Call PaintEvent in for loop

Tony123

Hi guys,

I want to make a scroll test performance. It have to work like that you scroll down a document by 1px then you call paintEvent and this have to happened 1000 times. So in for loop I write:

for(int i=0;i<1000;++i){
    document->m_preview->Scroll(0,1); // perform 1pixel scroll

}

Problem is that the paintEvent is called only on the last scroll call. How can I force to repaint after all calling of scroll function?

update(); repaint() doesn't seems to work!

THX

JonB

@Tony123
What Qt class is your document->m_preview?

jsulm

@Tony123 Well, you need to learn one thing: loops block Qt event loop (applies to all event driven applications) as long as they are executed. So, what you see is what you should expect. You should use a timer (QTimer) instead of a blocking loop.

Asperamanca

Try calling QCoreApplication::processEvents() after each 1px scroll

jsulm

@Asperamanca Please not! This is bad design. OK, for testing I would agree :-)

Asperamanca

@jsulm
OP states that this is just for a test. Of course such constructs should be avoided in production code.

mrjj

hehe
If you are feeling guilty ;) , you can always show how to do the
same thing with QTimer and a lambda.
But for pure testing, i agree. no harm using QCoreApplication::processEvents()

Gojir4

If your goal is to measure how many time it takes to paint a widget 1000 times, so QWidget::repaint() is the right choice, otherwise, I would recommend to use QWidget::update() instead of QWidget::repaint(). In this case, using loop and QCoreApplication::processEvents(), I think it will be the same. But with another design, this will let the event-loop repaint the widget only when it's necessary.

Asperamanca

@mrjj said in Call PaintEvent in for loop:

hehe
If you are feeling guilty , you can always show how to do the
same thing with QTimer and a lambda.
But for pure testing, i agree. no harm using QCoreApplication::processEvents()

Not feeling guilty, because for a performance test, combining QTimer and a lambda would make it much harder to measure the actual time spent on repainting after scroll, assuming I understood the OP's intentions here correctly.

QElapsedTimer measureTimer;
measureTimer.start();
for(int i=0;i<1000;++i){
    document->m_preview->Scroll(0,1); // perform 1pixel scroll
    QCoreApplication::processEvents();
}
qDebug() << measureTimer.elapsed();

and you are done, while still being very close to what the production code would do.

Tony123

@Asperamanca thank you ..yes exactly this is what I did this helps for purpose of testing of course ty