Possible to 'pass-through' values that are not proper JSON?

[fkettelhoit]

I am working with lots of different data classes that need to get serialized to/from Map<String, Object>, which mostly contain serializable data types as values, but also functions and other classes that cannot and should not be converted to/from JSON-compatible types but just 'passed through' as they are (but not ignored either). Since I'm not using the resulting Map as a proper JSON, I don't want the serializer to touch any type that it does not know how to handle but not drop it during a 'serialization' round trip either. Is it possible to do something like this using json_serializable?

I tried to use the following JsonConverter:

class Passthrough<T> implements JsonConverter<T, Object> {
  const Passthrough();

  @override
  T fromJson(Object json) => json as T;

  @override
  Object toJson(T json) => json;
}

typedef SomeFnType = void Function();

@JsonSerializable()
class Foo {
  @JsonKey()
  @Passthrough()
  final SomeFnType someFn;

  Foo(this.someFn);
}

However this causes the build runner to fail (I assume because the resulting type is still not JSON-compatible):

[SEVERE]json_serializable:json_serializable on lib/src/some_file.dart: Could not generate `fromJson` code for `someFn`.
None of the provided `TypeHelper` instances support the defined type.
package:some_package/src/some_file.dart:44:20
   ╷
44 │   final SomeFnType someFn;
   │                    ^^^^^^
   ╵

Is it possible to do this using TypeHelpers? If so, is there any documentation on how to provide custom TypeHelpers?

[kevmoo]

Have you tried a custom toJson that just returns the value?

[fkettelhoit]

That wouldn't work since I would have to write a separate toJson for every type (and the types are usually function types so there's not much opportunity for sharing a toJson implementation). I tried passing an identity function but even that doesn't work since JsonKey(toJson: ...) only works with a dynamically typed toJson implementation, not a generically typed one and thus the generated files are full of type errors.

Is it still possible to use TypeHelpers in the current version of json_serializable? I read that JsonConverters were introduced to reduce the need for TypeHelpers, but I don't think JsonConverters or toJson work in this scenario, so any pointers to relevant documentation about TypeHelpers would be appreciated.

[hacker1024]

#783