Timers cannot be stopped from another thread Qt - multithreading

Question

I am working on Qt application. There I am using two threads, one for the GUI and one for do the processing.
I have worker class which has QTimer as member class.
.h file:
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QMainWindow>
#include <QTimer>
#include <QThread>
class Worker : public QObject
{
Q_OBJECT
public:
Worker();
QTimer t;
public slots:
void process();
void startWorker();
};
namespace Ui {
class MainWindow;
}
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = nullptr);
~MainWindow();
private:
QThread workerThread;
Worker wt;
};
#endif // MAINWINDOW_H
cpp file
#include "mainwindow.h"
#include <QDebug>
#include <iostream>
Worker::Worker() : t(this)
{
connect(&t, SIGNAL(timeout()), this, SLOT(process()));
}
void Worker::process()
{
std::cout << "triggering timer" << std::endl;
}
void Worker::startWorker()
{
t.start(1000);
}
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent)
{
wt.moveToThread(&workerThread);
qDebug() << "worker thread " << wt.thread();
qDebug() << "timer thread " << wt.t.thread();
connect(&workerThread, SIGNAL(started()), &wt, SLOT(startWorker()));
connect(&workerThread, &QThread::finished, &workerThread, &QObject::deleteLater);
workerThread.start();
}
MainWindow::~MainWindow()
{
workerThread.quit();
workerThread.wait();
}
I can start the thread with no error. However when I close the application I am getting warning message.
QObject::killTimer: Timers cannot be stopped from another thread
QObject::~QObject: Timers cannot be stopped from another thread
If QTimer is child of worker class and it has been moved to thread why Qt is complaining about stopping it from different thread?
Note: I have added logs to print thread id and it outputs same value in both cases:
worker thread QThread(0x72fdf0)
timer thread QThread(0x72fdf0)
Can someone please explain? I do not understand what it's happening here
Thanks in advance

Answer 1

I finally was able to fix the error by:
Converting QTimer to pointer
Adding slot stopWorker as suggested by #Amfasis
In that slot not only stop QTimer but also delete it
Thanks all

Answer 2

You should stop the timer before QObject deletes it itself
in .h file, add the destructor:
class Worker : public QObject
{
Q_OBJECT
public:
Worker();
~Worker();
private:
QTimer t;
public slots:
void process();
void startWorker();
void stopWorker(); //this line was added
};
in .cpp file, add:
Worker::stopWorker()
{
t.stop();
}
and in constructor
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent)
{
...
connect(&workerThread, &QThread::finished, &wt, &Worker::stopWorker); //add this line!
...
}

