Extracting an internal JSON object from another JSON object - json

Question

I'm trying to show a list of tweets using Twitter API, using the fromJson factory.
Each tweet object has an extended_entities object which is an array of media objects.
If you're not familiar with the Twitter API you can see all the different objects here.
Here are the models i created in order to achieve this:
class Tweet {
final String createdAt;
final int id;
final String idStr;
final String text;
final String inReplyToStatusIdStr;
final String inReplyToUserIdStr;
final TweetExtendedEntities tweetExtendedEntities;
Tweet(
{this.createdAt,
this.id,
this.idStr,
this.text,
this.inReplyToStatusIdStr,
this.inReplyToUserIdStr,
this.tweetExtendedEntities});
factory Tweet.fromJson(Map<String, dynamic> json) {
return new Tweet(
createdAt: json['created_at'] as String,
id: json['id'] as int,
idStr: json['id_str'] as String,
text: json['text'] as String,
inReplyToStatusIdStr: json['in_reply_to_status_id_str'] as String,
inReplyToUserIdStr: json['in_reply_to_user_id_str'] as String,
tweetExtendedEntities: json['extended_entities'] as TweetExtendedEntities,
);
}
}
class TweetExtendedEntities {
final List<TweetMedia> tweetMedia;
TweetExtendedEntities({this.tweetMedia});
factory TweetExtendedEntities.fromJson(Map<String, dynamic> json) {
return new TweetExtendedEntities(
tweetMedia: json['media'] as List<TweetMedia>);
}
}
class TweetMedia {
final String mediaType;
final String mediaUrl;
TweetMedia({this.mediaType, this.mediaUrl});
factory TweetMedia.fromJson(Map<String, dynamic> json) {
return new TweetMedia(
mediaType: json['type'] as String,
mediaUrl: json['media_url'] as String,
);
}
}
Before i tried to get the extended_entities object everything was fine and i successfully got the JSON data and parsed it, but when i try to get the media objects using the code above, i get this error:
I/flutter (29538): type '_InternalLinkedHashMap<String, dynamic>' is not a subtype of type 'TweetExtendedEntities' in type cast where
I/flutter (29538): _InternalLinkedHashMap is from dart:collection
I/flutter (29538): String is from dart:core
I/flutter (29538): TweetExtendedEntities is from package:pubg_companion/models/tweet.dart
How can i get nested JSON objects using factory or any other way?

Answer 1

Dart has no idea that your JSON structure will correspond to your object, or how. You can't directly cast the JSON (which is probably a Map<String, dynamic>, but could also be other things) to your objects. #betorcs answer is a start in the right direction but needs a bit more.
This line:
tweetExtendedEntities: json['extended_entities'] as TweetExtendedEntities,
Needs to be
tweetExtendedEntities: TweetExtendedEntities.fromJson['extended_entities'],
And your TweetExtendedEntities method should look more like this:
factory TweetExtendedEntities.fromJson(Map<String, dynamic> json) {
return new TweetExtendedEntities(
tweetMedia: createTweetMediaList(json['media']));
}
static List<TweetMedia> createTweetMediaList(List json) {
if (json == null) return null;
if (json.isEmpty) return [];
return json.map((tweetMediaJson) => TweetMedia.fromJson(tweetMediaJson)).toList();
}
You could also certainly look into json_serializable if your needs start getting more complicated and you want to try to generate some of this code.

Answer 2

Your json parameter is Map<String, dynamic>, dynamic is not TweetExtendedEntities, but it can be cast to Map.
factory Tweet.fromJson(Map<String, dynamic> json) {
return new Tweet(
createdAt: json['created_at'] as String,
id: json['id'] as int,
idStr: json['id_str'] as String,
text: json['text'] as String,
inReplyToStatusIdStr: json['in_reply_to_status_id_str'] as String,
inReplyToUserIdStr: json['in_reply_to_user_id_str'] as String,
tweetExtendedEntities: TweetExtendedEntities.fromJson(json['extended_entities'] as Map),
);
}

Answer 3

In flutter, this error will be thrown when you pass a string without json.decode() to the fromjson factory
eg:
Map bodyJson = json.decode(loginResponse.body);
var login = new LoginResponse.fromJson(bodyJson);
The LoginResponse class
class LoginResponse {
int responseCode;
String message;
String responseObject;
LoginResponse();
factory LoginResponse.fromJson(Map<String, dynamic> json) => _$LoginResponseFromJson(json);
}
JSON and serialization in Flutter