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Question about ThreadSafeFunction #536
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Hi @marco-ms,
I think an AsyncWorker might work for you? Take a look at the example.
Hi @marco-ms,
I think an AsyncWorker might work for you? Take a look at the example.
AsyncWorker doesn't seem to fit, as I need to call my c++ code in the main thread and then it'll callback to the worker thread.
AsyncWorker calls native code in the worker thread, which is not what I want.
I need to call c++ call in the main thread, I receive callback in the worker thread and need to go back to the main thread to post result to JS layer.Actually seems my request to have it optional has been implemented already on N-API, but looks like made it only to Node 12.6.0 and yet to be released 10.17.0? Too bad it'll never get to Node 8.x I guess.
nodejs/node#27791Anyhow, is there a way to support this directly under node-addon-api and create a bogus callback? I tried passing Napi::Function() to ThreadSafe, but it errors out. Suggestions welcome.
cc @mhdawson
Code looks like this, from the JS this function is called directly passing a Promise and getting back some result.
Napi::Value MyFunction(const Napi::CallbackInfo &info) { Napi::Env env = info.Env(); Napi::HandleScope scope(info.Env()); tsfn = Napi::ThreadSafeFunction::New( info.Env(), Napi::Function(), // bogus function "Resource Name", 0, 1 ); const Napi::Promise::Deferred deferred = Napi::Promise::Deferred::New(env); GetObject()->DoSomethingNativeCode([deferred](Result &result) { // DoSomethingNativeCode() will call this callback on a worker thread auto callback = [deferred, result](Napi::Env env, Napi::Function jsCallback) { // Generate a jsResult object deferred.Resolve(jsResult); }; // Wait for the callback in the JS thread to be done. napi_status status = tsfn.BlockingCall(callback); if (status != napi_ok) { // Handle error } // Release the thread-safe function tsfn.Release(); }); return deferred.Promise(); }It throws:
Uncaught Exception: Error: Invalid Argument
If instead I modify
Napi::Function()toinfo[1].As<Napi::Function>()and pass from JavaScript an empty function, then it works fine.
And anyway even if I can get a bogus function, the code is not as reusable, since with a lambda callback I am not sure how to encapsulate the code really.
I tried a functor, but didn't succeed, anyway that is a separate issue.Calling
Napi::Function()will create an empty NapiFunctionwrapper and not actually point to anything. You need to create a function by either a lambda:Function::New(env, [](const CallbackInfo& cbinfo){ return cbinfo.Env().Undefined(); } )
or a global static
Value Noop( const CallbackInfo& info ) { return info.Env().Undefined(); } Function::New(env, Noop) // pass this to ThreadSafeFunction::New
If you will be creating many TSFNs, you could create one function, store it in a reference, and pass that reference's value:
Napi::FunctionReference ref = Napi::Persistent(Function::New(.../* from above */)); ref.Value() // pass this to ThreadSafeFunction::New
The optional
Functionargument will eventually make its way into node-addon-api.Adding a question here as I have been following this thread.
I'm doing something similar, and using
ThreadSafeFunction. I'm a little lost as to how I can get a reference to the JSthisinside of the callback.Function::Callworks inside of the c++ callback, but I cannot get anything that requiresthisto work.You want the
Functionthat's passed toThreadSafeFunction::Newto have a specificthis(when accessingcallbackInfo.This()for C++ native functions or thethisobject in a JS function)?Well, I guess you could pass a function that was bound to a specific
thisvia callingbindon theFunction?@KevinEady I'm just extending EventEmitter,
inherits(MyObject, EventEmitter);I want to call
MyObject.emitinside async thread, and need to have correctNapi::CallbackInfo::This()available to call it I believe.@CraigMason sooo I would think, you are passing either the complete
MyObjectorMyObject.emitto your native module?- If you are passing the complete object, you should have a reference to the object somewhere, such that when you use
Call, you can pass that object as the receiver - If you are passing
MyObject.emitdirectly, then could you passMyObject.emit.bind(MyObject)instead?
What are you expecting to call
MyObject.emit? As the ThreadSafeFunction'scallbackparameter,finalizeCallbackparameter, or within a[Non]BlockingCall'scallbackparameter?- If you are passing the complete object, you should have a reference to the object somewhere, such that when you use
I should probably explain the goal in a more abstract sense, rather than querying a specific implementation - as I'm new to native modules and I could be taking the wrong approach here.
I have a device which has an SDK, which dispatches events via a callback on new threads. I ultimately want to encapsulate that object as a JS Object that emits events, so I can deal with remaining logic in Node.
So having read through some example and the docs here, my conclusion was to extend EventEmitter (
inherits(DeviceModel, EventEmitter);), grab a function reference toemitin the constructor:Napi::Function emit = info.This().As<Napi::Object>().Get("emit").As<Napi::Function>(); this->_emitRef = Napi::Reference<Napi::Function>::New(emit, 1);Then create a
ThreadSafeFunctionto eventually call theemitfunction, which accessesEventEmitterclass vars. Thus, I figure I need to ensurethisis correct - which is where I'm a bit lost.Thank you,
@CraigMason I see...
One thing is that if you pass
emitas the TSFNcallbackfunction, it can only be called with at most one argument: that value which is returned through the[Non]BlockingCall()'s optionalCallbackparameter. This means that you'll only be able to pass the stringeventNameto thisemitfunction and never any additional data args... is that okay?I'm not 100% sure if it would work, but try
binding the emit function like so:Napi::Function emit = info.This().As<Napi::Object>().Get("emit").As<Napi::Function>(); Napi::Function bound = emit.Get("bind").As<Function>().Call(emit, { info.This() }).As<Function>(); this->_emitRef = Napi::Reference<Napi::Function>::New(bound, 1);
Reacted by Craig Mason@KevinEady Thank you :) Calling
binddid indeed work. I hadn't even thought of doing it that way.Regarding single parameter.. hmm, I'm not sure yet, I need to consider the remaining architecture first.
The optional
Functionargument will eventually make its way into node-addon-api.Thanks, your fix works for me. I wonder if node-addon-api can anticipate this optional argument by creating a noop function itself. That way would work also on older Node 8.x that won't probably get the fix backported. But I understand this may go out of scope.
You can keep this item open to track whatever work you are going to plan to make optional the function or feel free to close this.
Hi,
I am doing some c++ native code that returns some values via a callback to the add-on on a separate thread. I am using ThreadSafeFunction to go back to the main thread and post result to the JavaScript, however to do that I am using Promises, so I don't really need a function callback.
https://github.com/nodejs/node-addon-api/blob/master/doc/threadsafe_function.md#new
But in all Napi::ThreadSafeFunction::New() there is no way to NOT pass a Function, which I don't really need. Am I using the right tool here to go from a worker thread back to the main thread or is there an alternative that doesn't require a Function?
I think this likely should be called ThreadSafeCallback and accept either a Function or a Promise.