Weird issue with QGraphicsScene

0
votes

My objective: I want to draw a simple rectangle of length and breadth 100, at position x = 0, y = 0 on QGraphicsView. That should look something like this This is how I want

What i have done so far I have created a object (on heap) called block_realiser in constructor of homepage(MainWindow), it accepts QGraphicsView as parameter in constructor. I created a QGraphicsScene (on heap) in block realiser constructor, and in its constructor itself I set this scene to the view. There is a function called drawRect in block_realiser that is supposed to draw a rect of 100x100 at (0,0). Code is

Block_Realiser::Block_Realiser(QGraphicsView *view, QObject *parent) :
    QObject(parent)
{
    m_View = view;
    m_Scene = new QGraphicsScene;
    m_View->setScene(m_Scene);
}

void Block_Realiser::drawRect()
{
    m_Scene->addRect(m_View->x(), m_View->y(),
                 100, 100);
}

Now coming to my problem. There are two ways of calling the function drawRect in constructor of homepage. One is through timer (after 100 ms delay) and another is to call directly 1) Through timer, code is 

HomePage::HomePage(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::HomePage)
{
    ui->setupUi(this);
    realiser = new Block_Realiser(ui->graphicsView);

    QTimer *timer = new QTimer(this);
    connect(timer, SIGNAL(timeout()), realiser, SLOT(drawRect()));
    connect(timer, SIGNAL(timeout()), timer, SLOT(deleteLater()));
    timer->setSingleShot(true);
    timer->start(100);
}

Output is

enter image description here

2) Calling function directly

Code is 

HomePage::HomePage(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::HomePage)
{
    ui->setupUi(this);
    realiser = new Block_Realiser(ui->graphicsView);
    realiser->drawRect();
}

Output is enter image description here

So can anyone explain me what is going on in the above two cases? And how I can achieve my objective?. I have previously subclassed qwidget and reimplemented its paintEvent to achieve the same result of my objective. But this is not happening in qgraphicsscene. Please help me out. If any details missing, let me know.

2
c++qtqt5qgraphicsviewqgraphicsscene
Is your problem that the rectangle is not in the upper left corner? That has to do with the scene bounding rect. By default, QGraphicsView will center the scene. Try ui->graphicsView->setAlignment(Qt::AlignLeft|Qt::AlignTop); – king_nak

2 Answers

1
votes

AddRect coordinates are relative to the item, not to the widget containing the qgraphicscene.

You should call

  m_Scene->addRect(0,0,100,100);
1
votes

The views position is recalculated when its shown. When you use the timer, the m_View->x() and m_View->y() values are probably different than when you call your drawRect method directly. This would mean different width and height values. I don't understand why do you use the views position + 100 to calculate the size of your rect.

If you want your rect to be in the upper left corner, just set the alignment to your view:

m_View->setAlignment(Qt::AlignLeft | Qt::AlignTop);