How to overcome the Qthread synchronization issue here?

I have a main thread that loads the web-page and displays it. I have another thread running, that will just print the debug message on to the console.However, I am seeing that on running the QT-Thread, the web-page is not getting loaded. I tried putting the loading of web-page on to the constructor of thread, but this is also not helping here.Here is the code.

class MyJavaScriptOperations : public QObject {
    Q_OBJECT
public:

    Q_INVOKABLE qint32 MultOfNumbers(int a, int b) {
        qDebug() << a * b;
        return (a*b);
    }
};


#if 1

class MyThread : public QThread
{
    Q_OBJECT

public:
    MyThread();

public:
    void run();
};

MyThread::MyThread()
{
    qDebug()<<"Constructor called";
    QWebView *view = new QWebView();
    view->resize(400, 500);
    view->page()->mainFrame()->addToJavaScriptWindowObject("myoperations", new MyJavaScriptOperations);
    view->load(QUrl("./shreyas.html"));
    view->show();
    this->run();
}



void MyThread::run()
{
    qDebug()<<"Thread running";

    while(1)
     {
       qDebug()<<"Fire Callback now";
     }

}
#endif


int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    MyThread t;
    //t.run();



    return a.exec();
}

Solution

Just because code is in a subclass of QThread doesn't mean that code is executed in that thread. Your main thread constructs the object, and that constructor calls run(). This means that the code of the run method is still executed in the main thread, and - as it is blocking - the line a.exec() is never called, and the main thread never gets an event loop which is required for paint events and the like.

What you need to do is start the thread and wait for run() being executed:

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    // TODO: Code for your web view goes here. You will probably need to
    //       pass the created web view into the constructor of MyThread

    MyThread t;
    // start the thread - this will put an event in the main event loop
    t.start();

    // start the event loop - this will lead to MyThread::run() being called
    return a.exec();
}

This is enough to get your example running, but you will get errors when closing the web view as your way of using threads is not the intended one: If you want to make your code stable, put the code of your run() method in a separate worker class and use a default QThread for managing that, without subclassing QThread.

I recommend reading the Qt5 Documentation on threads, which is also applicable for earlier versions of Qt.