[Feature Request] Custom fromJson for JsonEnum

[farukprogrammer]

I want to track when unknownValue occurs when parsing fromJson for enum type, so that I can fix the model or tell the BE if something unexpected happen.

In order to do that, I guess we need to be able to inject custom deserialization method, not just mapping. Just like fromJson and toJson in JsonKey.

[jellynoone]

I have a similar use case, I'd like to handle unknown type or be able to specify an on unknown type value. Something along the lines:

@JsonEnum(onUnknownEnumValue: MyFormType.unknown)
enum MyFormType {
  checkbox, 
  radio,
  unknown,
}

Meaning, if the value in serialized format is 'hidden', its converted to MyFormType.unknown.

I actually see the $enumDecode has an unknownValue paramete that does just that. I just don't know how to cause the generator to use it.

Also:
From previous issues: #778, #653
There seems to be a want to be able to convert enums manually as well ie. have a generated function. Would it be possible to mark a the decoding and encoding functions to be public? Or maybe be able to set them a name?
This way we could optimize away the $enumDecode function and reuse the decoders/encoders throughout the codebase.

Example:

// my_form_type.dart

part 'my_form_type.g.dart';

@JsonEnum(
  alwaysCreate: true,
  onUnknownEnumValue: MyFormType.unknown,
  generateEncoderWithName: 'myFormTypeToValue',
  generateDecoderWithName: 'myFormTypeFromValue',
)
enum MyFormType {
  checkbox,
  radio,
  unknown,
}

// my_form_type.g.dart
MyFormType myFormTypeToValue(String json) {
  switch (json) {
    case 'checkbox':
      return MyFormType.checkbox;
    case 'radio':
      return MyFormType.radio;
  }

  return MyFormType.unknown;
}

String myFormTypeFromValue(MyFormType object) {
  switch (object) {
    case MyFormType.checkbox:
      return 'checkbox';

    case MyFormType.radio:
      return 'radio';

    case MyFormType.unknown:
      return 'unknown';
  }
}

[jellynoone]

After further digging I saw I missed the JsonKey.unknownEnumValue property. Ignore my previous comment unless the recommended improvements make sense.

I do think unifying enum conversion would be a nice have.