Running QtConcurrent on member function resulting in error

wcneill

Re: Creating a new thread with QtConcurrent did not work

I'll caveat this question with this: I am new to C++ (interpret pointers boggle my mind)

I am following the documentation for QtConcurrent::run here: https://doc.qt.io/qt-6/qtconcurrentrun.html#additional-api-features

I am attempting to run the following code:

    // This is a pointer to an Engine object from third party library LUI. 
    auto engine = lui::QueryInterop<wise::Engine>(lui::GetLUI());
    if (engine) {

        connect(&m_modelsLoadedWatcher, &QFutureWatcher<bool>::finished, this, &ConfigDialog::onNNModelsLoaded);

        // Begin loading models in new thread. 
        m_modelsLoadedFuture = QtConcurrent::run(&wise::Engine::loadPytorchModelsAsync, engine);
        m_modelsLoadedWatcher.setFuture(m_modelsLoadedFuture);

 
    } else {
       // do some stuff.
    }

However, I am consistently getting the following error.

I figure something in the called member function must not be compatible. This is what it looks like:

void Engine::loadPytorchModelsAsync() { 
    QString scriptModulesPath = QDir("plugins:").absolutePath() + QString("/wisePlugin/PyTorchModels/");

    // spread out the processing of our heavy NN model loads over multiple threads
    auto pytorchLoadFunctions = wise::nn::GetPyTorchModelLoaders(scriptModulesPath);
    auto numberOfModelsToLoad = pytorchLoadFunctions.size();
    auto progress = new QProgressDialog("Loading Neural Network Models...", "", 0, (int)pytorchLoadFunctions.size(),
                                        lui::GetLUI()->getUI()->mainWindow()->centralWidget());

    progress->setCancelButton(nullptr);
    progress->setMinimumDuration(0);
    progress->setValue(0);

    // Define lambda function which will update progress bar when a model is loaded.
    wise::IEngine::SubTaskCompleteFunction actionOnComplete = [progress, numberOfModelsToLoad](int numComplete) {
        // invoke on the main thread
        QMetaObject::invokeMethod(qApp, [progress, numComplete] {
            progress->raise(); // bring the widget to the front of the app
            progress->setValue(numComplete);
        });
    };

    // Queue load functions
    QueueSimpleBlockingTaskGroup(pytorchLoadFunctions, wise::IEngine::eTaskBlockType::BlockAWorkerThread, actionOnComplete);
}

I'm not sure where to go from here honestly. It seems that I am following the documentation perfectly but I know that can't be true or this would be working. Can someone show me what I need to correct?

wcneill

@SGaist

I got it to work by making the QFuture member variable of the class that was handling the GUI stuff, then just accessing it from the class I was working on above.

if (engine) {

        // Engine has a QFuture<void> member for the models being loaded. We will watch it here. 
        connect(&m_modelsLoadedWatcher, &QFutureWatcher<bool>::finished, this, &ConfigDialog::onNNModelsLoaded);

        // We know that the implementation of IEngine is Engine, so cast. 
        m_modelsLoadedWatcher.setFuture(dynamic_cast<wise::Engine*>(engine)->m_modelsLoadedFuture);

    }

Im curious however about the invokeMethod you were talking about. I started a new thread asking about it, as I did try it out: https://forum.qt.io/topic/140834/how-to-combine-qtconcurrent-with-qmetaobject-invokemethod.

SGaist

Hi and welcome to devnet,

To rule out the obvious, can you build the example successfully ?

On a side note: you code snippet does not match the error you show in the image.

wcneill

This post is deleted!

wcneill

@SGaist As it turns out, I was looking at the docs for the wrong version. The arguments just needed to be flipped. Thanks for pointing out the difference.

SGaist

Now that you mention it, I remember that I thought that there would some trouble with that. Nice catch !

wcneill

@SGaist said in Running QtConcurrent on member function resulting in error:

Now that you mention it, I remember that I thought that there would some trouble with that. Nice catch !

Hah, a co-worker taught me how to use intellisense to filter out the 96 overrides of QtConcurrent::run to discover the error :D

I'm running into new problems now (widgets must be created in GUI thread)... I found this thread which you participated in here, and take its contents to imply that I am calling QtConcurrent::run from a thread that is not the GUI thread, so any widgets created by that call will make this error occur.

Unfortunately, I'm not sure which thread is the GUI thread, so I guess I just have to move the run call around into different cpp files until I find the right one?

I feel out of my depth here. I normally write models in Python and someone else does all this C++ work for me. This is what I get for volunteering to learn something new :D

SGaist

The correct way is to not do GUI access in a method called through QtConcurrent::run.

Emit a signal or use invokeMethod but no direct access.

wcneill

@SGaist

I got it to work by making the QFuture member variable of the class that was handling the GUI stuff, then just accessing it from the class I was working on above.

if (engine) {

        // Engine has a QFuture<void> member for the models being loaded. We will watch it here. 
        connect(&m_modelsLoadedWatcher, &QFutureWatcher<bool>::finished, this, &ConfigDialog::onNNModelsLoaded);

        // We know that the implementation of IEngine is Engine, so cast. 
        m_modelsLoadedWatcher.setFuture(dynamic_cast<wise::Engine*>(engine)->m_modelsLoadedFuture);

    }

Im curious however about the invokeMethod you were talking about. I started a new thread asking about it, as I did try it out: https://forum.qt.io/topic/140834/how-to-combine-qtconcurrent-with-qmetaobject-invokemethod.