Qt QThread stop when I need

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?

Please edit your question to provide a [mcve] that can be used to reproduce the problem. — G.M., Dec 10 '20 at 13:10

score 0 · Accepted Answer · edited Dec 12 '20 at 13:32

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(); });

And I retry your code and there is a little errors: `connect(ui->stopButton, &QPushButton::clicked, [work]{ work->setStatus(); });` I change it to :`connect(ui->stopButton, &QPushButton::clicked, [this] { work->setStatus(); });` After I change `work` to `this` ,your answer is right. [link](https://stackoverflow.com/questions/7895879/using-member-variable-in-lambda-capture-list-inside-a-member-function) — liuyulvv, Dec 12 '20 at 13:26

score 0 · Answer 2 · answered Dec 12 '20 at 13:03

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)

How is this different from my own answer? You make use of `requestInterruption` whereas I used an atomic bool to request interruption. It should amount to the same thing. Can you show how `MainWindow::stop` is called? The `connect` call? — G.M., Dec 12 '20 at 13:09
`connect(ui->stopButton, &QPushButton::clicked, this, &MainWindow::stop);` — liuyulvv, Dec 12 '20 at 13:25

Qt QThread stop when I need

2 Answers2