emit event as thread safe callback

[corymickelson]

Hello, first thank you for publishing this module. I am trying to call the emit function on a ObjectWrapped class that inherits from EventEmitter, in which i am passing to ThreadSafeCallback the emit function. Stepping through the code I can see the cb->call(...) is getting called but the event is never making it back to the javascript side. Below is the code in question. Any help is welcome, thank you.

Napi::Value
Signer::ReadForSignature(const CallbackInfo& info)
{
  vector<int> opts =
    AssertCallbackInfo(info, { { 0, { nullopt, option(napi_number) } } });
  if (opts[0] == 1) {
    signer->SetSignatureSize(info[0].As<Number>().Uint32Value());
  } else {
    signer->SetSignatureSize(4096);
  }

  auto emit = this->Value().Get("emit").As<Function>(); // <--  get the emit fn
  auto cb = make_shared<ThreadSafeCallback>(emit); // <-- pass emit to threadsafecallback
  field->SetSignature(*signer->GetSignatureBeacon());
  doc.WriteUpdate(signer.get(), !output.empty());
  if (!signer->HasSignaturePosition()) {
    Error::New(info.Env(),
               "Cannot find signature position in the document data")
      .ThrowAsJavaScriptException();
    return {};
  }
  signer->AdjustByteRange();
  signer->Seek(0);
  auto shareSigner = signer.get();
  thread([cb, shareSigner] {
    size_t sigBuffer = 65535 * 2, sigBufferLen;
    char* sigData;
    while (static_cast<void>(sigData = reinterpret_cast<char*>(
                               malloc(sizeof(char) * sigBuffer))),
           !sigData) {
      sigBuffer = sigBuffer / 2;
      if (!sigBuffer)
        break;
    }
    if (!sigData) {
      cb->call([](Napi::Env env, vector<napi_value>& args) {
        args = { String::New(env, "error"), String::New(env, "out of memory") };
      });
      return;
    }
    while (sigBufferLen = shareSigner->ReadForSignature(sigData, sigBuffer),
           sigBufferLen > 0) {
      cb->call([sigData, sigBuffer](Napi::Env env, vector<napi_value>& args) {
        args = { String::New(env, "data"),
                 Buffer<char>::Copy(env, sigData, sigBuffer) };
      });
    }
    free(sigData);
    cb->call([](Napi::Env env, vector<napi_value>& args) {
      args = { String::New(env, "end"), env.Undefined() };
    });
  })
    .detach();
  return this->Value();
}

....
let signer = new npdf.Signer(doc, signedPath)
signer.signatureField = field
let buf: Buffer
signer.readForSignature() // .on funcs never are invoked
signer.on('data', chunk => {
         buf = Buffer.concat(chunk)
    })
signer.on('end', () => {
        console.log(buf.length)
        return resolve()
    })
signer.on('error', e => {
        console.log(e)
        return reject(e)
    })

[mika-fischer]

Hey, sorry for the late response, was on vacation…

I think the problem might be that the callback is invoked before the listeners are attached. Could you check this? Put a printf inside the lamda before args = { …. And on the JS side, also sprinkle some console.log's. Or just call readForSignature after attaching the listeners.

Also, N-API now has native support for what this project does, so it might be wiser to use that instead:
https://nodejs.org/dist/latest-v11.x/docs/api/n-api.html#n_api_asynchronous_thread_safe_function_calls

[mika-fischer]

Additionally, I think you need to use the receiver constructor of ThreadSafeCallback and provide this->Value() as the receiver for the callback, otherwise this is undefined in the emit function...

[jonathan-roy]

Hi @mika-fischer,

I am having a similar issue, could you please elaborate on this:

you need to use the receiver constructor of ThreadSafeCallback and provide this->Value() as the receiver for the callback, otherwise this is undefined in the emit function...

Thank you!

[mika-fischer]

If in JS you would do

cb = () => val.emit()

then in C++ you need to do:

auto cb = make_shared<ThreadSafeCallback>(val, val.Get("emit").As<Function>()); // 2 arguments!

Or conversely, if you do this in C++:

auto cb = make_shared<ThreadSafeCallback>(val.Get("emit").As<Function>()); // 1 argument!

it's like doing this in JS:

cb = () => val.emit.call(undefined)

[jonathan-roy]

Ok I see! I didn't know how to set the this when calling emit, thank you very much.