c++ - 在不调用 QThread::start() 的情况下使用 QThread 是否有意义？

Question

一段时间以来，我一直在使用 Qt 开发一个应用程序，在该应用程序中我必须从相机中抓取帧。相机将在与应用程序其余部分不同的线程中运行。我遵循了以下建议:

http://mayaposch.wordpress.com/2011/11/01/how-to-really-truly-use-qthreads-the-full-explanation/

和

https://www.qt.io/blog/2010/06/17/youre-doing-it-wrong

不要子类化 QThread。相反，我创建了一个工作对象 CCameraCapture 并将其移动到 QThread。相机的帧抓取由连接到抓取帧槽的 QTimer 控制。将 CCameraCapture 移动到 QThread 后，可以通过启动计时器来开始捕获。我的问题是:我必须调用 QThread 类的 start() 吗？我的方法无需调用即可工作。循环由定时器执行，worker 实际上在另一个线程中工作，而没有调用 start。所以我想知道如果使用定时器，调用 start() 是否有意义？为了清楚起见，我将我的 Worker 类放在下面，它是在主应用程序中创建的:

m_CameraCaptureThread= new QThread();
m_CameraCapture = new CCameraCapture(30);
m_CameraCapture->moveToThread(m_CameraCaptureThread);

//Connect error signal
QObject::connect(m_CameraCapture, SIGNAL(error(QString,QString)), this, SLOT(reportError(QString,QString)));

//Connect started() of QThread to the starting function of the worker class
connect(m_CameraCaptureThread, SIGNAL(started()), m_CameraCapture, SLOT(startGrabbing()));

//Connect the finished signal of the worker class to the thread for quitting the loop
connect(m_CameraCapture, SIGNAL(finished()), m_CameraCaptureThread, SLOT(quit()));

//This connections guarantees that the *m_CVideoCapture is automatically deleted if the event loop of the thread is terminated. Therefore, m_CVideoCapture does not need to be released manually if the capturing process is stopped.
QObject::connect(m_CameraCaptureThread, SIGNAL(finished()), m_CameraCaptureThread, SLOT(deleteLater()));
QObject::connect(m_CameraCapture, SIGNAL(finished()), m_CameraCapture, SLOT(deleteLater()));

//Connect sendFrame to update frame for displaying the current frame
QObject::connect(m_CameraCapture, SIGNAL(sendFrame(QImage)), this, SLOT(receiveFrame(QImage)));
/** 

所以到目前为止，还没有调用 m_CameraCaptureThread->start() 来启动处理循环。但是如果我调用 CameraCapture->startGrabbing() 它会很好地工作。 grabFrame() 槽由计时器触发，帧被发送到主应用程序。然而，到目前为止我看到的所有代码示例都调用了 start()，即使它们使用了计时器:例如: http://blog.debao.me/2013/08/how-to-use-qthread-in-the-right-way-part-1/ (用法2-1在页尾)

这是我的 CCameraCapture.h:

/** 

@file CCameraCapture.h 
@brief this file contains the definition of the class CCameraCapture.


**/ 

#ifndef CCameraCapture_H
#define CCameraCapture_H

//Required Qt libs
#include <QObject>
#include <QImage>
#include <QTimer>
#include <QString>
#include <QMutex>
#include <QDebug>

//Required OpenCV libs
#include <opencv2\opencv.hpp>

/** 
@class CCameraCapture 

@brief This class defines a video capture object which should be moved to thread.

class CCameraCapture : public QObject{

Q_OBJECT

public:
    /** 
    @brief Constructor of CCameraCapture.
    @param frameRate Desired frame rate (if possible)
    */
    CCameraCapture(double frameRate=30);
    /**
    @brief Destructor of CCameraCapture.
    */
    ~CCameraCapture();
    /**
    @brief This function terminates the thread. 
    */
    void exitThread();
    bool startGrabbing();
    void stopGrabbing();
    /**
    @brief Check if camera is running
    @return Returns true if camera is running
    */
    bool isGrabbing();

private:
    //The openCV capturing object to access the camera
    cv::VideoCapture m_Cap;
    // Device index
    int m_IdxDevice;
    // Timer for triggring grab frame
    QTimer m_Timer;
    //The most recent frame
    QImage m_Frame;
    //Mutex to lock variables
    QMutex m_Mutex;

private slots:
    /**
    @brief This slot grabs a frame from the camera. It is triggered by the timer m_Timer.
    */
    void grabFrame();

signals:
    /**
    @brief This signal needs to be connected to the slot in the main application which should receive the images.
    @param img The most recent frame.

