Qt QThread stop when I need - c++

Question

//Work.h
#ifndef WORK_H
#define WORK_H
#include <QDebug>
#include <QObject>
#include <QThread>
class Work : public QObject {
Q_OBJECT
public:
explicit Work(QObject *parent = nullptr);
public slots:
void snap();
void setStatus();
signals:
private:
bool status;
};
#endif // WORK_H
//Work.cpp
#include "Work.h"
Work::Work(QObject *parent) : QObject(parent) { status = true; }
void Work::snap() {
status = true;
while (true) {
if (status) {
qDebug() << "Work thread: " << QThread::currentThreadId();
} else {
qDebug() << "STOP";
break;
}
}
}
void Work::setStatus() { status = false; }
//MainWindow.h
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QMainWindow>
#include <QThread>
#include "Work.h"
QT_BEGIN_NAMESPACE
namespace Ui {
class MainWindow;
}
QT_END_NAMESPACE
class MainWindow : public QMainWindow {
Q_OBJECT
public:
MainWindow(QWidget *parent = nullptr);
~MainWindow();
private:
Ui::MainWindow *ui;
Work *work;
QThread thread;
};
#endif // MAINWINDOW_H
//MainWindow.cpp
#include "MainWindow.h"
#include "ui_MainWindow.h"
MainWindow::MainWindow(QWidget *parent)
: QMainWindow(parent), ui(new Ui::MainWindow) {
ui->setupUi(this);
work = new Work();
work->moveToThread(&thread);
thread.start();
connect(ui->startButton, SIGNAL(clicked()), work, SLOT(snap()));
connect(ui->stopButton, SIGNAL(clicked()), work, SLOT(setStatus()));
}
MainWindow::~MainWindow() {
thread.terminate();
delete ui;
}
//main.cpp
#include <QApplication>
#include "MainWindow.h"
int main(int argc, char *argv[]) {
QApplication a(argc, argv);
MainWindow w;
qDebug() << QThread::currentThreadId();
w.show();
return a.exec();
}
I use MainWindow to display, Work to do something. And I use work->moveToThread(&thread).
Click start button to execute snap function in Work, what I want to do is when I click stop button, the snap function output STOP. And I can still start and stop whenever I like.
But I fail. It seems impossible to change the status during the while loop. Work doesn't get the stopButton clicked signal. Is it because of priority?
Could anyone give me some advices?

Answer 1

Firstly consider your Work::snap implementation...
void Work::snap() {
status = true;
while (true) {
if (status) {
qDebug() << "Work thread: " << QThread::currentThreadId();
} else {
qDebug() << "STOP";
break;
}
}
}
Once started it never yields control to the Qt event loop. Now consider the connect call...
connect(ui->stopButton, SIGNAL(clicked()), work, SLOT(setStatus()));
Since ui->stopButton and work have different thread affinities this is effectively a queued connection and requires the receiver to have an active event loop. Hence the call to setStatus will remain pending forever.
A better way to achieve your goal might be to make use of a simple atomic bool...
std::atomic<bool> status;
and change the connect call to modify status directly using a lambda (untested)...
connect(ui->stopButton, &QPushButton::clicked, [this]{ work->setStatus(); });

Answer 2

I solve it.
I add a slot and a signal in MainWindow and change stop slot.
connect(ui->startButton, &QPushButton::clicked, this, &MainWindow::start);
connect(this, &MainWindow::startSnap, work, &Work::snap);
// start slot
void MainWindow::start() {
thread.start();
emit startSnap();
}
void MainWindow::stop() {
if (thread.isRunning()) {
thread.requestInterruption();
}
}
And change the codes in Work::snap
void Work::snap() {
while (true) {
if (QThread::currentThread()->isInterruptionRequested()) {
qDebug() << "STOP";
QThread::currentThread()->exit();
return;
} else {
qDebug() << "Work thread: " << QThread::currentThreadId();
}
}
}
The key codes are:
thread.requestInterruption();(MainWindow::stop)
QThread::currentThread()->exit();(Work::snap)
thread.start();(MainWindow::start)