Determine if the hand-written well-known types are necessary

[apple]

Determine if the hand-written well-known types are necessary

[tbkka]

Wrappers could easily be implemented as a generated base + extensions for the custom JSON behaviors. Someone recently asked me about "easy starter projects" -- I think that would be one.

I suspect Struct and even Any could also be done that way, though Any might require some adjustments to the rest of the library to expose the right customization points.

[allevato]

The NullValue enum in struct.proto is going to make this a little trickier. Since it's an enum and the json property is generated directly on the type, there's no way to have an extension replace it. (Aside: Even if we extract JSON support into separate extensions, we can't introduce our own hand-written extension on the same type without it causing an ambiguity.)

I haven't thought too deeply about how to solve this yet, but the options that come to mind feel like hacks:

1. Add a method to ProtoEnum like remapJSONName(_ value: Self, _ name: String) and have the generated code call it, passing it the JSON string for the case. The default implementation in the protocol extension would just return the string verbatim. A hand-written extension on concrete enum types could then override it and return something else (in this case, "null").
2. Just add a special case for Google_Protobuf_NullValue in the JSON serialization code.
3. Other options...?

Any option will pick will likely have a small cost in terms of runtime library size, generated code size, and execution speed. Hopefully they're negligible; in terms of execution speed at least, surely the rest of the string processing will dominate whatever we add.

[tbkka]

Since it's an enum and the json property is generated directly on the type, there's no way to have an extension replace it.

The technique I used to allow overriding generated methods on messages could work here. In the library, define:

extension ProtobufEnum {
   var json: String {return _protoc_generated_json}
}

The generator then emits a definition for _protoc_generated_json instead of defining json directly. Then we can override json in an extension (struct/enum extensions can override protocol extensions).

[thomasvl]

@allevato is working on this for Struct. Once that lands, I've got an idea for how we might be able to do this for Any.

[thomasvl]

#387 removed the Any version. This is now done.

[allevato]

🎉 🎉 🎉

[tbkka]

Good work!