Extracting an internal JSON object from another JSON object

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?

Dan Field · Accepted Answer · 2018-04-12 03:17:47Z

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.

betorcs · Accepted Answer · 2018-04-12 02:47:33Z

0

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),
    );
  }

answered Apr 12, 2018 at 2:47

betorcs

2,84124 silver badges29 bronze badges

1 Comment

Dan Field Over a year ago

OP also has to do something similar in TweetExtendedEntities.fromJson - can't cast it directly to a List<TweetMedia>. This answer would be a bit more helpful if it at least pointed out how that might work

Shyju M · Accepted Answer · 2018-04-12 06:13:09Z

0

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

answered Apr 12, 2018 at 6:13

Shyju M

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