[Solved] Trouble creating an arrow between two QGraphicsItems

[Guigui]

I am trying to implement some kind of node graph GUI. I started the work based on the "Diagram Scene":http://qt-project.org/doc/qt-5.0/qtwidgets/graphicsview-diagramscene.html and the "Elastic Node":http://qt-project.org/doc/qt-5.0/qtwidgets/graphicsview-elasticnodes.html examples. I have some difficulties however in customizing the nodes and draw arrows between them.

So far, I subclassed the QGraphicsProxyWidget class to create some kind of dialog looking node. The node is further customized by adding labels and connectors, which are subclasses of QGraphicsPathItem.

!http://i41.tinypic.com/67uy9s.jpg(Nodes)!

The problem is that the arrow never appear. I managed however to make it work between the connectors alone.

!http://i43.tinypic.com/339mums.jpg(Nodes and arrow)!

Here's my code to draw the arrow (almost the same as the Diagram Scene example).

@void NodeLinkItem::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
{
Q_UNUSED(option)
Q_UNUSED(widget)

painter->setRenderHint(QPainter::HighQualityAntialiasing, true);
painter->setPen(pen());
painter->setBrush(Qt::white);

// Don't paint the arrow if both connectors are superposed
if (m_sourceConnector->collidesWithItem(m_destConnector))

return;

// Draw the line part of the arrow
QLineF centerLine(m_sourceConnector->pos(), m_destConnector->pos());
QPolygonF endPolygon = m_destConnector->path().toFillPolygon();
QPointF p1 = endPolygon.first() + m_destConnector->pos();

QPointF p2;
QPointF intersectPoint;
QLineF polyLine;

for (int i = 1; i < endPolygon.count(); ++i) {
p2 = endPolygon.at(i) + m_destConnector->pos();
polyLine = QLineF(p1, p2);
QLineF::IntersectType intersectType = polyLine.intersect(centerLine, &intersectPoint);
if (intersectType == QLineF::BoundedIntersection)
break;
p1 = p2;
}

setLine(QLineF(intersectPoint, m_sourceConnector->pos()));

// Draw the arrowhead part of the arrow
double angle = std::acos(line().dx() / line().length());

if (line().dy() >= 0)
angle = (PI * 2) - angle;

qreal arrowSize = 10;
QPointF arrowP1 = line().p1() + QPointF(std::sin(angle + PI / 3) * arrowSize,
                                        std::cos(angle + PI / 3) * arrowSize);
QPointF arrowP2 = line().p1() + QPointF(std::sin(angle + PI - PI / 3) * arrowSize,
                                        std::cos(angle + PI - PI / 3) * arrowSize);

m_arrowhead.clear();
m_arrowhead << line().p1() << arrowP1 << arrowP2;

painter->drawLine(line());
painter->drawPolygon(m_arrowhead);

if (isSelected()) {

painter->setPen(QPen(Qt::black, 1, Qt::DashLine));
QLineF linkLine = line();
double a = linkLine.angle();
double d = 4;

    linkLine.translate(d * std::sin(a / 180 * PI), d * std::cos(a / 180 * PI));
    painter->drawLine(linkLine);
    linkLine.translate(2 * d * std::sin((a - 180) / 180 * PI), 2*d*std::cos((a - 180) / 180 * PI));
    painter->drawLine(linkLine);

}
}
@

Any idea why it wouldn't work when the connector is inside another node? Is it possible that the coordinates returned around lines 15-17 are in parent coordinates, aka. coordinates inside the node?

[podsvirov]

From documentation:

Items live in their own local coordinate system.

• QGraphicsItem::pos() - Returns the position of the item in parent coordinates.
• QGraphicsItem::scenePos() - Returns the item's position in scene coordinates.

Items NodeLinkItem, m_sourceConnector, m_destConnector have the same parent?

[Guigui]

I was just about to try reparenting the objects:

• I don't set any parent for NodeLinkItem.
• m_sourceConnector and m_destConnector are of type NodeConnectorItem.
• NodeConnectorItem are instanciated when NodeItem objects are created, and I set the new node as parent for every of it's Connectors.

So I suppose since the connector has a NodeItem as parent, connectors return coordinates relative to this node. I'll try setting no parents to the connectors.

[Guigui]

_<

For unknown reasons, the connectors are not painted if I don't set their parent node. However, using scenePos() instead of pos() fixed everything. Thank you for your answer.

[podsvirov]

Always happy to help!