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# edge.cpp Example File
graphicsview/elasticnodes/edge.cpp

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     #include <QPainter>

     #include "edge.h"
     #include "node.h"

     #include <math.h>

     static const double Pi = 3.14159265358979323846264338327950288419717;
     static double TwoPi = 2.0 * Pi;

     Edge::Edge(Node *sourceNode, Node *destNode)
         : arrowSize(10)
     {
         setAcceptedMouseButtons(0);
         source = sourceNode;
         dest = destNode;
         source->addEdge(this);
         dest->addEdge(this);
         adjust();
     }

     Edge::~Edge()
     {
     }

     Node *Edge::sourceNode() const
     {
         return source;
     }

     void Edge::setSourceNode(Node *node)
     {
         source = node;
         adjust();
     }

     Node *Edge::destNode() const
     {
         return dest;
     }

     void Edge::setDestNode(Node *node)
     {
         dest = node;
         adjust();
     }

     void Edge::adjust()
     {
         if (!source || !dest)
             return;

         QLineF line(mapFromItem(source, 0, 0), mapFromItem(dest, 0, 0));
         qreal length = line.length();
         QPointF edgeOffset((line.dx() * 10) / length, (line.dy() * 10) / length);

         prepareGeometryChange();
         sourcePoint = line.p1() + edgeOffset;
         destPoint = line.p2() - edgeOffset;
     }

     QRectF Edge::boundingRect() const
     {
         if (!source || !dest)
             return QRectF();

         qreal penWidth = 1;
         qreal extra = (penWidth + arrowSize) / 2.0;

         return QRectF(sourcePoint, QSizeF(destPoint.x() - sourcePoint.x(),
                                           destPoint.y() - sourcePoint.y()))
             .normalized()
             .adjusted(-extra, -extra, extra, extra);
     }

     void Edge::paint(QPainter *painter, const QStyleOptionGraphicsItem *, QWidget *)
     {
         if (!source || !dest)
             return;

         // Draw the line itself
         QLineF line(sourcePoint, destPoint);
         painter->setPen(QPen(Qt::black, 1, Qt::SolidLine, Qt::RoundCap, Qt::RoundJoin));
         painter->drawLine(line);

         // Draw the arrows if there's enough room
         double angle = ::acos(line.dx() / line.length());
         if (line.dy() >= 0)
             angle = TwoPi - angle;

         QPointF sourceArrowP1 = sourcePoint + QPointF(sin(angle + Pi / 3) * arrowSize,
                                                       cos(angle + Pi / 3) * arrowSize);
         QPointF sourceArrowP2 = sourcePoint + QPointF(sin(angle + Pi - Pi / 3) * arrowSize,
                                                       cos(angle + Pi - Pi / 3) * arrowSize);
         QPointF destArrowP1 = destPoint + QPointF(sin(angle - Pi / 3) * arrowSize,
                                                   cos(angle - Pi / 3) * arrowSize);
         QPointF destArrowP2 = destPoint + QPointF(sin(angle - Pi + Pi / 3) * arrowSize,
                                                   cos(angle - Pi + Pi / 3) * arrowSize);

         painter->setBrush(Qt::black);
         painter->drawPolygon(QPolygonF() << line.p1() << sourceArrowP1 << sourceArrowP2);
         painter->drawPolygon(QPolygonF() << line.p2() << destArrowP1 << destArrowP2);
     }