QtConcurrent::run for loading HTML page crashes app

MH24

Hello,

I have been trying for several days to speed up the load process of an HTML file from disk. using Qt6.5.1 on Ubuntu 22.04LTS and Qt Creator 10.0.2.

In mainwindow.h I define:

QWebEnginePage *page = new QWebEnginePage(); //as a public object

private:
void workerFunction();

In constructor of mainwindow.cpp this is the relevant code:

ui->setupUi(this);
QFuture<void> t1 = QtConcurrent::run([this](){

qDebug()<<"In thread";
workerFunction();
});
connect(page,SIGNAL(loadFinished(bool)),this,SLOT(readPage(bool))); //couldn't get QWebEnginePage::loadFinished(bool) to work //so using the ancient SIGNAL/SLOT

And the workerFunction() is defined:

void MainWindow::workerFunction(){
page->load(QUrl("qrc:/html_page.html"));

}

readPage(bool) is a slot where html is parsed and ui is set so I keep this away from the separate thread.

The app however crashes. I have tried many variants of QConcurrent::run, but they always gave errors. This doesn't give errors but it crashes.
I tried QFuture<void> t1 = QtConcurrent::run(workerFunction); with extern added before void in definition of function as per official documentation. It said Reference to non-static member function must be called..., so I ditched it and tried:

QThreadPool pool;
QFuture<void> t1 = QtConcurrent::run(&pool, workerFunction);

which is also from official docs and got same error as above. I tried creating a worker class as per this answer but it also crashed. I used t1.waitForFinished() but it also gives same error.
If I just have a qDebug() in lambda function it runs fine. As soon as I add page->load... it crashes.

How do I use concurrency and threads to speed up my page load (if not parsing) from disk?

Christian Ehrlicher

You must not access gui elements from outside the main (gui) thread and there is no need since QWebEnginePage::load() is asynchronous (like all such functions).

MH24

@Christian-Ehrlicher Okay thank you. But how do I speed the loading up? File is just 50KB. I load it, parse the HTML and display on listWidget on UI. On 16GB RAM ubuntu it loads well (could be better), but on Windows 11 with 8GB RAM, it takes a few seconds.

Christian Ehrlicher

@MH24 I don't know but a thread will (when it would work) also not speed it up.