moving child widget inside parent widget without repainting - c++

Question

I am trying to make a child widget to be aligned to the right-bottom corner of a parent widget. The repainting of the child widget is costly so I want to avoid it. Because I am only moving the child widget with a static content, theoretically no repainting is necessary. But I do not know how to achieve that. I react to resizeEvent(), in which I update the child's position to the corner. I have set the widget attributes (trial-error method) to minimize the paint events as far as I was able to. But there are still some repaint events called for the child when the parent is being resized - when enlarged and shrinked as well. When being resized slowly and only in one direction (x or y), it seems only 1 pixel wide band is repainted. When resized faster and in both directions at the same time, the child widget is repainted all. Is it possible to tweak the code to avoid the repainting of the child widget completely? I would like to avoid write manual double-buffering algorithm for this case myself, I hope Qt is able to solve this somehow for me. Is it?
#include <QApplication>
#include <QDebug>
#include <QPainter>
#include <QPaintEvent>
#include <QWidget>
class ChildWidget : public QWidget
{
public:
ChildWidget(QWidget *parent) : QWidget(parent)
{
setAttribute(Qt::WA_StaticContents);
setAttribute(Qt::WA_OpaquePaintEvent);
}
protected:
void paintEvent(QPaintEvent *event) override
{
qDebug() << "paintEvent" << event->rect();
QPainter painter(this);
painter.fillRect(event->rect(), QBrush(QColor("red")));
}
};
class ParentWidget : public QWidget
{
public:
ParentWidget()
{
m_childWidget = new ChildWidget(this);
m_childWidget->resize(100, 100);
setAttribute(Qt::WA_StaticContents);
resize(200, 200);
}
protected:
void resizeEvent(QResizeEvent *event) override
{
QWidget::resizeEvent(event);
m_childWidget->move(width() - 100, height() - 100);
}
private:
ChildWidget *m_childWidget;
};
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
ParentWidget w;
w.show();
return a.exec();
}
Note: In my final use case, this parent widget is intended to be covered completely by several non-overlapping child widgets and each of these child widgets will be opaque and will take care of painting its whole area. I.e. no special handling of alpha channel is needed. This might be important information for setting the widget attributes.
Note: I tried a trick, not moving the child immediately, but postpone it with QTimer::singleShot(0, [this]{m_childWidget->move(width() - 100, height() - 100);}); This prevents repainting during parent widget resizing but only in cases when the size of the parent grows. When it shrinks, it is the same as before.