Why is my QGraphicsLine in the wrong place

Question

I want to draw a line to connect two circles (QGraphicsEllipseItem), but I find that I don't get the desired result with this way of writing.

    //they have been initialized to the correct place
    QGraphicsEllipseItem* nodeu;
    QGraphicsEllipseItem* nodev;

    this->addLine(nodeu->x(), nodeu->y(), nodev->x(), nodev->y());

The result of executing these codes is that only two circles appear, but no lines appear. like this

My rough inference is the problem of coordinate transformation, but I just can't solve it.
thank you!

Answer 1

you should first add one QGraphicsView in your UI or :

  QGraphicsView *graphicsView;
  QGridLayout *gridLayout;
  gridLayout = new QGridLayout(centralwidget);
  gridLayout->setSpacing(0);
  gridLayout->setObjectName(QString::fromUtf8("gridLayout"));
  graphicsView = new QGraphicsView(centralwidget);
  graphicsView->setObjectName(QString::fromUtf8("graphicsView"));

  gridLayout->addWidget(graphicsView, 0, 0, 1, 1);

then :

    QGraphicsScene *_scene = new QGraphicsScene(this);
    ui->graphicsView->setScene(_scene);
    ui->graphicsView->setRenderHints(QPainter::Antialiasing);

    QGraphicsEllipseItem *nodeu = new QGraphicsEllipseItem;
    nodeu->setRect(20, 10, 20, 20);

    _scene->addItem(nodeu);


    QGraphicsEllipseItem *nodev = new QGraphicsEllipseItem;
    nodev->setRect(80, 60, 20, 20);
    _scene->addItem(nodev);

    QGraphicsLineItem *_lineItem = new QGraphicsLineItem;

    _lineItem->setLine(nodeu->rect().x() + nodeu->rect().width() / 2.0, nodeu->rect().y() + nodeu->rect().height() / 2.0,
                       nodev->rect().x() + nodev->rect().width() / 2.0, nodev->rect().y() + nodev->rect().height() / 2.0);
    _scene->addItem(_lineItem);

this is the output: