Say one has an abstract Car class with a derived Cabrio class.
From a REST api he recieves a JSON with data
abstract class Car {
int id;
String name;
String description;
Car({
this.id,
this.name,
this.description,
});
factory Car.fromJson(Map<String, dynamic> json, String type) {
Car car;
if (type == 'cabrio') car = Cabrio.fromJson(json);
// other possible if-statements
car.id = int.parse(json['id']);
car.name = json['name'];
car.description = json['description'];
return car;
}
class Cabrio extends Car {
String roofMaterial;
String someOtherProp;
Cabrio({
id,
name,
this.roofMaterial,
this.someOtherProp
}) : super(
id: id,
name: name,
description: description);
factory Cabrio.fromJson(Map<String, dynamic> json) =>
Cabrio(
roofMaterial: json['roof_material'],
someOtherProp: json['some_other_prop']
);
}
dynamic dyn = jsonDecode(response.body);
Cabrio cabrio = Car.fromJson(dyn[0], 'cabrio');
cabrio.roofMaterial // null
cabrio.someOtherProp // null
return cabrio;
Why is cabrio.roofMaterial or cabrio.someOtherProp null ?
Why I am taking this approach
I didn't like seeing for example
id: json['id']
in all derived classes. This approach is to prevent such redundancy
What I know
according to the debugger, the properties of the derived class Cabrio are set correctly by the json values in it's fromJson
when inspecting the car object at car.name = json['name'] the derived class' properties (like cabrio.roofMaterial) are already null
What I consider to be a problem
at
if (type == 'cabrio') car = Cabrio.fromJson(json, type);
I am 'pushing' a cabrio object into a Car object (which has less properties than Cabrio). But that should not be wrong since it's just a parent class type
What you're needing in this example is an explicit type cast after you call Car.fromJson(...) to ensure that the type you're dealing with is Cabrio which has the extra fields rather than an instance of Car
final cabrio = Car.fromJson(json, 'cabrio') as Cabrio;
I spent some time to update your code to a newer version of Dart with the changes required to ensure that these fields were no longer null
https://gist.github.com/MarkOSullivan94/60ce6625538e16f373c5c1d6254952e9
Related
im new to Dart-flutter.
i have watching a tutorial video from udemy course writing in dart pad
so i also wrote in dartpad. but it showing error in it.
this is the code wrote in udemy..
import 'dart:convert';
void main(){
var rawJson = '{"url": "https://helo.com","id": 2}';
var parsedJson = json.decode(rawJson);
var imageModel = new ImageModel.fromJson(parsedJson);
print(imageModel.url);
}
class ImageModel{
int id;
String url;
ImageModel.fromJson(parsedJson) {
id = parsedJson['id'];
url = parsedJson['url'];
}
ImageModel(this.id, this.url);
}
in that video it runs, but for me it shows error as
Error compiling to JavaScript:
Info: Compiling with sound null safety
Warning: Interpreting this as package URI, 'package:dartpad_sample/main.dart'.
lib/main.dart:15:3:
Error: This constructor should initialize field 'id' because its type 'int' doesn't allow null.
ImageModel.fromJson(parsedJson) {
^
lib/main.dart:12:7:
Info: 'id' is defined here.
int id;
^^
lib/main.dart:15:3:
Error: This constructor should initialize field 'url' because its type 'String' doesn't allow null.
ImageModel.fromJson(parsedJson) {
^
lib/main.dart:13:10:
Info: 'url' is defined here.
String url;
^^^
Error: Compilation failed.
i have no idea what the problem is..
can you guys help me to troubleshoot the error
Since you're compiling with null safety, you may change to this:
int? id;
String? url;
For further info about null safety, please refer to this link https://dart.dev/null-safety
Try using null safe code:
eg:
int? id;
String? url;
You will try like this
import 'dart:convert';
class ImageModel{
final int id;
final String url;
ImageModel({required this.id, required this.url});
factory ImageModel.fromJson(Map<String, dynamic> parsedJson) {
return ImageModel(
id: parsedJson['id'] as int,
url: parsedJson['url'] as String,
);
}
}
void main() {
var rawJson = '{"url": "https://helo.com","id": 2}';
var parsedJson = json.decode(rawJson);
print(parsedJson['url']);
print(parsedJson['id']);
}
You are probably watching tutorial that was recorded before null-safety introduction in Dart (null-safety is a Dart feature that helps prevent NullRefference exceptions in runtime).
You can either go with nullable values (like #Sowat Kheang suggested in another answer) for id and url in ImageModel class (change String to String? and int to int?). Then you will be fine, but it defeats the purpose of null-safety.
