Motivation for multithreading in my App

JonB

@Absurd
I didn't say anything about sub-classing! You would only need to do that if you needed to replace/change the behaviour of QThread::start/run/exec(), which you do not, since the default behaviour already suits you.

@MrShawn
From your code sample I can see that invoking moduleThread->start() will send a SIGNAL to MyModule::start(), but what should MyModule::start() do in order to be kept alive?

QThread moduleThread;  //event loop in another thread
...
moduleThread->start();

That's exactly as I said. I suggested you look at theQThread reference links I posted. You would see that QThread::start() calls QThread::run() calls QThread::exec() by default, unless you do something else about it. It's the QThread::exec() that is providing the event loop/keep alive which I think you are struggling with.

I don't know, maybe if you read carefully through the really simple usage in https://wiki.qt.io/QThreads_general_usage, which is the same as @MrShawn's, the explanation would help.

P.S.
I wonder if you're thinking myModule->moveToThread(&moduleThread); "replaces" the QThread. It does not, it just moves your module into the thread (its variables/slots/where it runs).

QObject::connect(&moduleThread,&QThread::started,myModule,&MyModule::start);  //start may be your function that instantiates some other objects within your module.

This is just going to call your slot via a signal when the thread starts, for you to do your own initializations, at the end of which your slot should simply return not call something else. It will still be going QThread::start() -> QThread::run() -> QThread::exec(). MyModule::start has nothing to do with QThread::start. Try renaming to MyModule::myStart and going QObject::connect(&moduleThread,&QThread::started,myModule,&MyModule::myStart); and it will work just the same. Does that clarify for you?

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());
     }
}