How to properly position a QGraphicsTextItem on a QGraphicsScene

Question

The probelm I have is that I am trying to position a QGraphicsTextItem on a specific location of my QGraphicsScene , to be precise in the center of the speedometer below. But nothing I tried seems to work properly. Despite I clearly provided the coordinates I want the QGraphicsTextItem, it still anchored on top-left as shown below: Below the snippet of code:

speedwidget.cpp

SpeedWidget::SpeedWidget(QWidget *parent)
    : QWidget(parent)
    , ui(new Ui::SpeedWidget)
{
    ui->setupUi(this);
    mScene = new Scene(this);
    ui->graphicsView->setScene(mScene);

    io = new QGraphicsTextItem;
    io->setPos(210,240);
    QFont *f = new QFont;
    f->setPointSize(18);
    io->setFont(*f);
    mScene->addText("Odometer :")->setDefaultTextColor(Qt::red);
    mScene->addItem(io);
}

speedwidget.h

class SpeedWidget : public QWidget
{
    Q_OBJECT
public:
    SpeedWidget(QWidget *parent = nullptr);
    ~SpeedWidget();

private:
    Ui::SpeedWidget *ui;
    Scene *mScene;
    QGraphicsTextItem *io;

};
#endif // SPEEDWIDGET_H

What I tried so far to solve the problem was:

1. I consulted this post which explained the idea but I don't need to change the background color in this example.

2. This one had a good snippet, but I am not trying to highlight the color. I am only trying to position the text on a specific location.

3. If I change io->setPos(210,240); and give different numbers, I still see it anchored on top-left.

Thanks for pointing to the right direction for solving this issue.

Answer 1

The Qt Graphics View framework implements a parent-child mechanism. Simply make your QGraphicsTextItem become a child of your odometer's dial item. From then on, your text label's position will be relative to the one of the dial:

io = new QGraphicsTextItem(this);

// Position text item so that it's centered on the dial
...

Furthermore, note that this line:

mScene->addText("Odometer :")->setDefaultTextColor(Qt::red);

Means that you create yet another text item. A pointer to the newly created item is being returned. Either use & retain that one or use the one that you explicitly created with the new expression.

Furthermore you might want to reconsider your choice of using a QGraphicsScene for one simple "widget". While the Qt graphics framework is not necessarily heavy, it certainly brings quite a lot of overhead for just rendering an odometer. You can simply overwrite the QWidget::paint() function in your SpeedWidget subclass and render everything using QPainter. That will be a lot lighter and also a lot less hassle.