Unable to skip empty List when serializing JSON with JsonConverter

[NoBrainer]

Context

I got my desired behavior to work with @JsonKey, but I couldn't get it to work with a JsonConverter. (I think this should be a solution to a related issue, but it doesn't work as I expect.)

Here is my source code:

// my_model.dart

import 'package:equatable/equatable.dart';
import 'package:json_annotation/json_annotation.dart';

part 'my_model.g.dart';

@JsonSerializable(
  explicitToJson: true,
  converters: [
    StringTrimConverter(),
    IgnoreEmptyListConverter(),
    IgnoreEmptyNullableListConverter(),
  ],
  includeIfNull: false,
)
class MyModel extends Equatable {
  final List<int> intList;
  final List<String> strList;
  final String str;
  final String? strOptional;

  @JsonKey(toJson: listToJson)
  final List<int> intListWithJsonKey;

  const MyModel({
    this.intList = const [],
    this.intListWithJsonKey = const [],
    this.strList = const [],
    this.str = '',
    this.strOptional = '',
  });

  @override
  List<Object?> get props => [
    intList,
    intListWithJsonKey,
    strList,
    str,
    strOptional,
  ];

  factory MyModel.fromJson(Map<String, dynamic> json) =>
      _$MyModelFromJson(json);

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

List? listToJson(List list) => list.isEmpty ? null : list;

/// Ensure Strings are trimmed when serialized and deserialized.
class StringTrimConverter implements JsonConverter<String, String> {
  const StringTrimConverter();

  @override
  String fromJson(String json) => json.trim();

  @override
  String toJson(String str) => str.trim();
}

/// Ensure list keys are skipped when serializing JSON
class IgnoreEmptyListConverter implements JsonConverter<List, List?> {
  const IgnoreEmptyListConverter();

  @override
  List fromJson(List? json) => json ?? [];

  @override
  List? toJson(List list) => list.isEmpty ? null : list;
}

/// Ensure list keys are skipped when serializing JSON
class IgnoreEmptyNullableListConverter implements JsonConverter<List?, List?> {
  const IgnoreEmptyNullableListConverter();

  @override
  List? fromJson(List? json) => json ?? [];

  @override
  List? toJson(List? list) => (list == null || list.isEmpty) ? null : list;
}

And here is my generated code (after running dart run build_runner build):

// my_model.g.dart

// GENERATED CODE - DO NOT MODIFY BY HAND

part of 'my_model.dart';

// **************************************************************************
// JsonSerializableGenerator
// **************************************************************************

MyModel _$MyModelFromJson(Map<String, dynamic> json) => MyModel(
  intList:
      (json['intList'] as List<dynamic>?)
          ?.map((e) => (e as num).toInt())
          .toList() ??
      const [],
  intListWithJsonKey:
      (json['intListWithJsonKey'] as List<dynamic>?)
          ?.map((e) => (e as num).toInt())
          .toList() ??
      const [],
  strList:
      (json['strList'] as List<dynamic>?)
          ?.map((e) => const StringTrimConverter().fromJson(e as String))
          .toList() ??
      const [],
  str: json['str'] == null
      ? ''
      : const StringTrimConverter().fromJson(json['str'] as String),
  strOptional:
      _$JsonConverterFromJson<String, String>(
        json['strOptional'],
        const StringTrimConverter().fromJson,
      ) ??
      '',
);

Map<String, dynamic> _$MyModelToJson(MyModel instance) => <String, dynamic>{
  'intList': instance.intList,
  'strList': instance.strList.map(const StringTrimConverter().toJson).toList(),
  'str': const StringTrimConverter().toJson(instance.str),
  'strOptional': ?_$JsonConverterToJson<String, String>(
    instance.strOptional,
    const StringTrimConverter().toJson,
  ),
  'intListWithJsonKey': ?listToJson(instance.intListWithJsonKey),
};

Value? _$JsonConverterFromJson<Json, Value>(
  Object? json,
  Value? Function(Json json) fromJson,
) => json == null ? null : fromJson(json as Json);

Json? _$JsonConverterToJson<Json, Value>(
  Value? value,
  Json? Function(Value value) toJson,
) => value == null ? null : toJson(value);

The Good

• StringTrimConverter works on a String.
• StringTrimConverter works on a List<String>.
• includeIfNull: false works on a String?.
• @JsonKey(toJson: listToJson) works on an individual List field.
  • Note: This is the behavior I want with a JsonConverter.

The Problem

• IgnoreEmptyListConverter is not used at all.
• IgnoreEmptyNullableListConverter is not used at all.

1. How do I do make this use one of the unused converters? (In other words, how do I do what I'm doing with @JsonKey without having to put @JsonKey on every List field?)
2. Is there something I need to change with one of the converters to make it work?

Other Details

Here's the relevant part of my pubspec.yaml:

name: app
description: "demo"
publish_to: 'none'

version: 0.0.0+1

environment:
  sdk: ^3.10.8

dependencies:
  flutter:
    sdk: flutter

  equatable: ^2.0.8
  json_annotation: ^4.11.0
  meta: ^1.17.0
  yaml: ^3.1.2
  yaml_writer: ^2.0.0

dev_dependencies:
  flutter_test:
    sdk: flutter

  flutter_lints: ^6.0.0

  build_runner: ^2.10.5
  json_serializable: ^6.12.0