How to properly use adjustSize() in Qt - c++

Question

I'm new to Qt, and this issue about auto resizing has driven me crazy.
I create a class called RenderArea that inherits QWidget. In its paintEvent(), I use a QPainter to draw an image. In order for the whole window to scale with the image, I resize before painting. The relevant code is
if (image && !image->isNull())
{
resize(image->size());
painter.drawImage(image->rect(), *image, image->rect());
}
However, RenderArea will stretch too much through other widgets (like buttons and menus). It is contained in a centralWidget with a vertical layout. But when I call centralWidget->adjustSize() it does not scale everything together, but instead shrinks RenderArea t and hides the image.
How do I instruct the central widget as well as the window to scale with the new size of my customized widget? I know I could use a QLabel and set its scaledContents to be true, but I need a lot of other sophisticated rendering so a simple QLabel is not enough.

Answer 1

The sizeHint function should return recommended widget's size. So RenderArea should return image size as its sizeHint. When the image is changed the updateGeometry function should be called to update a cached sizeHint value:
class RenderArea : public QWidget
{
public:
explicit RenderArea(const QImage &image, QWidget *parent = 0)
: QWidget(parent), m_image(image)
{
}
QSize sizeHint() const
{
return m_image.isNull() ? QWidget::sizeHint() : m_image.size();
}
void paintEvent(QPaintEvent *event)
{
QWidget::paintEvent(event);
if (!m_image.isNull())
{
QPainter painter(this);
painter.drawImage(rect(), m_image, m_image.rect());
}
}
void setImage(const QImage &image)
{
m_image = image;
updateGeometry();
}
private:
QImage m_image;
};
When the child widget is resized, the parent widget isn't doing it automatically. Fortunately there is QWidget::adjustSize function which lets us to resize the parent widget to fit its content:
class Window : public QWidget
{
Q_OBJECT
private slots:
void onImageChanged(const QString &fileName)
{
m_area->setImage(QImage(fileName));
adjustSize();
}
private:
RenderArea *m_area;
};