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 i

Answer 1

When going through QWidget::grab() I noticed this part:

if (r.width() < 0 || r.height() < 0) {
    // For grabbing widgets that haven't been shown yet,
    // we trigger the layouting mechanism to determine the widget's size.
    r = d->prepareToRender(QRegion(), renderFlags).boundingRect();
    r.setTopLeft(rectangle.topLeft());
}

QWidget::grab() has been introduced in Qt 5.0, and this test function with a QDialog containing a layout seems to work on Qt 5.5.1

int layoutTest_2(QApplication& a)
{
    CLayoutTestDlg dlg; // initial dlg size can also be set in the constructor
        // https://stackoverflow.com/questions/21635427

    QPixmap pixmap = dlg.grab(); // must be called with default/negative-size QRect
    bool savedOK = pixmap.save("E:/temp/dlg_img.png");
        // saving is not necessary, but by now the layout should be done

    dlg.show();
    return a.exec();
}