What was missing was the implementation of showEvent(). This is now done and it fits well, even when not resizing.

Yes, For widgets it is showEvent() which is first called when it is shown. For anything "size" that is better than in constructor.

@odelaune
But nonetheless you still say it does work when you do the fitInView() in the resizeEvent(), but not in the constructor, right??

Yes, it does work in resizeEvent().

Then I would basically read it as: fitInView() does not work "at all" in constructor before the view/scene is really shown, and that's just how it is.

It makes sense.

Assuming it's good in resizeEvent() you have your solution now anyway? (If it's still not right very first time when shown but no resizeEvent() is called, you can do it initially in showEvent() too?)

What was missing was the implementation of showEvent(). This is now done and it fits well, even when not resizing.

Here is the working example code

test.cpp

// Qt headers
#include <QPixmap>
#include <QGraphicsPixmapItem>
#include <QGraphicsRectItem>
#include <QPainter>
#include <QPen>
#include <QDebug>
#include <QResizeEvent>

// Other headers
#include "test.h"

Test::Test(QWidget* parent /*=nullptr*/):
    QMainWindow(parent)
{
    m_view = new QGraphicsView(this);

    // draw border of the QGraphicsView
    m_view->setStyleSheet("border: 5px solid red");

    m_scene = new QGraphicsScene(m_view);

    m_view->setRenderHint(QPainter::Antialiasing, true);
    m_view->setScene(m_scene);
    m_view->setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
    m_view->setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff);

    QPixmap img("tmp.svg");
    QGraphicsPixmapItem *item = m_scene->addPixmap(img);

    // draw border of the QGraphicsScene
    QGraphicsRectItem *rect_item = m_scene->addRect(m_scene->sceneRect());
    rect_item->setParentItem(item);
    QPen pen = QPen(Qt::black);
    pen.setWidth(10);
    rect_item->setPen(pen);

    // set the mapView as the central widget
    setCentralWidget(m_view);

    m_view->show();
}

// destructor
Test::~Test()
{
}


void Test::resizeEvent(QResizeEvent* event)
{
    if(!m_scene)
        return;
    QRectF bounds = m_scene->itemsBoundingRect();
    m_view->fitInView(bounds, Qt::IgnoreAspectRatio);
}


void Test::showEvent(QShowEvent* event)
{
    if(!m_scene)
        return;
    QRectF bounds = m_scene->itemsBoundingRect();
    m_view->fitInView(bounds, Qt::IgnoreAspectRatio);
}

Thank you very much for your support.

Then I would basically read it as: fitInView() does not work "at all" in constructor before the view/scene is really shown, and that's just how it is. Assuming it's good in resizeEvent() you have your solution now anyway? (If it's still not right very first time when shown but no resizeEvent() is called, you can do it initially in showEvent() too?)

For example, debug out what is returned from m_scene->itemsBoundingRect() for that line where you call it in the constructor, my guess is it's not the size it will end up being?

Actually, they are the same (QRectF(-5,-5 472x349)) both in the constructor and in the resizeEvent function. The value doesn't change either when I resize the window (still QRectF(-5,-5 472x349)). The same with sceneRect().

Size is only "correct" when the object has been shown, since that is when Qt can calculate/set it. So it would not be surprising if you have to redo this line in every resizeEvent....

main.cpp

// Qt headers
#include <QApplication>

#include "test.h"

int main(int argc, char *argv[])
{
    QApplication application(argc, argv);

    Test *applicationWindow = new Test();
    applicationWindow->setMinimumWidth(400);
    applicationWindow->setMinimumHeight(400);
    applicationWindow->show();

    return application.exec();
}

test.cpp

// Qt headers
#include <QPixmap>
#include <QGraphicsPixmapItem>
#include <QGraphicsRectItem>
#include <QPainter>
#include <QPen>
#include <QResizeEvent>

// Other headers
#include "test.h"

Test::Test(QWidget* parent /*=nullptr*/):
    QMainWindow(parent)
{
    m_view = new QGraphicsView(this);

    // draw border of the QGraphicsView
    m_view->setStyleSheet("border: 5px solid red");

    m_scene = new QGraphicsScene(m_view);

    m_view->setRenderHint(QPainter::Antialiasing, true);
    m_view->setScene(m_scene);
    m_view->setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
    m_view->setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff);

    QPixmap img("tmp.svg");
    QGraphicsPixmapItem *item = m_scene->addPixmap(img);

    // draw border of the QGraphicsScene
    QGraphicsRectItem *rect_item = m_scene->addRect(m_scene->sceneRect());
    rect_item->setParentItem(item);
    QPen pen = QPen(Qt::black);
    pen.setWidth(10);
    rect_item->setPen(pen);

    // set the mapView as the central widget
    setCentralWidget(m_view);

    m_view->show();

    m_view->fitInView (m_scene->itemsBoundingRect(),Qt::IgnoreAspectRatio); // useless here

}

// destructor
Test::~Test()
{
}


void Test::resizeEvent(QResizeEvent* event)
{
    if(!m_scene)
        return;
    QRectF bounds = m_scene->itemsBoundingRect();
    m_view->fitInView(bounds, Qt::IgnoreAspectRatio);
}

test.h

#ifndef TEST_H
#define TEST_H

#include <QMainWindow>
#include <QGraphicsView>
#include <QGraphicsScene>

class Test : public QMainWindow
{
    Q_OBJECT
public:
    explicit Test(QWidget* parent = nullptr);
    ~Test() override;
    void resizeEvent(QResizeEvent* event) override;

public slots:

private:
    QGraphicsView *m_view;
    QGraphicsScene *m_scene;
};

#endif // TEST_H

And here is what I get when I start the application:

So clearly not what I am expecting.

However, I get what I want when I resize the window:

The trick is done via the resizeEvent method. But I do not understand how can I do the same in the same in the constructor. Indeed

m_view->fitInView (m_scene->itemsBoundingRect(),Qt::IgnoreAspectRatio)

does not do the job when it is placed at the end of the constructor but works fine in resizeEvent.

Any idea?

m_view = new QGraphicsView();

m_view = new QGraphicsView( this );

QGraphicsScene *scene = new QGraphicsScene( m_view or this );
m_view->setRenderHint(QPainter::Antialiasing, true);
m_view->setScene(scene); ///The view does not take ownership of scene with this call.

Try to change resize policy of scene

check this example: https://code.qt.io/cgit/qt/qtbase.git/tree/examples/widgets/graphicsview/elasticnodes?h=6.3

@JonB said in QGraphicsScene does not fit QGraphicsView full size:

• Maybe your scene is already the full size of the view (or vice versa). Maybe you actually need to enlarge the scene/view to fill the whole window.

Why not but I am not sure to know how to do it.

• I would try with a QWidget instead of a QMainWindow.
• Maybe your scene is already the full size of the view (or vice versa). Maybe you actually need to enlarge the scene/view to fill the whole window.

That's all i know.

How would you rewrite my code to get what I look for?

Ah, actually for the whole scene: void QGraphicsView::fitInView(const QRectF &rect, Qt::AspectRatioMode aspectRatioMode = Qt::IgnoreAspectRatio)

ui->view->fitInView( scene->sceneRect(),Qt::KeepAspectRatio);

?