Or you can change your code so it will handle null values in proper way. Code will look something like this:
import 'dart:convert';
void main(){
var rawJson = '{"url": "https://helo.com","id": 2}';
var parsedJson = json.decode(rawJson);
var imageModel = ImageModel.fromJson(parsedJson);
print(imageModel.url);
}
class ImageModel{
// Make your fields 'final' if you don't plan to change them later
// this will make your life easier and is a good practice.
final int id;
final String url;
// we add curly braces to make arguments named
// and add 'required' keyword to indicate that constructor can't be called
// without actually passing arguments into it
ImageModel({required this.id, required this.url});
// here we use factory constructor that will call our default constructor
// it's also a good practice to specify type of a variable if you know it (Map<String, dynamic> in this case)
factory ImageModel.fromJson(Map<String, dynamic> parsedJson) {
// since we don't know for sure if 'parsedJson' has key 'id'
// we would add '??' — null checking operator.
// So if parsedJson['id'] is null, value -1 will be passed
// to the constructor. Same with 'parsedJson['url']'.
return ImageModel(
id: parsedJson['id'] ?? -1,
url: parsedJson['url'] ?? '');
}
}
var rawJson = '{"url": "https://helo.com","id": 2}';
Actually it is a Sting Json and Encoding is not perform properly So follow the step..
Map data={"url": "https://helo.com","id": 2};
Encode that data. json.encode(data);
Now the Encoding is done then you can send the data any server if you have..
When you get back data then... json.decode(rawData)
Then parsing work properly
I want to convert a nested model to the correct model depending on the type. I know that #JsonKey can be used to specifically handle certain properties. However, how can I access other properties on fromJson? The fromJson method has to be static, but then I cannot access the other properties. Does anyone have any idea how to solve this?
#JsonSerializable(explicitToJson: true, nullable: true)
class Model {
int type;
#JsonKey(
name: 'action',
fromJson: _convertActionToModel,
)
dynamic action;
Model({this.type, this.action});
factory Model.fromJson(Map<String, dynamic> json) =>
_$ModelFromJson(json);
Map<String, dynamic> toJson() => _$ModelToJson(this);
static dynamic _convertActionToModel(dynamic json) {
switch (type) { // How can i get this type?
case 0:
return OtherModel.fromJson(json as Map<String, dynamic>);
break;
....
}
}
How can I get the type for the switch case?
Your json is just a map<String, Object>, you could do:
var type = json['typeKey'] as String (or int, bool);
I'm working with some complex json in dart, and I have an issue creating objects before I know what type they'll be.
I appreciate the suggestions, but I don't think I completely understand. In the given answer:
var entity = Model();
castToEntity(entity, {'test': 10});
Don't I need to know that it will be a Model class?
What if I have the below two classes:
#JsonSerializable(explicitToJson: true, includeIfNull: false)
class Location {
String id;
String resourceType;
Location({#required this.id, this.resourceType})
factory Location.fromJson(Map<String, dynamic> json) => _$LocationFromJson(json);
Map<String, dynamic> toJson() => _$LocationToJson(this);
}
class Reference {
String reference;
String resourceType;
Location({#required this.reference, this.resourceType}
factory Reference.fromJson(Map<String, dynamic> json) => _$ReferenceFromJson(json);
Map<String, dynamic> toJson() => _$ReferenceToJson(this);
}
And then I query the server, and I don't know what kind of class it will be. It could be a Location, or a Reference, or if it's a list, it could be multiple of both, and I don't know until I've requested it.
var myBundle = Bundle.fromJson(json.decode(response.body));
Each "myBundle.entry" is another resource. I'd like to be able to use information from that resource to define itself. So I could do something like:
myBundle.entry.resourceType newResource = new myBundle.entry.resourceType();
What I'm doing right now is sending it to a function that has all of the possible options predefined:
var newResource = ResourceTypes(myBundle.entry[i].resource.resourceType,
myBundle.entry[i].resource.toJson());
dynamic ResourceTypes(String resourceType, Map<String, dynamic> json) {
if (resourceType == 'Location') return (new Location.fromJson(json));
if (resourceType == 'Reference') return (new Reference.fromJson(json));
}
It was said that there's not reflection in dart, so I didn't know any other way to do it.
As far as I know, it's not possible since Dart doesn't have Reflection like c#, the most close that I can imagine, is using an abstract class that enforces your entity to implement fromJson, and, in that method, you read the Map and put values into fields, like the code below:
abstract class Serializable {
void fromJson(Map<String,dynamic> data);
}
class Model implements Serializable {
int test;
#override
void fromJson(data) {
test = data['test'];
}
}
Serializable castToEntity(Serializable entity, Map<String, dynamic> data) {
return entity..fromJson(data);
}
Now, when you read you database and have the Map, you can call a method generic like:
var entity = Model();
castToEntity(entity, {'test': 10});
print(entity.test);
Where entity is an empty model.
Note: Your fields on entity, cannot be final, since fromJson is an instance method and not a factory method.
