Generic JsonConverter Not Working

[yousinix]

• json_annotation: ^4.4.0
• json_serializable: ^6.0.0
• Reproduction repo

---

If we have a simple generic JsonConverter like this:

class IndexedConverter<T> implements JsonConverter<List<T>, List<dynamic>> {
  const IndexedConverter(this.fromJsonConverter);

  final T Function(Map<String, dynamic> onMap) fromJsonConverter;

  @override
  List<T> fromJson(List<dynamic> jsonItems) {
    return jsonItems.asMap().entries.map((entry) {
      final index = entry.key;
      final json = entry.value;
      return fromJsonConverter({'index': index, ...json});
    }).toList();
  }

  // ...
}

Then it was used on a class field like this:

@JsonSerializable()
class Question {
  // ...
  @IndexedConverter(Choice.fromJson)
  final List<Choice> choices;
}

The actual output is:

Question _$QuestionFromJson(Map<String, dynamic> json) => Question(
      choices: (json['choices'] as List<dynamic>)
          .map((e) => Choice.fromJson(e as Map<String, dynamic>))
          .toList(),
    );

While the expected output is:

Question _$QuestionFromJson(Map<String, dynamic> json) => Question(
      choices: IndexedConverter(Choice.fromJson).fromJson(json['choices'] as List<dynamic>),
    );

[qevka]

I had the same problem. Still looking for a solution.

[kevmoo]

Try using JsonKey(fromJson) instead?

@JsonSerializable()
class Question {
  const Question({
    required this.choices,
  });

  @JsonKey(fromJson: _choiceListFromJson)
  final List<Choice> choices;
}

List<Choice> _choiceListFromJson(List<dynamic> items) =>
    const IndexedConverter<Choice>(Choice.fromJson).fromJson(items);

[yousinix]

This is a workaround, but it doesn't fix the issue.

This issue is about annotating members directly with IndexedConverter, without the need to define a custom fromJson function for each one. Using this workaround contradicts with the idea of Custom Converters.

[DevNico]

I just wanted to open a similar issue. I don't feel like this is resolved. Can this issue get some attention again please?

[kevmoo]

Please post the code you'd like to write here ⬇️

[yousinix]

@kevmoo I have attached a reproduction repo in the issue's description, it might be helpful.

[kevmoo]

Could you try upgrading to Dart 2.17?

I just ran your reproduction on my machine – everything ran cleanly!

[tlserver]

I am facing the same issue in json_annotation 4.8.1, json_serializable 6.7.1 and dart 3.1.5. Here is a MRE:

import 'package:json_annotation/json_annotation.dart';

part 'test.g.dart';

typedef JsonObject = Map<String, dynamic>;

// A data class to be converted
class UselessGeneric<T> {
  final T value;

  const UselessGeneric(this.value);
}

// The json converter which does not work currently
class UselessGenericJsonConverter<T>
    extends JsonConverter<UselessGeneric<T>, T> {
  const UselessGenericJsonConverter();

  @override
  UselessGeneric<T> fromJson(T json) => UselessGeneric(json);

  @override
  T toJson(UselessGeneric<T> object) => object.value;
}

// Workaround json converter
class StringUselessGenericJsonConverter
    extends UselessGenericJsonConverter<String> {
  const StringUselessGenericJsonConverter();
}

@JsonSerializable()
@UselessGenericJsonConverter<String>() // Not Work : Exception when `dart run build_runner build`
// @StringUselessGenericJsonConverter() // This work
class Example {
  final UselessGeneric<String> text;

  Example(this.text);

  factory Example.fromJson(JsonObject json) => _$ExampleFromJson(json);

  Map<String, dynamic> toJson() => _$ExampleToJson(this);
}

I think @UselessGenericJsonConverter<String>() and @StringUselessGenericJsonConverter() should have exactly the same behavior, but actually not.

[black-rusuz]

Having the same problem.

I wrote in my model.dart:

    @Default(0) @cDouble double price,
    @cDouble double? oldPrice,

But in model.g.file:

      price: (json['price'] as num?)?.toDouble() ?? 0,
      oldPrice: cDouble.fromJson(json['old_price']),

What's interesting is that price has @Default annotation, but oldPrice doesn't.
Here's my converter:

const _digits = 6;
const cDouble = DoubleConverter();

class DoubleConverter extends JsonConverter<double?, dynamic> {
  const DoubleConverter();

  @override
  double? fromJson(dynamic json) {
    return parseDouble(json);
  }

  @override
  dynamic toJson(double? object) {
    final string = object?.toStringAsFixed(_digits) ?? '';
    return double.tryParse(string);
  }
}

double? parseDouble(dynamic value) {
  return value is num
      ? double.parse(value.toStringAsFixed(_digits))
      : double.tryParse(value.toString());
}

UPD: versions
Flutter 3.16.0
Dart 3.2.0
json_serializable: 6.7.1