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

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.

Answer 2

The layout calculation of a widget can be forced by calling invalidate() followed by activate() on its layout, even if the widget is hidden. This also causes the widget's size() and sizeHint() functions to return correct and updated values, even if show() has not yet been called on that widget.

It is however necessary to care about all child widgets and layouts recursively, as a layout recalculation request doesn't automatically propagate to the childs.

The following code shows how to do this.

/**
 * Forces the given widget to update, even if it's hidden.
 */
void forceUpdate(QWidget *widget) {
    // Update all child widgets.
    for (int i = 0; i < widget->children().size(); i++) {
        QObject *child = widget->children()[i];
        if (child->isWidgetType()) {
            forceUpdate((QWidget *)child);
        }
    }

    // Invalidate the layout of the widget.
    if (widget->layout()) {
        invalidateLayout(widget->layout());
    }
}

/**
 * Helper function for forceUpdate(). Not self-sufficient!
 */
void invalidateLayout(QLayout *layout) {
    // Recompute the given layout and all its child layouts.
    for (int i = 0; i < layout->count(); i++) {
        QLayoutItem *item = layout->itemAt(i);
        if (item->layout()) {
            invalidateLayout(item->layout());
        } else {
            item->invalidate();
        }
    }
    layout->invalidate();
    layout->activate();
}