Nested Generics

[pythoneer]

i think this his highly related to this issue

but i have some trouble using it for my needs. I either use it wrong or it is currently not possible.

this is an example of what i want to parse

{
  "status": "success",
  "message": null,
  "code": 0,
  "data": {
    "timestamp": 0,
    "result": [
      {
        "id": 409,
        "active": true,
        "accessToken": "12345_abc",
      }
    ]
  }
}

and here the corresponding classes

@JsonSerializable()
class WsResponse<T> extends Object with _$WsResponseSerializerMixin<T> {

  String status;
  String message;
  int code;
  @JsonKey(fromJson: _dataFromJson, toJson: _dataToJson)
  WsData<T> data;

  WsResponse({this.status, this.message, this.code, this.data});

  factory WsResponse.fromJson(Map<String, dynamic> json) => _$WsResponseFromJson<T>(json);
}

here i have an outer "class" with status message and code that works like every ordinary class.

@JsonSerializable()
class WsData<T> extends Object with _$WsDataSerializerMixin<T> {
  int timestamp;
  @JsonKey(fromJson: _resultFromJson, toJson: _resultToJson)
  List<T> result;

  WsData({this.timestamp, this.result});

  factory WsData.fromJson(Map<String, dynamic> json) => _$WsDataFromJson<T>(json);
}

@JsonSerializable()
class WsLogin extends Object with _$WsLoginSerializerMixin {
  int id;
  bool active;

  String accessToken;

  WsLogin({this.id, this.active, this.accessToken,});

  factory WsLogin.fromJson(Map<String, dynamic> json) => _$WsLoginFromJson(json);
}

I omitted the fromJson etc. definitions because i don't know how to express them right either (may be the part where i am wrong)

but if i try _dataFromJson for example

WsData<T> _dataFromJson<T>(Map<String, dynamic> input) {
  return input['data'] as WsData<T>;
}

i get an error with

[SEVERE] json_serializable on lib/webservice/webservice.dart:
Error running JsonSerializableGenerator
Error with `@JsonKey` on `data`. The `fromJson` function `_dataFromJson` return type `WsData<T>` is not compatible with field type `WsData<T>`.
package:crewlove/webservice/webservice.dart:31:13
  WsData<T> data;
            ^^^^

which is surprising for me at first. Omitting the @JsonKey annotations does not help either as i think its necessary for generics. But i don't know how to express my intent here.

My intent is to use it like this

var wsLoginResp = new WsResponse<WsLogin>.fromJson(json.decode(resp));  

[kevmoo]

Update this code:

WsData<T> _dataFromJson<T>(Map<String, dynamic> input) {
  return input['data'] as WsData<T>;
}

to

T _dataFromJson<T>(Map<String, dynamic> input) {
  return input['data'] as T;
}

(Although you'll need to put in some sort of helper here – since the input JSON is not WsData<T> or whatever....

Closing this to get it off my queue. Please reopen if you still have issues...

[aegis123]

I'm trying to achieve something similar to create a generic object that can be used for endpoints that return a collection/page of objects.

{
  "page": 1,
  "results": [
     .... objects ....
  ],
  "total_results": 14,
  "total_pages": 1
}

@JsonSerializable()
class ResponseCollection<T> extends Object
    with _$ResponseCollectionSerializerMixin<T> {
  @JsonKey(name: "page")
  final int page;
  @JsonKey(name: "total_results")
  final int totalResults;
  @JsonKey(name: "total_pages")
  final int totalPages;
  @JsonKey(fromJson: resultFromJson, toJson: _resultToJson)
  final List<T> results;

  ResponseCollection(
      this.page, this.totalResults, this.totalPages, this.results);

  factory ResponseCollection.fromJson(Map<String, dynamic> json) =>
      _$ResponseCollectionFromJson<T>(json);
}

List<T> resultFromJson<T>(List input) {
  // looking for a way to check what T is we can call the correct Class.formJson()
  return input.map((f) {
    if (T is MovieSearchResult) {
      return MovieSearchResult.fromJson(f);
    } else if (T is Movie) {
      return Movie.fromJson(f);
    } else {
      throw Exception("Unknow type");
    }
  });
}

Map<String, dynamic> _resultToJson<T>(List<T> input) => {'results': input};

[kevmoo]

@aegis123 – does that work for you? You'd need to be running the VM w/ --preview-dart-2 so that the generic parameter to resultFromJson shows up...

[aegis123]

Thats should be enable by default in beta 3 right? Or do I have to add --preview-dart-2 to flutter packages pub run build_runner build make it use the dart 2 preview?

[kevmoo]

You need a version of Flutter where build_runner is running w/ --preview-dart-2. That should be coming soon...

[aegis123]

Is that in preview 1 available?

[kevmoo]

I don't think so...

[mecseid]

@kevmoo or @aegis123 : Do you have any working example where the class has a List of generics?

I can't achieve that @aegis123 wants (generic object that can be used for endpoints that return a collection/page of objects).

I checked the examples and the tests of json_serializable, but I can't figure out how to use it with List of generics.

[kevmoo]

Let me hack on this now...

[kevmoo]

I'm working on a fix for this now...

[kevmoo]

Closing this out for #308 – trying to brainstorm a solution. Please comment!