QThread cannot be stop after QCamera->start()

QtTester

hey guys.
6.9.3+vs2022.
I wanna capture a usb cam video. it works find but the thread for capturing cannot be stop.
when I exit the app, thread.wait() cannot return after runing run() function ,even i already run stop().
If I donot do anything , wait() works fine and can return.
why wait() cannot return?

mainwindow.cpp, two button, one for start , another for stop:

#include "mainwindow.h"
#include "ui_mainwindow.h"

#include <QCamera>
#include <QDebug>
#include <QCameraDevice>
#include <QMediaDevices>

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    connect(this,&MainWindow::sigStart,&m_cam,&CamOperator::run);
    connect(this,&MainWindow::sigStop,&m_cam,&CamOperator::stop);

    //
    qDebug() <<"ui:"<<QThread::currentThread();
}

MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::on_pushButton_clicked()
{
    emit sigStart();
}

void MainWindow::on_pushButton_2_clicked()
{
    emit sigStop();
}

wrapper for qcamera:

#include "camoperator.h"
#include <QDebug>
#include <QImage>
#include <QDateTime>
#include <QTimer>

#define T_DBG qDebug()
#define T_ERR qDebug()

CamOperator::CamOperator(QObject *parent)
    : QObject{parent}
{
    moveToThread(&m_bgThread);
    m_bgThread.start();
}

CamOperator::~CamOperator()
{
    stop();

    if(m_bgThread.isRunning()){
        T_DBG<<"stop cam...";
        m_bgThread.requestInterruption();
        m_bgThread.quit();
        if(1){
            T_DBG<<"wait thread end...";

            // 3000 right here, after run(), it cannot return anymore.
            m_bgThread.wait();
        }
        T_DBG<<"thred end";
    }

}

void CamOperator::run()
{
    auto cam = QMediaDevices::defaultVideoInput();

    T_DBG <<QThread::currentThread();
    if(cam.isNull()){
        T_ERR <<"no usb cam";
        goto fail;
    }
    m_camera = new QCamera(cam);
    m_captureSession.setCamera(m_camera);
    m_imageCapture = new QImageCapture;
    m_captureSession.setImageCapture(m_imageCapture);
    connect(m_imageCapture,&QImageCapture::imageCaptured,this,[=](int id,const QImage &i){
        emit sigImageReady(i);
    });
    connect(m_imageCapture,&QImageCapture::readyForCaptureChanged,this,[=](bool ready){
        if(ready){
            m_imageCapture->capture();
            //qDebug()<<"ready";
        }
    });
    emit sigErrorReport("openCamOk");
    T_DBG <<"start capture";
    m_camera->start();
    goto end;

fail:
    emit sigErrorReport("openCamFail");
end:
    T_DBG <<"start cap end";
}

void CamOperator::stop()
{
    if(m_camera){
        m_camera->stop();
        disconnect(m_imageCapture);
        delete m_imageCapture;
        delete m_camera;
        m_camera = nullptr;
    }
    emit sigErrorReport("camClosed");
    T_DBG <<"stop end";
}

#ifndef CAMOPERATOR_H
#define CAMOPERATOR_H

#include <QThread>
#include <QCamera>
#include <QImageCapture>
#include <QCameraDevice>
#include <QMediaDevices>
#include <QMediaCaptureSession>

class ICameraWrapper;
class CamOperator : public QObject
{
    Q_OBJECT
public:
    explicit CamOperator(QObject *parent = nullptr);
    ~CamOperator();

signals:
    void sigErrorReport(const QString &);
    void sigImageReady(const QImage &);

public slots:
    void run();
    void stop();

private:
    QString m_camType;
    QCamera *m_camera{};
    QImageCapture *m_imageCapture;
    QMediaCaptureSession m_captureSession;
    QThread m_bgThread;
};

#endif // CAMOPERATOR_H

Christian Ehrlicher

@QtTester said in QThread cannot be stop after QCamera->start():

m_captureSession

This lives in the wrong thread since you don't pass a parent.
Why a thread at all? Please follow the official examples when you really want a thread here.

