class CustomIntParser extends JsonConverter<int, Object?> {
  const CustomIntParser();
  @override
  int fromJson(Object? json) {
    if (json is int) return json;
    if (json is String) return int.parse(json);
    if (json is double) return json.toInt();

    throw FormatException("can't parse $json to int");
  }

  @override
  Object? toJson(int object) {
    return object.toString();
  }
}

class CustomNullableIntParser extends JsonConverter<int?, Object?> {
  const CustomNullableIntParser();
  @override
  int? fromJson(Object? json) {
    if (json == null) return null;
    return CustomIntParser().fromJson(json);
  }

  @override
  Object? toJson(int? object) {
    return object?.toString();
  }
}

import 'package:json_annotation/json_annotation.dart';
import 'package:your_package/lib/utils/parsers/custom_nullable_int_parser.dart';

part 'some_model.g.dart';

@JsonSerializable()
class SomeModel {
  const SomeModel({
    required this.id,
    required this.amount,
  });

  factory SomeModel.fromJson(Map<String, Object?> json) => _$SomeModel(json);

  @CustomIntParser()
  final int id;

  @CustomNullableIntParser()
  final int? amount;

  Map<String, Object?> toJson() => _$SomeModelToJson(this);
}