Qt multi-threaded display synchronization problem - c++

Question

We developed an image processing and display program based on Qt. We use sub-threads for image processing and the main thread as UI display. In order to display synchronization, we use the Qt::BlockingQueuedConnection parameter when connecting the signal-slot. There are other threads that may overwrite the image data, so we use locks.
code show as below:
class ProcessThread : public QThread
{
Q_OBJECT
public:
ProcessThread(QObject *parent = nullptr);
void run() override
{
int i=0;
while (1)
{
if(isQuit)break;
mutex.lock();
QString filename=QString("/home/%1.bmp").arg((i++)%2000);
data=imread(filename.toStdString());
//dosomthing...
emit processCompleted(&data);
mutex.unlock();
}
}
void setIsQuit(bool v){isQuit=v;}
signals:
void processCompleted(const Mat *data);
private:
bool isQuit=false;
Mat data;
QMutex mutex;
};
class Widget : public QWidget
{
Q_OBJECT
public:
Widget(QWidget *parent = nullptr): QWidget(parent)
{
thread=new ProcessThread();
connect(thread,SIGNAL(processCompleted(const Mat *)),
this,SLOT(processCompleted(const Mat *)),Qt::BlockingQueuedConnection);
thread->start();
}
~Widget()
{
thread->setIsQuit(true);
thread->quit();
thread->wait();
delete thread;
}
void paintEvent(QPaintEvent *event) override
{
Q_UNUSED(event)
QPainter p(this);
p.drawImage(QPoint(0,0),image);
}
private slots:
void processCompleted(const Mat *data)
{
if(data!=nullptr)
{
if(data->channels()==1)
{
image=QImage(data->data,
data->cols,
data->rows,
data->step,
QImage::Format_Grayscale8);
}
else
{
image=QImage(data->data,
data->cols,
data->rows,
data->step,
QImage::Format_RGB888);
}
}
else
{
image=QImage();
}
repaint();
}
private:
QImage image;
ProcessThread *thread;
};
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
Widget w;
w.show();
return a.exec();
}
When I exit the program, it will be stuck in thread->wait() in the ~Widget() function; it cannot continue to execute;
If I comment out thread->wait(); it will not get stuck when exiting, but it will prompt: "QMutex: destroying locked mutex" and "QThread: Destroyed while thread is still running" errors;
If the Qt::BlockingQueuedConnection parameter is not used when connecting the signal-slot, the first problem above will not occur, but the display will not be synchronized.
We need to display synchronization, how can we do that? Thanks!