Qt: How to force a hidden widget to calculate its layout?

Question

What I am trying to do is render a qwidget onto a different window (manually using a QPainter)

I have a QWidget (w) with a layout and a bunch of child controls. w is hidden. Until w is shown, there is no layout calculations happening, which is expected.

When I call w->render(painter, w->mapToGlobal(QPoint(0,0)), I get a bunch of controls all overlapping each other.
w->layout()->activate();w->layout()->update() doesn't seem to do anything.

Is there a way to force the layout to happen without showing w?

Answer 1

Try with the QWidget::sizeHint() method, which is supposed to return the size of the widget once laid out.

Answer 2

QWidget::adjustSize works for me. But I did some tricky things: because widget is invisible, I have to explicitly set to visible. Sometimes calling adjustSize of the top most widget doesn't work, but call children first then call top-most widget works. Good Luck!

Answer 3

Forcing a layout calculation on a widget without showing it on the screen:

widget->setAttribute(Qt::WA_DontShowOnScreen);
widget->show();

The show() call will force the layout calculation, and Qt::WA_DontShowOnScreen ensures that the widget is not explicitly shown.

Very useful for command line unit tests, where you do not want the user's desktop to be cluttered with random widgets popping up left and right.