I'm wondering how can I parse a nested json to a class with generic types. My intention is to wrap responses from the backend (like loginRespose that contains a token) with a code and a message
I have
class BaseResponse<T>{
int code;
String message;
T responseObject;
BaseResponse.fromJson(Map<String, dynamic> parsedJson)
: code = parsedJson['Code'],
message = parsedJson['Message'],
responseObject = T.fromJson(parsedJson['ResponseObject']); //This is what I'd like to do
}
Obviously the last line throws an error because T doesn't has a named constructor "fromJson".
I tried adding some restrictions to the Type but I didn't find any solutions. Do you have any idea on how to pull this off?
You can't do such thing, at least not in flutter. As dart:mirror is disabled and there's no interface for classes constructors.
You'll have to take a different route.
I'll recommend using POO instead. You would here give up on deserializing responseObject from your BaseResponse. And then have subclass of BaseResponse handles this deserialization
Typically you'd have one subclass per type:
class IntResponse extends BaseResponse<int> {
IntResponse.fromJson(Map<String, dynamic> json) : super._fromJson(json) {
this.responseObject = int.parse(json["Hello"]);
}
}
You can then hide this mess away by adding a custom factory constructor on BaseResponse to make it more convenient to use.
class BaseResponse<T> {
int code;
String message;
T responseObject;
BaseResponse._fromJson(Map<String, dynamic> parsedJson)
: code = parsedJson['Code'],
message = parsedJson['Message'];
factory BaseResponse.fromJson(Map<String, dynamic> json) {
if (T == int) {
return IntResponse.fromJson(json) as BaseResponse<T>;
}
throw UnimplementedError();
}
}
Then either instantiate the wanted type directly, or use the factory constructor :
final BaseResponse foo = BaseResponse.fromJson<int>({"Hello": "42", "Code": 42, "Message": "World"});
You can achieve this with the built_value package (you'll also need built_value_generator and build_runner). Your class will look something like this:
part 'base_response.g.dart';
abstract class BaseResponse<T> implements Built<BaseResponse<T>, BaseResponseBuilder<T>> {
int code;
String message;
T responseObject;
factory BaseResponse([updates(BaseResponseBuilder<T> b)]) = _$BaseResponse<T>;
static Serializer<BaseResponse> get serializer => _$baseResponseSerializer;
}
You will have to run flutter packages pub run build_runner build to make the generated file. Then you use it like this:
BaseResponse baseResponse = serializers.deserialize(
json.decode(response.body),
specifiedType: const FullType(BaseResponse, const [FullType(ConcreteTypeGoesHere)])
);
There's just one more bit of boilerplate you have to take care of. You need another file called serializers.dart. You need to manually add all the classes you want to deserialize here, and also an addBuilderFactory function for each class that takes a type parameter - and for each concrete type you want to use.
part 'serializers.g.dart';
#SerializersFor(const [
BaseResponse,
ConcreteTypeGoesHere,
])
final Serializers serializers = (_$serializers.toBuilder()
..addBuilderFactory(
FullType(BaseResponse, const [const FullType(ConcreteTypeGoesHere)]),
() => new BaseResponseBuilder<ConcreteTypeGoesHere>()
)
..addPlugin(StandardJsonPlugin()))
.build();
Then re-run flutter packages pub run build_runner build
Makes me wish for Gson... :S
Here is my approach:
class Wrapper<T, K> {
bool? isSuccess;
T? data;
Wrapper({
this.isSuccess,
this.data,
});
factory Wrapper.fromJson(Map<String, dynamic> json) => _$WrapperFromJson(json);
Map<String, dynamic> toJson() => _$WrapperToJson(this);
}
Wrapper<T, K> _$WrapperFromJson<T, K>(Map<String, dynamic> json) {
return Wrapper<T, K>(
isSuccess: json['isSuccess'] as bool?,
data: json['data'] == null ? null : Generic.fromJson<T, K>(json['data']),
);
}
class Generic {
/// If T is a List, K is the subtype of the list.
static T fromJson<T, K>(dynamic json) {
if (json is Iterable) {
return _fromJsonList<K>(json) as T;
} else if (T == LoginDetails) {
return LoginDetails.fromJson(json) as T;
} else if (T == UserDetails) {
return UserDetails.fromJson(json) as T;
} else if (T == Message) {
return Message.fromJson(json) as T;
} else if (T == bool || T == String || T == int || T == double) { // primitives
return json;
} else {
throw Exception("Unknown class");
}
}
static List<K> _fromJsonList<K>(List<dynamic> jsonList) {
return jsonList?.map<K>((dynamic json) => fromJson<K, void>(json))?.toList();
}
}
In order to add support for a new data model, simply add it to Generic.fromJson:
else if (T == NewDataModel) {
return NewDataModel.fromJson(json) as T;
}
This works with either generic objects:
Wrapper<Message, void>.fromJson(someJson)
Or lists of generic objects:
Wrapper<List<Message>, Message>.fromJson(someJson)
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?
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.
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),
);
}
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