Understanding GraphicsView Viewport Size

[vgrunert]

Hi,

I know that I found a similar topic somewhere this for ex. but it does not help me.
Anyway, I really can't figure out why the graphicsview always returns QSize(98,28) as it's size, geometry and viewport size upon construction.

It's a problem because I intend to draw something on the scene based on the actual viewport coordinates.
This of course is not possible since I get the wrong viewport size results.

Here is a small snippet of what I did and would like to do:

//inside a QMainWindow constuctor: 
Mainwindow::Mainwindow(QWidget *parent)
     : QMainWindow(parent)
 
{
...
QGraphicsScene *scene = new QGraphicsScene(this); 
QGraphicsView *view = new QGraphicsView(this);
view->setScene(scene);
view->show(); 
drawSomething();
qDebug() << view->viewport()->size();
qDebug() << view->geometry();
...
}
 
...
void Mainwindow::drawSomething()
{
     //draw 5 lines equally spaced aroud the width of the viewport
     //does not work because I get the 98 pixels as the width of the viewport width
     qreal intervall = view->viewport()->width() / 5;
     for(int i = 0; i < 5; ++i)
     {
          scene->addLine(intervall*i, 0, intervall*i, 1);    
     }
}
 
// returns 
QSize(98, 28)
QRect(0,0 100x30)

//inside a QMainWindow constuctor: 
Mainwindow::Mainwindow(QWidget *parent)
     : QMainWindow(parent)

{
...
QGraphicsScene *scene = new QGraphicsScene(this); 
QGraphicsView *view = new QGraphicsView(this);
view->setScene(scene);
view->show(); 
drawSomething();
qDebug() << view->viewport()->size();
qDebug() << view->geometry();
...
}

...
void Mainwindow::drawSomething()
{
     //draw 5 lines equally spaced aroud the width of the viewport
     //does not work because I get the 98 pixels as the width of the viewport width
     qreal intervall = view->viewport()->width() / 5;
     for(int i = 0; i < 5; ++i)
     {
          scene->addLine(intervall*i, 0, intervall*i, 1);    
     }
}

// returns 
QSize(98, 28)
QRect(0,0 100x30)

To copy to clipboard, switch view to plain text mode 

These values are not the actual values of the Graphicsview of course. Now the problem is that I need to draw lines for ex.
based on the actual viewport size.

Many thanks.

[wysota]

Anyway, I really can't figure out why the graphicsview always returns QSize(98,28) as it's size, geometry and viewport size upon construction.

Widgets have bogus sizes until they are shown for the first time (simply calling show() does not make them visible).

It's a problem because I intend to draw something on the scene based on the actual viewport coordinates.

That rarely makes sense. Are you sure you don't want to draw it as the view's background?

[vgrunert]

Thank you for the quick reply. No, in fact I am not sure.
But I want the lines to be scaled along with changes in resize events.
Hence I thought that the right approach would be to calculate the lines expressed in their viewport coordinates.


Btw

...
view->setVisible(true);
...

...
view->setVisible(true);
...

To copy to clipboard, switch view to plain text mode 

does not work either.

So how do I draw the view at it's construction?

[wysota]

Thank you for the quick reply. No, in fact I am not sure.
But I want the lines to be scaled along with changes in resize events.
Hence I thought that the right approach would be to calculate the lines expressed in their viewport coordinates.

If you want something to be in viewport coordinates then it doesn't seem to belong in the scene. In my opinion you should implement it in QGraphicsView::drawBackground() reimplementation.

Btw

...
view->setVisible(true);
...

...
view->setVisible(true);
...

To copy to clipboard, switch view to plain text mode 

does not work either.

because that's not the point. show() only says "I want to be visible". The actual magic happens when the application processes that event.

So how do I draw the view at its construction?

You don't.

[vgrunert]

Thank you again. Yes that looks about right. I'll give it a try and post my results once done.

bw

[d_stranz]

If you want to get the actual size of a widget, you can override the showEvent(). By the time it gets there, the initial size and layout has been calculated and dimensions are correct. If you want to respond to changes in size, you also need to handle resizeEvent(). But beware, resizeEvent() can be called before showEvent(), so you have to wait until showEvent() to ensure that resizeEvent() contains correct widget dimensions.