Motivation for multithreading in my App

Absurd

@JonB Yes!
That clears everything up.

Thank you

Absurd

Ok, this is how I transformed my code:

In main.cpp:

int main(int argc, char* argv[]) {
    Controller controller(argc, argv);
    return controller.runApplication();
}

And in Controller:

Controller::Controller(int argc, char* argv[]) :  _app(argc, argv) {
    // does nothing
}

void Controller::initModule() {
    _devicesManager = new DevicesManager;
    _moduleThread = new QThread(this); // Controller will be the parent of QThread - and will be responsible for deleting it
    _devicesManager->moveToThread(_moduleThread);

    _moduleThread->start();
}

void Controller::initView() {
    _gui = new GUI;
}

int Controller::runView() {
    _gui->show();
    return _app.exec();
}

void Controller::connectSignals() {
    connect(_moduleThread, SIGNAL(started()), _devicesManager, SLOT(start()));
    // more to come, for now just connecting QThread's 'started' signal to  DevicesManager::start()
}

int Controller::runApplication() {
    initModule();
    initView();
    connectSignals();
    return runView();
}

For now, DeviceManager does absolutely nothing, except this:

void DevicesManager::start() {
    qDebug() << "Starting!";
}

(that's why I didn't bother to connect QThread::finished to DeviceManager::deleteLater for now - because it holds no memory allocation)

But I am experiencing two problems:

When I run it, the GUI runs fine, but then when I close the GUI by clicking the x button of the window, I get:
QThread: Destroyed while thread is still running
I don't see the "Starting!" printed out to the console, which makes me think that start didn't really run, not to mention running on a different thread...

J.Hilk

hi, @Absurd

you start your QThread object before you do the connection -> started signal is emitted before the connection is made -> slot gets not invoked.

_moduleThread->start();

Absurd

@J.Hilk ohh... how silly of me... :
Thank you.

Do you know what could be the cause for the second thing?
QThread: Destroyed while thread is still running

kshegunov

@Absurd said in Motivation for multithreading in my App:

@J.Hilk ohh... how silly of me... :
Thank you.

Do you know what could be the cause for the second thing?
QThread: Destroyed while thread is still running

Call QThread::quit and after that QThread::wait before you exit the application.

Addendum:

What is a good justification to use threads anyway?

Latency. Having a slow, blocking or otherwise high latency operation you don't need or want to wait for - you need to thread it, otherwise you do not.

Absurd

@kshegunov thanks for your reply. appreciate it.

@kshegunov said in Motivation for multithreading in my App:

Call QThread::quit and after that QThread::wait before you exit the application.

I should call QThread::quit & QThread::wait from Controller's destructor, right?

kshegunov

@Absurd said in Motivation for multithreading in my App:

I should call QThread::quit & QThread::wait from Controller's destructor, right?

Yes, that is correct. You should also free your worker objects:

QObject::connect(_moduleThread, &QThread::finished, _devicesManager, &QObject::deleteLater);

Absurd

@kshegunov why not just delete _deviceManager after Qthread::wait() has returned?

@MrShawn said:

No, the first QString is like a "key" for your message type, so that you know what keys to look for or are available in your QVariantMap.

Can you please give a usage example for that?
I couldn't figure out why isn't void message(QString, QVariant) enough...

kshegunov

@Absurd said in Motivation for multithreading in my App:

@kshegunov why not just delete _deviceManager after Qthread::wait() has returned?

It's possible, but it still belongs to the worker thread, you may get warnings.

MrShawn

@Absurd

void message(QString msgType, QVariantMap data);

QVariantMap not QVariant.

QVarantMap is a map with QStrings for keys, so like say you emit a signal that is declared like above, it may look something like this.

QVariantMap params;
int param1 = 2;
double param2 = 2.2;
params.insert("param1",param1);
params.insert("param2",param2);
emit message("runSomeSpecificMethod",params);

Then the receiving slot may look something like this

void getMessage(QString messageType, QVariantMap data)
{
     if (messageType.compare("runSomeSpecificMethod",Qt::CaseInsensitive) == 0){
          SomeSpecificMethod(data.value("param1").toInt(),data.value("param2").toDouble());
     }
}