[WIP] tsfn: Add context to [Non]BlockingCall callback

[KevinEady]

This is WIP, currently missing the overload functionality for [Non]BlockingCall(DataType*, Callback), to allow the Callback to have either signature void(Napi::Env, Function, DataType*) [existing functionality] or void(Napi::Env, Function, DataType*, void*) [new context param]

Todo:

• Add [Non]BlockingCall with DataType* overloads
• Update docs

Fixes: #653

[KevinEady]

I ran into this problem when finishing up the PR and now stuck...

Basically, the [Non]BlockingCall with data overload currently has this signature:

template <typename DataType, typename Callback>
inline napi_status ThreadSafeFunction::BlockingCall(
    DataType* data, Callback callback)

and is used something like this:

tsfn.BlockingCall(data, [](Napi::Env env, Function cb, DataType* data) { ... })

and we want to be able to do this...

tsfn.BlockingCall(data, [](Napi::Env env, Function cb, DataType* data, void* context) { ... })

Similar to how I modified this CallbackWrapper to be a class with two constructor signatures (with or without void* context), I think I would need to create a new template class for this specific Data callback...

template <typename DataType>
class DataCallbackWrapper {
    private:
      using ContextlessCallback = std::function<void(Env, Function, DataType*)>;
      using ContextedCallback = std::function<void(Env, Function, DataType*, void *)>;

      ContextlessCallback ctxless;
      ContextedCallback ctxfull;

    public:
      CallbackWrapper(ContextlessCallback arg) : ctxless(arg){};
      CallbackWrapper(ContextedCallback arg) : ctxfull(arg){};

      void operator()(Napi::Env env, Napi::Function callback, DataType* data, void *context) {
        ctxfull ? ctxfull(env, callback, data, context) : ctxless(env, callback, data);
      };
    };
}

It just seems like a lot of wrapping and objects, I don't know if this is the right approach. Hopefully we can discuss in today's NAPI meeting.

[KevinEady]

Closing this in favor of #687