnswers": [
"0-554-50421-9",
"0-294-02768-8"
],
"body": "Experience family training.",
"ans": [
{
"id": "0-8260-5153-7",
"body": "Mouth exist kid."
},
{
"id": "0-294-02768-8",
"body": "Agreement factor."
},
{
"id": "0-554-50421-9",
"body": "Down race professional show."
},
{
"id": "1-124-45547-7",
"body": "Most such onto strategy."
}
]
}
]
}
I tried using this tool after failing to properly do this myself.
https://javiercbk.github.io/json_to_dart/
This is the class definitions I got(I changed some things because of VSCode suggestions).
class Video {
int? id;
String? videoUrl;
List<Question>? questions;
Video({this.id, this.videoUrl, this.questions});
Video.fromJson(Map<String, dynamic> json) {
id = json['id'];
videoUrl = json['videoUrl'];
if (json['questions'] != null) {
questions = <Question>[];
json['questions'].forEach((v) {
questions!.add(Question.fromJson(v));
});
}
}
Map<String, dynamic> toJson() {
final Map<String, dynamic> data = <String, dynamic>{};
data['id'] = id;
data['videoUrl'] = videoUrl;
if (this.questions != null) {
data['questions'] = questions!.map((v) => v.toJson()).toList();
}
return data;
}
}
class Question {
String? id;
String? type;
String? body;
String? correctAnswerId;
List<Ans>? ans;
List<String>? correctAnswers;
Question(
{this.id,
this.type,
this.body,
this.correctAnswerId,
this.ans,
this.correctAnswers});
Question.fromJson(Map<String, dynamic> json) {
id = json['id'];
type = json['type'];
body = json['body'];
correctAnswerId = json['correctAnswerId'];
if (json['ans'] != null) {
ans = <Ans>[];
json['ans'].forEach((v) {
ans!.add(Ans.fromJson(v));
});
}
correctAnswers = json['correctAnswers'].cast<String>();
}
Map<String, dynamic> toJson() {
final Map<String, dynamic> data = <String, dynamic>{};
data['id'] = id;
data['type'] = type;
data['body'] = body;
data['correctAnswerId'] = correctAnswerId;
if (ans != null) {
data['ans'] = ans!.map((v) => v.toJson()).toList();
}
data['correctAnswers'] = correctAnswers;
return data;
}
}
class Ans {
String? id;
String? body;
Ans({this.id, this.body});
Ans.fromJson(Map<String, dynamic> json) {
id = json['id'];
body = json['body'];
}
Map<String, dynamic> toJson() {
final Map<String, dynamic> data = <String, dynamic>{};
data['id'] = id;
data['body'] = body;
return data;
}
}
I load the JSON like this.
getVid() async {
final String response = await rootBundle.loadString('assets/videos.json');
final data = await json.decode(response)['videos'];
List<Video> vids = List<Video>.from(data.map((x) => Video.fromJson(x)));
}
But end up with this error message inside of the console.
Uncaught (in promise) Error: NoSuchMethodError: 'cast'
Dynamic call of null.
Receiver: null
Arguments: []
Your help is greatly appreciated!

Answer 1

This line of code correctAnswers = json['correctAnswers'].cast<String>(); is causing error, as it is not able to cast it as list of string
Try using List.from() or map() or List.of()methods
correctAnswers = List<String>.from(json['correctAnswers']);
Or
correctAnswers = json['correctAnswers'].map((answer) => answer.toString()).toList();
Or
correctAnswers = List<String>.of(json['correctAnswers']);