genericArgumentFactories: support any variable name for fromJsonT/toJsonT

[rrousselGit]

Currently, it appears that genericArgumentFactories works only if the mappers use a very specific name format

Consider the following example:

@JsonSerializable(genericArgumentFactories: true)
class Either<First, Second> {
  Either(this.first, this.second);

  factory Either.fromJson(
    Map<String, Object?> json,
    First Function(Object?) fromJsonFirst,
    Second Function(Object?) fromJsonSecond,
  ) {
    throw UnimplementedError();
  }

  First first;
  Second second;
}

@JsonSerializable()
class Another {
  Another(this.value);

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

  final Either<int, String> value;
}

If we renamed fromJsonFirst to anything else, then json_serializable would fail with:

Expecting a `fromJson` constructor with exactly one positional parameter. The only extra parameters allowed are functions of the form `T Function(Object?) fromJsonT` where `T` is a type parameter of the target type.
package:generic_argument_factory_integration/models.dart:46:18
   ╷
46 │   factory Either.fromJson(

It would be great if we could use any other variable name instead, such as:

  factory Either.fromJson(
    Map<String, Object?> json,
    First Function(Object?) first,
    Second Function(Object?) second,
  ) {
    throw UnimplementedError();
  }

[kevmoo]

PRs welcome! 😄

[rrousselGit]

I may raise a PR, as the current implementation throws a RangeError if the toJson is defined in an interface (which it will with Freezed)

What do you think about simply checking the number of arguments, without checking what they are? Worse case scenario, we'll have a compilation error.

This may improve the developer experience too, because the compilation error will show what the problem is.

For example at some point I made a mistake in the syntax for "toJson". And instead of Object Function(T) I did T Function(Object), but the error message wasn't helpful. On the other hand, a compilation error will clearly indicate the issue.

[kevmoo]

I don't like guessing. I guess i could just look for one function that matches what is needed here.

[Saxorman]

Hello! I have the same warning during code generation.

@JsonSerializable(genericArgumentFactories: true, createToJson: false)
class InheritWrapperField<T> {
  final T value;
  final bool inherit;

  InheritWrapperField(this.value, this.inherit);

  factory InheritWrapperField.fromJson(
          Map<String, dynamic> json, T Function(Object? json) fromJson) =>
      _$InheritWrapperFieldFromJson(json, fromJson);
}

Expecting a `fromJson` constructor with exactly one positional parameter. The only extra parameters allowed are functions of the form `T Function(Object?) fromJsonT` where `T` is a type parameter of the target type.
*/options_field_wrapper.dart:27:31
   ╷
27 │   factory InheritWrapperField.fromJson(
   │                               ^^^^^^^^

If change class name to InheritWrapper - all ok