Freezed window

  • Hi all,
    is my first time in a form so, if I made mistakes, sorry...
    I wrote a simply program that get images from udp channel and show them in my MainWindow, also I show some counter.
    The program work fine and fast but MainWindows is not sizable, is not movable, clicking button nothing happens.

    Now I try to explain my problem:
    there are 2 classes, one for mainwindows and one for udp channel; these classes are connected by two signals and two slots

    @int main(int argc, char *argv[])
    QApplication a(argc, argv);
    Camera *camera = new Camera();

    udpCameraManager *cameraManager = new udpCameraManager();
    camera->cameraManager = cameraManager;
    return a.exec();


    Clicking a button the first GetImage happen and the download starts. GetImage is:

    @ void GetImage()
    QByteArray Data;
    Data.append("Hello from UDP");

      // Sends the datagram datagram
      // to the host address and at port.
      // qint64 QUdpSocket::writeDatagram(const QByteArray & datagram,
      //                      const QHostAddress & host, quint16 port)
      QHostAddress RemoteHost;
      connect(socket, SIGNAL(readyRead()), this, SLOT(slotReadData()));
      socket->writeDatagram(Data, RemoteHost, 11000);


    then slotReadData emit ImageReadySignal that is connected to DoDrawImage; DoDrawImage create image, call QWidget::repaint() and emit Ready signal; Ready signal is connected to GetImage.
    My paintEvent is:

    @void Camera::paintEvent(QPaintEvent *event)
    QPainter painter(this);

    if (!image.isNull())
        double difference = (time.elapsed() / 1000);
        QRect rect = QRect(670, 140, 180, 20);
        painter.drawText(rect,Qt::AlignLeft,"difference: " + QString::number(difference,'f',2));
        rect = QRect(670, 170, 180, 20);
        if(difference > 0)
            double fps = (double)ImgCounter/(double)difference;
            painter.drawText(rect,Qt::AlignLeft,"fps: " + QString::number(fps,'f',2));
        rect = QRect(670, 200, 180, 20);
        painter.drawText(rect,Qt::AlignLeft,"Images: " + QString::number(ImgCounter));


  • For me that sounds like an infinite loop of your signal and slot connections. Is that possible?

  • Yes it is, I need to download and show continuosly, but I dont know how to solve the freeze, I'm new on Qt programming.

  • Your slots should do as little work as possible because staying in a slot blocks the event queue. You can use QThread for example to download the image. When it is finished call a slot to display it. If it fits your needs you can also use QTimer to update you UI every x milliseconds.
    There are a few possibilities to periodically update your UI but a major rule is to never block the event queue.

  • OK,
    I'll try it and I'll answer as soon as possible.
    Thanks alot for your suggestions.

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