QTimer not triggering slot

[grinqt]

Hi,

I am able to create a QTimer, start it, see that it is running, but it is not activating my slot. Here is my code:

#include "desktopserialca.h"
#include <QDebug>
#include <QThread>

DesktopSerialCA::DesktopSerialCA(AppDevice appDevice, QObject *parent)
    : QObject{parent}
{
    QTimer* timer1 = new QTimer(this);
    connect(timer1, &QTimer::timeout, this, &DesktopSerialCA::readSerialData);
    timer1->start(2000);

    qDebug() << "Timer id: " << timer1->timerId();
    qDebug() << "Is active: " << timer1->isActive();
    qDebug() << "Remaining time: " << timer1->remainingTime();
}

void DesktopSerialCA::connectSerialDevice()
{

}

void DesktopSerialCA::readSerialData() {
    qDebug() << "Slot activated!";
}

.h file:

#ifndef DESKTOPSERIALCA_H
#define DESKTOPSERIALCA_H

#include <QObject>
#include <QSerialPort>
#include <QSerialPortInfo>
#include <QTimer>
#include "appdevice.h"
#include "testsigslots.h"

class DesktopSerialCA : public QObject
{
    Q_OBJECT
    QSerialPort serialPort;
    QTimer* timer;
    void connectSerialDevice();
public:
    explicit DesktopSerialCA(AppDevice appDevice, QObject *parent = nullptr);

signals:

public slots:
    void readSerialData();
};

#endif // DESKTOPSERIALCA_H

main.cpp:

#include <QApplication>
#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQmlContext>
#include <QCoreApplication>
#include <QSerialPort>
#include <QSerialPortInfo>
#include <QIODevice>

#include "appdevice.h"
#include "desktopserialca.h"


int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);
    QQmlApplicationEngine engine;
    AppDevice appDevice = AppDevice(); // device the app is running on

    if (appDevice.isDesktop()) {
        DesktopSerialCA serialCA = DesktopSerialCA(appDevice);
    }

    engine.load(QUrl(QStringLiteral("qrc:/Main.qml")));
    if (engine.rootObjects().isEmpty())
        return -1;
    return app.exec();
}

I'm deploying to Desktop Qt 6.7.0. Here is my application output:

15:44:04: QML debugging is enabled. Only use this in a safe environment.
Timer id:  1
Is active:  true
Remaining time:  2100

As you can see, the slot is not being triggered. Any help would be appreciated!

[ChrisW67]

In main.cpp:

    if (appDevice.isDesktop()) {
        DesktopSerialCA serialCA = DesktopSerialCA(appDevice);
    }

The lifespan of the DesktopSerialCA object is scoped by the enclosing braces. So, the object gets constructed giving the output you see, and then immediately destroyed.

[Pl45m4]

@grinqt

Fix:

    DesktopSerialCA serialCA;
    if (appDevice.isDesktop()) {
        serialCA = DesktopSerialCA(appDevice);
    }

Also what is AppDevice? Do you really want to pass it by value instead of passing the pointer/a ref?

[grinqt]

Thank you for the responses, I'm going to read up a bit more on object lifespan to solidify my understanding.

[Pl45m4]

@grinqt

This is basic and simple C/C++.
When a scope ends, all locally declared (stack-) variables become invalid.
(Heap variables as well, but then you leak memory, if you don't clean up properly)

[grinqt]

@Pl45m4 I am pretty new to C++. I understand why the code wasn't working in my case, but I need to figure out how to change my code. When I try the code you recommended:

    DesktopSerialCA serialCA;
    if (appDevice.isDesktop()) {
        serialCA = DesktopSerialCA(appDevice);
    }

It does not work since it is trying to call a default constructor here:

DesktopSerialCA serialCA;

Would you recommend I use a pointer instead? Like so:

    DesktopSerialCA* serialCA;
    if (appDevice.isDesktop()) {
        serialCA = new DesktopSerialCA(appDevice);
    }

But then where would I delete serialCA? If I do it in main, won't the object be deleted before execution?

[JonB]

@grinqt
Then you should delete it explicitly prior to any return from main(). Or QScopedPointer could do it for you.

[ChrisW67]

@grinqt DesktopSerialCA is a QObject so you could just use the QObject hierarchy to manage its lifetime.

DesktopSerialCA* serialCA = nullptr;  // always initialise pointers
if (appDevice.isDesktop()) {
    serialCA = new DesktopSerialCA(appDevice, qApp);
}

(See qApp if that is unknown to you).

If I do it in main, won't the object be deleted before execution?

If you explicitly delete after app.exec() returns then you should be good because your GUI is done:

...
int returnVal = app.exec();
delete serialCA;
return returnVal;

but @JonB's suggestion of QScopedPointer would be better if you do not use QObject hierarchy.

[grinqt]

@JonB @ChrisW67 Thanks! I'll use a QScopedPointer!