QtTester

@Christian-Ehrlicher
thanks for replying.
I have not found any thread programing for QCamera/QMediaCaptureSession,
the example in Examples\Qt-6.9.3\multimedia\camera\ use UI thread ?
can you suggest some web-link?

Pl45m4

@QtTester said in QThread cannot be stop after QCamera->start():

I have not found any thread programing for QCamera/QMediaCaptureSession,

It's not dedicated to QCamera only. I think @Christian-Ehrlicher was referring to the general Qt Multithreading examples and documentation

jsulm

@QtTester And the question is: why do you need a thread?

QtTester

@jsulm for not stucking the UI? I donot know whether it is appropriate to use a thread.

JKSH

@QtTester said in QThread cannot be stop after QCamera->start():

@jsulm for not stucking the UI? I donot know whether it is appropriate to use a thread.

Please run and study the official Camera Example: https://doc.qt.io/qt-6/qtmultimedia-camera-example.html

It shows that you don't need threads -- QCamera/QMediaCaptureSession does not block the UI.

QtTester

@JKSH I read the code.
but what I am curious is : why qthread.wait() not return after qcamera.start() ?

Christian Ehrlicher

@QtTester said in QThread cannot be stop after QCamera->start():

why qthread.wait() not return after qcamera.start()

I already told you what you might did wrong

m_captureSession

This lives in the wrong thread since you don't pass a parent.

QtTester

@Christian-Ehrlicher
if i donot new the class, wait() works fine, event i call qcamera.start():

void CamOperator::run()
{
    auto cam = QMediaDevices::defaultVideoInput();

    T_DBG <<QThread::currentThread();
    if(cam.isNull()){
        T_ERR <<"no usb cam";
        goto fail;
    }

    m_camera.setCameraDevice(cam);
    m_captureSession.setCamera(&m_camera);
    m_captureSession.setImageCapture(&m_imageCapture);
    m_captureSession.setVideoSink(&m_vs);
    connect(&m_imageCapture,&QImageCapture::imageCaptured,this,[=](int id,const QImage &i){
        emit sigImageReady(i);
    });
    connect(&m_vs, &QVideoSink::videoFrameChanged, [&]( const QVideoFrame &frame ) {
        m_imageCapture.capture();
    });
    emit sigErrorReport("openCamOk");
    T_DBG <<"start capture";
    m_camera.start();
    goto end;

fail:
    emit sigErrorReport("openCamFail");
end:
    T_DBG <<"start cap end";
}

but if i new the class and pass parent,wait() won't return anymore, why the parent will stuck it?:

void CamOperator::run()
{
    auto cam = QMediaDevices::defaultVideoInput();

    T_DBG <<QThread::currentThread();
    if(cam.isNull()){
        T_ERR <<"no usb cam";
        goto fail;
    }

    m_camera = new QCamera(cam,this);
    m_camera->setCameraDevice(cam);
    m_captureSession = new QMediaCaptureSession(this);
    m_captureSession->setCamera(m_camera);
    m_imageCapture = new QImageCapture(this);
    m_captureSession->setImageCapture(m_imageCapture);
    m_captureSession->setVideoSink(&m_vs);
    connect(m_imageCapture,&QImageCapture::imageCaptured,this,[=](int id,const QImage &i){
        emit sigImageReady(i);
        //qDebug()<<"cap:"<<QThread::currentThread();
    });
    connect(&m_vs, &QVideoSink::videoFrameChanged, [&]( const QVideoFrame &frame ) {
        m_imageCapture->capture();
    });
    emit sigErrorReport("openCamOk");
    T_DBG <<"start capture";
    m_camera->start();
    goto end;

fail:
    emit sigErrorReport("openCamFail");
end:
    T_DBG <<"start cap end";
}