How to properly stop QTimer from another thread - qt

Question

I see that such topic was discussed many times, but can't find clear answer to simple situation. I have Worker class running in the own thread where I create timer and want to stop it due to some condition.
But I am getting the error:
Timers cannot be stopped from another thread
I fill that am missing some core logic in thread working in Qt. Can someone please explain how to resolve that? Thank you.
Here is the main.cpp
#include <QCoreApplication>
#include <QObject>
#include <QtDebug>
#include "worker.h"
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
Worker w;
return a.exec();
}
Worker.h
#ifndef WORKER_H
#define WORKER_H
#include <QObject>
#include <QTimer>
#include <QThread>
#include <QtDebug>
class Worker : public QObject
{
Q_OBJECT
public:
explicit Worker(QObject *parent = nullptr);
private:
QThread t;
QTimer *timer;
int count = 1;
public slots:
void dataTimerFunction();
void onStopTimer(QTimer *t);
signals:
void stopTimer(QTimer *t);
};
#endif // WORKER_H
Worker.cpp
#include "worker.h"
Worker::Worker(QObject *parent) : QObject(parent)
{
this->moveToThread(&t);
QObject::connect(&t, &QThread::finished, this, &QObject::deleteLater);
t.start();
// are we in the new thread from this point, right?
timer = new QTimer();
QObject::connect(timer, &QTimer::timeout, this, &Worker::dataTimerFunction);
QObject::connect(this, &Worker::stopTimer, this, &Worker::onStopTimer);
// QObject::connect(this, &Worker::stopTimer, this, &Worker::onStopTimer, Qt::QueuedConnection); doesn't work as well
timer->start(200);
}
void Worker::dataTimerFunction()
{
qDebug()<<count;
count++;
if (count>5){
emit stopTimer(timer);
//timer->stop();
}
}
void Worker::onStopTimer(QTimer *t)
{
t->stop();
}

Answer 1

The problem is indeed that the timer is not moved to your thread.
Here is a modified worker class, that does what you might want to achieve. It moves the timer also to the thread and then starts and stops it by using signals / slots.
worker.h
#ifndef WORKER_H
#define WORKER_H
#include <QObject>
#include <QTimer>
#include <QThread>
#include <QtDebug>
class Worker : public QObject
{
Q_OBJECT
public:
explicit Worker(QObject *parent = nullptr);
private:
QThread t;
QTimer *timer;
int count = 1;
signals:
void stopTimer();
void startTimer(int msec);
public slots:
void dataTimerFunction();
};
#endif // WORKER_H
worker.cpp
#include "worker.h"
Worker::Worker(QObject *parent) : QObject(parent)
{
// are we in the new thread from this point, right?
timer = new QTimer(nullptr);
this->moveToThread(&t);
timer->moveToThread(&t);
QObject::connect(&t, &QThread::finished, this, &QObject::deleteLater);
t.start();
QObject::connect(timer, &QTimer::timeout, this, &Worker::dataTimerFunction);
QObject::connect(this, &Worker::stopTimer, timer, &QTimer::stop, Qt::QueuedConnection);
QObject::connect(this, &Worker::startTimer, timer, static_cast<void (QTimer::*)(int)>(&QTimer::start), Qt::QueuedConnection);
startTimer(200);
}
void Worker::dataTimerFunction()
{
qDebug()<<count;
count++;
if (count>5){
stopTimer();
}
}

Answer 2

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