#include <QtCore>
#include <QtGui>
#include <QtWidgets>

class ModalDialog : public QDialog
{
   Q_OBJECT

public:
   ModalDialog(QWidget *parent = 0);

public slots:
   void slotLongOperation();
};

class MainWindow : public QMainWindow
{
    Q_OBJECT
public:
    MainWindow(QWidget *parent = 0);

public slots:
    void slotShowModalDialog();
};

ModalDialog::ModalDialog(QWidget *parent)
  : QDialog(parent)
{
   setWindowTitle("ModalDialog");
   setModal(true);
   resize(400, 400);

   QPushButton *btnLO = new QPushButton("Begin Long Task!", this);
   btnLO->resize(200, 35);
   connect(btnLO, SIGNAL(clicked()), this, SLOT(slotLongOperation()));
}

void ModalDialog::slotLongOperation()
{
   qApp->setOverrideCursor(QCursor(Qt::WaitCursor));
   QThread::sleep(3);
   qApp->restoreOverrideCursor();


   QMessageBox mBox(this);
   mBox.setText("Long Task Completed!");
   mBox.exec();
}

MainWindow::MainWindow(QWidget *parent)
  : QMainWindow(parent)
{
    setWindowTitle("MainWindow");
    
    QPushButton* btn = new QPushButton("Open Dialog", this);
    btn->resize(200, 35);

    QHBoxLayout *layout = new QHBoxLayout;
    layout->setMargin(10);
    layout->addWidget(btn);
    setLayout(layout);

    connect(btn, SIGNAL(clicked()), this, SLOT(slotShowModalDialog()));
}

void MainWindow::slotShowModalDialog()
{
   ModalDialog dlg(this);
   dlg.exec();
}

#include "main.moc"
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    MainWindow w;
    w.resize(800, 400);
    w.show();

    return a.exec();
}