Cannot connect signal and slot from different thread.

Wuzi

Hello,

I'm trying to create an udp socket in a different thread. After I created the socket, I try to connect QUdpSocket::readyRead with my own function readData() from another thread. When I initialize in the UDP constructor QObject with the default constructor with no arguments, the output result is, that the first connect works fine and I get data, the disconnect works fine, because I don't get anymore data, but the reconnect does not work anymore.
When I initialize in the UDP constructor QObject with MainWindow as parent(QObject(parent)), then the first connect does not work, but the reconnect works fine, and I get data in.

Why the different initialisation of QObject changes the behavior?
Why not every connect is successful and I don't get data everytime?[0_1537179976696_UDPClient.zip](Uploading 100%)

Second test I made was to disable QThread:

// m_udp->moveToThread(&m_udp_thread);
// m_udp_thread.start();

with QObject() in UDP constructor:
first connect works fine.
disconnect works fine.
reconnect does not work, because I don't get anymore data in

with QObject(parent) in UDP constructor:
first connect seems not to work
reconnect works fine, because I get data in

Here you can find my UDP sender:
UDP Sender

Here is a small Test Project with this strange behavior:
Test Project

Why I don't get the behavior I expect?

sierdzio

Please take a look at the docs:

The object cannot be moved if it has a parent

https://doc.qt.io/qt-5/qobject.html#moveToThread

So, create your QUdpSocket without parent (not QObject() but a nullptr - that is important!), then move to thread. If you need it parented within the thread, you can call setParent() later.

Also, remember to use Qt::QueuedConnection for your inter-thread connections - then you don't have to worry about locking any mutexes and such.

Wuzi

@sierdzio said in Cannot connect signal and slot from different thread.:

Also, remember to use Qt::QueuedConnection for your inter-thread connections - then you don't have to worry about locking any mutexes and such.

Thank you sierdzio for your help! Qt::QueuedConnection will be made automatically when I create a connection between two threads or do I have to set it explizit?

I tried to init QObject(nullptr) in UDP, but it does not change anything. I don't understand why it will not work, when I remove the moveToThread function to use just one thread?

sierdzio

@Wuzi said in Cannot connect signal and slot from different thread.:

Qt::QueuedConnection will be made automatically when I create a connection between two threads or do I have to set it explizit?

It is done automatically if you connect after moving to thread. That's another important thing: connect signals and slots after you move to thread!

However, I do recommend setting it explicitly - then somebody who will read your code later will be 100% sure what your intention was.

Wuzi

@sierdzio said in Cannot connect signal and slot from different thread.:

However, I do recommend setting it explicitly - then somebody who will read your code later will be 100% sure what your intention was.

Ok thats an argument thank you.

Wuzi

@sierdzio said in Cannot connect signal and slot from different thread.:

That's another important thing: connect signals and slots after you move to thread!

Yes I do this :

mainwindow.cpp:

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    m_mutex = new QMutex;
    m_udp = new UDP(this, m_mutex);

    m_udp->init();
    m_udp->moveToThread(&m_udp_thread);
    m_udp_thread.start();

    qDebug() << "Connect";
    m_udp->connectDataReady();
    sleep(5);
    qDebug() << "Disconnect";
    m_udp->disconnectDataReady();
    sleep(5);
    qDebug() << "Reconnect";
    m_udp->connectDataReady();
}

UDP.cpp:

void UDP::connectDataReady(){
    qDebug() << connect(m_socket, &QUdpSocket::readyRead, this, &UDP::readData, Qt::ConnectionType::QueuedConnection);
}

void UDP::disconnectDataReady(){
    disconnect(m_socket, &QUdpSocket::readyRead, this, &UDP::readData);
}

I tried also to remove the direction connection

 m_udp->disconnectDataReady();

by using a queued connection, but it doesn't change something

J.Hilk

@Wuzi
question, why do you (want to) disconnect the signals from the socket to you slot?

Wuzi

@J.Hilk said in Cannot connect signal and slot from different thread.:

to you slot?

because I would like to stop logging data. You bring me to another idea I can try: this is only a part of my project udpLogger. When the readyRead signal is emitted, the UDP object reads the data in and emits a signal which signals the Plots object, that new data is available and the Plots object processes the new data. Now instead of disconnecting to readyRead, I can disconnect to the Plots object.

J.Hilk

@Wuzi
youc could also use a bool to check, before emitting the signal

if(logData)
   emit mySignal();

Wuzi

@J.Hilk said in Cannot connect signal and slot from different thread.:

if(logData)
emit mySignal();

Yes exactly i do know this and it works fine. Thank you Hilk

But it would be nice to know why disconnecting does not work.