Getting size of a QGraphicsScene

[amleto]

sorry, I edited my post a few times and took some questions out as I found the actual problem I think you are facing - see the end of my previous post.

[sedi]

Hi Amleto, you've made my week. Thinking about it, it's just logical. I had thought, "this->showMaximized();" would have put me onto the sunny side of a maximized window already - but, of course, the "mainWin.show();" is yet to come after all the stuff I did in the constructor. Actually I took a different approach in the details, but you are right, the real problem is: i have to wait for the resize. I've tried it out quickly with a single shot timer - BINGO.

QPointF tl(view->horizontalScrollBar()->value(), view->verticalScrollBar()->value());
    QPointF br = tl + view->viewport()->rect().bottomRight();
    QMatrix mat = view->matrix().inverted();
    QRectF visibleRect=mat.mapRect(QRectF(tl,br));
    rectItem->setPos(view->mapToScene(view->viewport()->pos()));
    visibleRect.setRect(visibleRect.x(),visibleRect.y(),visibleRect.width()-1,visibleRect.height()-1); //the "-1" is just for being able to see the red pen 
    rectItem->setPen(QPen(Qt::red));
    rectItem->setRect(visibleRect);

QPointF tl(view->horizontalScrollBar()->value(), view->verticalScrollBar()->value());
    QPointF br = tl + view->viewport()->rect().bottomRight();
    QMatrix mat = view->matrix().inverted();
    QRectF visibleRect=mat.mapRect(QRectF(tl,br));
    rectItem->setPos(view->mapToScene(view->viewport()->pos()));
    visibleRect.setRect(visibleRect.x(),visibleRect.y(),visibleRect.width()-1,visibleRect.height()-1); //the "-1" is just for being able to see the red pen 
    rectItem->setPen(QPen(Qt::red));
    rectItem->setRect(visibleRect);

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Edit:
To make it work in the initialisation of my real program it was not enough to hook into the resizeEvent, I also had to call the calculation from QEvent::show, as done here:

void CentralWidget::resizeEvent(QResizeEvent *)
{
    emit this->centralWidgetWasResized();
}
bool CentralWidget::event(QEvent * event)
{
    if (event->type()==QEvent::Show) emit this->centralWidgetWasResized();
    event->ignore();
    QWidget::event(event);
    return true;
}

void CentralWidget::resizeEvent(QResizeEvent *)
{
    emit this->centralWidgetWasResized();
}
bool CentralWidget::event(QEvent * event)
{
    if (event->type()==QEvent::Show) emit this->centralWidgetWasResized();
    event->ignore();
    QWidget::event(event);
    return true;
}

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Still, though, I'll have to keep in mind that the correct size is provided only after nearly all my constructors' work has been done.
Thank you very much for all the effort you've put into this!

[Spitfire]

To make it work in the initialisation of my real program it was not enough to hook into the resizeEvent, I also had to call the calculation from QEvent::show, as done here:

That is incorrect. If you had to do that - you did something wrong.

Take a look (as compile at try) at this example code:

// mainwindow.h
class QGraphicsView;
class QGraphicsRectItem;
 
class MainWindow : public QMainWindow
{
    Q_OBJECT
 
public:
    MainWindow( QWidget* parent = 0) ;
 
protected:
    void    resizeEvent( QResizeEvent* event );
 
private:
    QGraphicsView*      view;
    QGraphicsRectItem*  item;
};
 
//mainwindow.cpp
MainWindow::MainWindow( QWidget* parent )
    :
    QMainWindow( parent ),
    view( new QGraphicsView( this ) ),
    item( NULL )
{
    QGraphicsScene* scene = new QGraphicsScene( this );
    this->item = scene->addRect( 0,0,0,0, QPen(), Qt::blue );
 
    this->view->setScene( scene );
    this->setCentralWidget( view );
}
 
void    MainWindow::resizeEvent( QResizeEvent* e )
{
    this->view->scene()->setSceneRect( this->view->viewport()->rect() );
    this->item->setRect( this->view->viewport()->rect() );
}
 
// main
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.showMaximized(); // here you show the window, not in the constructor
 
    return a.exec();
}

// mainwindow.h
class QGraphicsView;
class QGraphicsRectItem;

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow( QWidget* parent = 0) ;

protected:
    void    resizeEvent( QResizeEvent* event );

private:
    QGraphicsView*      view;
    QGraphicsRectItem*  item;
};

//mainwindow.cpp
MainWindow::MainWindow( QWidget* parent )
    :
    QMainWindow( parent ),
    view( new QGraphicsView( this ) ),
    item( NULL )
{
    QGraphicsScene* scene = new QGraphicsScene( this );
    this->item = scene->addRect( 0,0,0,0, QPen(), Qt::blue );

    this->view->setScene( scene );
    this->setCentralWidget( view );
}

void    MainWindow::resizeEvent( QResizeEvent* e )
{
    this->view->scene()->setSceneRect( this->view->viewport()->rect() );
    this->item->setRect( this->view->viewport()->rect() );
}

// main
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.showMaximized(); // here you show the window, not in the constructor

    return a.exec();
}

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Resize event always has the new size of the window and is delivered after the show event. The is no reason for you to check size of the window in the show event.
Also, you should not show widget from within the widget, you've got main() for that.

Still, though, I'll have to keep in mind that the correct size is provided only after nearly all my constructors' work has been done.

Again not correct - window size is set after calling show (for first time or resize or any other method of changing window geometry) but only after execution flow goes back to the event loop.
So not only window size isn't set when you constructor work is entirely finished, it will not be set untill the system takes over (in this case when you call a.exec() ).

[sedi]

Thank you, I'll look int that!!