使用QThread而不调用QThread::start()有意义吗?

Question

一段时间以来，我现在与Qt在一个应用程序上工作，我必须从相机中抓取帧。相机将在不同于应用程序其他部分的线程中运行。我遵循以下建议:

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

不能子类化QThread。相反，我创建了一个工作对象ccamercapture并将其移动到QThread中。相机的抓帧由QTimer控制，QTimer连接到抓帧槽。在将ccamercapture移动到QThread之后，可以通过启动计时器来启动捕获。我的问题是:我是否必须调用QThread类的start() ?我的方法不用叫就能奏效。该循环由计时器执行，而worker实际上在另一个线程中工作，而无需调用start。所以我想知道如果使用定时器调用start()是否有任何意义?为了清晰起见，我把我的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。但如果我调用camercapture -> start抓取()，它会很好地工作。定时器触发grabFrame()槽，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在页面末尾)

这是我的ccamercapture .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 <opencv2opencv.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start没有被调用，我怀疑它没有在新线程上运行。

虽然你可能不需要事件循环(尽管QTimer需要)，Qt文档中说:-

通过调用run()开始执行线程。操作系统将根据优先级参数调度线程。如果线程已经在运行，则此函数不执行任何操作。

所以，你应该调用start，但是如果它已经在运行，那么再调用一次也没什么坏处。

你可以通过调用QObject的thread()函数来测试对象在哪个线程上运行，在你将对象移动到一个新线程之前和之后，并比较返回的指针。

Answer 2

好的，我想我找到了点，为什么我不必调用start()。对于计时器，我使用

connect(&m_Timer, SIGNAL(timeout()), this, SLOT(grabFrame()), Qt::DirectConnection);

直接连接将在没有启动事件循环的情况下工作。如果我把它改成

connect(&m_Timer, SIGNAL(timeout()), this, SLOT(grabFrame());

我必须打电话给

m_CameraCaptureThread->start();