Draw ellipse delay and draw another one - c++

Question

i need to draw a ellipse and after some delay draw another one.
I having trouble doing this.
This is a simplified version of the code that i'm actually doing, but i belive this will help me solve the problem
here is the code.
MyView::MyView()
{
sc = new QGraphicsScene();
this->setSceneRect(0,0,800,600);
this->setFixedSize(800,600);
this->setStyleSheet("QScrollBar {height:0px;}");
this->setStyleSheet("QScrollBar {width:0px;}");
sc->setSceneRect(0,0,800,600);
this->setScene(sc);
}
void MyView::mousePressEvent(QMouseEvent *event)
{
sc->addEllipse(event->x(),event->y(),10,10,QPen(),QBrush(Qt::red));
int i=0;
while(i < 1000000000) // SIMULATING DELAY
i++; //
sc->addEllipse(event->y(),event->x(),10,10,QPen(),QBrush(Qt::blue));
}
class MyView : public QGraphicsView
{
public:
MyView();
QGraphicsScene *sc;
public slots:
void mousePressEvent(QMouseEvent *event);
};
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MyView wv;
wv.show();
return a.exec();
}
the first ellipse is not showing up until the while over and the second one appears
it's doesn't matter how long is the while. Always draw the two ellipses at the same time.

Answer 1

In the main thread of the GUI you should not have tasks that take too much time since they block the eventloop preventing the GUI's own work from being performed. In your case, that while loop can be replaced by a QTimer:
void MyView::mousePressEvent(QMouseEvent *event)
{
QPointF p = mapToScene(event->pos());
sc->addEllipse(QRectF(p, QSizeF(10, 10)), QPen(), QBrush(Qt::red));
// 1000 is the delay in ms
QTimer::singleShot(1000, this, [this, p](){
sc->addEllipse(QRectF(p, QSizeF(10, 10)), QPen(), QBrush(Qt::blue));
});
}