Answer 3

You should new your timer in your task object slot, not in the task construtor. Stop the timer when your task object is deleted.
My task object header: Worker.h
#pragma once
#include <QObject>
class QTimer;
class Worker : public QObject
{
Q_OBJECT
public:
explicit Worker(QObject *parent = nullptr);
~Worker();
public slots:
void WorkerTaskStartSlot(void);
void TaskFinished(void);
private slots:
void TimerOutToDoSomethingSlot(void);
signals:
void WorkertResultSig(void);
private:
QTimer *m_pTimer = nullptr;
};
Worker.cpp:
#include "Worker.h"
#include <QDebug>
#include <QThread>
#include <QTimer>
Worker::Worker(QObject *parent) : QObject(parent)
{
qDebug()<<__FUNCTION__<<"threadid"<< QThread::currentThreadId();
}
Worker::~Worker()
{
TaskFinished();
qDebug()<<__FUNCTION__<<"threadid"<< QThread::currentThreadId();
}
void Worker::WorkerTaskStartSlot(void)
{
qDebug()<<__FUNCTION__<<"threadid"<< QThread::currentThreadId();
emit WorkertResultSig();
m_pTimer = new QTimer(this);
connect(m_pTimer,&QTimer::timeout,this,&Worker::TimerOutToDoSomethingSlot);
m_pTimer->start(1000);
}
void Worker::TaskFinished(void)
{
qDebug()<<__FUNCTION__<<"threadid"<< QThread::currentThreadId();
if(m_pTimer)
{
if(m_pTimer->isActive())
{
m_pTimer->stop();
qDebug()<<__FUNCTION__<<"stop timer"<< QThread::currentThreadId();
}
delete m_pTimer;
m_pTimer = nullptr;
}
}
void Worker::TimerOutToDoSomethingSlot(void)
{
qDebug()<<__FUNCTION__<<"threadid"<< QThread::currentThreadId();
}
The controller header: MainWindow.h
#pragma once
#include <QMainWindow>
class QPushButton;
class QWidget;
class Worker;
class QThread;
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
MainWindow(QWidget *parent = nullptr);
~MainWindow();
private:
void InitCtrl(void);
private slots:
void StartTaskBtnSlot(const bool &checked);
void WorkertResultSlot(void);
private:
QPushButton *m_pStartTaskBtn = nullptr;
QWidget *m_pCenterWidget = nullptr;
Worker *m_pWorker = nullptr;
QThread *m_pWorkerThread = nullptr;
};
MainWindow.cpp
#include "MainWindow.h"
#include <QVBoxLayout>
#include <QPushButton>
#include <QWidget>
#include <QDebug>
#include <QThread>
#include "Worker.h"
MainWindow::MainWindow(QWidget *parent)
: QMainWindow(parent)
{
InitCtrl();
qDebug()<<__FUNCTION__<<"mainwindow thread id"<< QThread::currentThreadId();
}
MainWindow::~MainWindow()
{
}
void MainWindow::InitCtrl(void)
{
m_pCenterWidget = new QWidget(this);
m_pStartTaskBtn = new QPushButton(QStringLiteral("Start"),this);
QVBoxLayout *pvertlayout = new QVBoxLayout();
pvertlayout->addWidget(m_pStartTaskBtn);
m_pCenterWidget->setLayout(pvertlayout);
setCentralWidget(m_pCenterWidget);
m_pStartTaskBtn->setCheckable(true);
connect(m_pStartTaskBtn,&QPushButton::clicked,this,&MainWindow::StartTaskBtnSlot);
}
void MainWindow::StartTaskBtnSlot(const bool &checked)
{
if(checked)
{
m_pStartTaskBtn->setText(QStringLiteral("Close"));
m_pWorkerThread = new QThread();
m_pWorker = new Worker();
// move the task object to the thread BEFORE connecting any signal/slots
m_pWorker->moveToThread(m_pWorkerThread);
connect(m_pWorkerThread, SIGNAL(started()), m_pWorker, SLOT(WorkerTaskStartSlot()));
connect(m_pWorker, SIGNAL(WorkertResultSig()), this, SLOT(WorkertResultSlot()));
// automatically delete thread and task object when work is done:
connect(m_pWorkerThread, SIGNAL(finished()), m_pWorker, SLOT(deleteLater()));
connect(m_pWorkerThread, SIGNAL(finished()), m_pWorkerThread, SLOT(deleteLater()));
m_pWorkerThread->start();
}
else
{
m_pStartTaskBtn->setText(QStringLiteral("Start"));
m_pWorkerThread->quit();
m_pWorkerThread->wait();
}
}
void MainWindow::WorkertResultSlot(void)
{
qDebug()<<__FUNCTION__<<"threadid"<<QThread::currentThreadId();
}
Finally, it will output result like this:
MainWindow::MainWindow mainwindow thread id 0x2bf0
Worker::Worker threadid 0x2bf0
Worker::WorkerTaskStartSlot threadid 0x4af0
MainWindow::WorkertResultSlot threadid 0x2bf0
Worker::TimerOutToDoSomethingSlot threadid 0x4af0
Worker::TimerOutToDoSomethingSlot threadid 0x4af0
Worker::TimerOutToDoSomethingSlot threadid 0x4af0
Worker::TimerOutToDoSomethingSlot threadid 0x4af0
Worker::TaskFinished threadid 0x4af0
Worker::TaskFinished stop timer 0x4af0
Worker::~Worker threadid 0x4af0