    */
    void sendFrame(QImage currentFrame);
    /** 
    @brief This signal is emitted if an error occurs
    @param errMsg QString contains the error message to be displayed.
    @param errTitle QString contains the title of the diplayed error message.
 
    This signal should be connected to a slot in the main application. It allows to send error reports back to the main application which can be displayed on screen
    */
    void error(QString errMsg,QString errTitle);
    void finished();
};
#endif //CCameraCapture_H

这是cpp文件:

/**
@file CCameraCapture.cpp 
@brief this file contains the function definitions of CCameraCapture.
**/ 

#include "CCameraCapture.h"

CCameraCapture::CCameraCapture(double frameRate):m_Mutex(),m_IdxDevice(0)
{
    //Connect timer to grabFrame
    connect(&m_Timer, SIGNAL(timeout()), this, SLOT(grabFrame()), Qt::DirectConnection);
    //Set framerate
    m_Timer.setInterval(1000/frameRate);
}


CCameraCapture::~CCameraCapture(void)
{
}

void CCameraCapture::grabFrame(){
    qDebug() << "Worker thread ID" << this->thread();
    //Lock this function
    QMutexLocker ml(&m_Mutex);
    //Local image storage
    cv::Mat cvFrameBGR,cvFrameRGB;
    //Get new frame from camera
    m_Cap>>cvFrameBGR;
    //Convert frame to RGB
    cv::cvtColor(cvFrameBGR, cvFrameRGB, CV_BGR2RGB);
    //Convert cv::Mat to QImage
    QImage m_Frame=QImage((uchar*)(cvFrameRGB.data),cvFrameRGB.cols,cvFrameRGB.rows,QImage::Format_RGB888);     
    //Send frame to receivers
    emit sendFrame(m_Frame);
}

bool CCameraCapture::startGrabbing(){
    //Lock this function
    QMutexLocker ml(&m_Mutex);
    //Check if camera is open
    if(!m_Cap.isOpened()){
        //Connect to camera
        if(!m_Cap.open(m_IdxDevice)){
            emit error(QString("Could not connect to Camera."),QString("Error: No camera detected"));
            return 0;
        }
        else{
            //Start grabbing
            m_Timer.start();
            return 1;
        }
    }
    else{
        //Start grabbing
        m_Timer.start();
        return 1;
    }
}

void CCameraCapture::stopGrabbing(){
    //Lock this function
    QMutexLocker ml(&m_Mutex);
    //Stop grabbing
    m_Timer.stop();
}

bool CCameraCapture::isGrabbing(){
    //Lock this function
    QMutexLocker ml(&m_Mutex);
    //Return true if timer is running and triggering grabFrame
    return m_Timer.isActive();
}

void CCameraCapture::exitThread(){
    //Lock this function
    QMutexLocker ml(&m_Mutex);
    //Stop grabbing
    stopGrabbing();
    //Release camera
    m_Cap.release();
    //Emit finished signal which should be connected to quit() of QThread and deleteLater() of this class;
    emit finished();
}

Answer 1

如果没有调用 start，我怀疑它没有在新线程上运行。

虽然您可能不需要事件循环(尽管 QTimer 需要)，但 Qt documentation状态:-

Begins execution of the thread by calling run(). The operating system will schedule the thread according to the priority parameter. If the thread is already running, this function does nothing.

因此，您应该调用 start，但如果它已经在运行，那么再次调用它也没有什么坏处。

您可以通过在将对象移动到新线程之前和之后调用 QObject 的 thread() 函数并比较返回的指针来测试该对象在哪个线程上运行。