QObject::startTimer: Timers cannot be started from another thread

Dummie1138

Hi. I have the following code/minimum reproducable example.

What the code does: There is an object that manages all device connections. In the controller, I am placing this object into a separate thread because I have issues with the Qt SerialPort data reception being blocked when the titlebar is moved.

controller.h

#ifndef CONTROLLER_H
#define CONTROLLER_H
#include <QString>
#include <QObject>
#include <QTimer>
#include <QTime>
#include <QQueue>
#include <QByteArray>
#include <QThread>
#include <cmath>

class DeviceConnection;

class Controller:public QObject
{
    Q_OBJECT
public:
    //singleton
    static Controller* get();
    Controller(QObject *parent = nullptr);
    QThread* devConThread = new QThread(this);  //Thread for DeviceConnection
    //There's more to this file but those objects have been omitted.
};

#endif // CONTROLLER_H

controller.cpp

Controller::Controller(QObject *parent) :
    QObject(parent){
    //NOTE Placing device connection on a new thread because of this:
    //FIXIT putting this onto a new thread causes issues with reconnecting.
    deviceConnection = new DeviceConnection;
    deviceConnection->moveToThread(devConThread);
    devConThread->start();
}

void Controller::reconnect(){
    deviceConnection->restartConnection();
}

deviceconnection.cpp

DeviceConnection::DeviceConnection(){
    connectingTimer = new QTimer(this);
    connectingTimer->start(3000);
    
    serialPort_Controller = new QSerialPort(this);
    connect(connectingTimer, SIGNAL(timeout()), this, SLOT(connectDevice()));
}

//ends the connection to the device and restarts the connection procedure with the next port
void DeviceConnection::restartConnection()
{
    serialPort_Controller->close();
    qDebug() << "restartConnection";
    connectingTimer->start(3000);
    qDebug() << "startedTimer";
    emit lostConnection();
}

When restartConnection is triggered, I get the following output:

restartConnection
QObject::startTimer: Timers cannot be started from another thread
startedTimer

My confusion comes from this: how am I starting this timer from another thread? My understanding of threads is that deviceConnection is currently stored in another thread, "devConThread". And since the timer connectingTimer is being started within the deviceConnection object, the timer should be started from the same thread as the deviceConnection. I am unsure what I am doing incorrectly here.

Please let me know if more information is required.

sierdzio

@Dummie1138 said in QObject::startTimer: Timers cannot be started from another thread:

void Controller::reconnect(){
deviceConnection->restartConnection();
}

Here you are calling the deviceConnection which runs in devConThread from your main thread.

To get rid of this problem, use signals and slots - send a signal asking to reconnect, and receive it in DeviceConnection and run your timer there in the slot. This way Qt will make sure it's all done properly.