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# arrow.cpp Example File
graphicsview/diagramscene/arrow.cpp

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     #include <QtGui>

     #include "arrow.h"
     #include <math.h>

     const qreal Pi = 3.14;

     Arrow::Arrow(DiagramItem *startItem, DiagramItem *endItem,
              QGraphicsItem *parent, QGraphicsScene *scene)
         : QGraphicsLineItem(parent, scene)
     {
         myStartItem = startItem;
         myEndItem = endItem;
         setFlag(QGraphicsItem::ItemIsSelectable, true);
         myColor = Qt::black;
         setPen(QPen(myColor, 2, Qt::SolidLine, Qt::RoundCap, Qt::RoundJoin));
     }

     QRectF Arrow::boundingRect() const
     {
         qreal extra = (pen().width() + 20) / 2.0;

         return QRectF(line().p1(), QSizeF(line().p2().x() - line().p1().x(),
                                           line().p2().y() - line().p1().y()))
             .normalized()
             .adjusted(-extra, -extra, extra, extra);
     }

     QPainterPath Arrow::shape() const
     {
         QPainterPath path = QGraphicsLineItem::shape();
         path.addPolygon(arrowHead);
         return path;
     }

     void Arrow::updatePosition()
     {
         QLineF line(mapFromItem(myStartItem, 0, 0), mapFromItem(myEndItem, 0, 0));
         setLine(line);
     }

     void Arrow::paint(QPainter *painter, const QStyleOptionGraphicsItem *,
               QWidget *)
     {
         if (myStartItem->collidesWithItem(myEndItem))
             return;

         QPen myPen = pen();
         myPen.setColor(myColor);
         qreal arrowSize = 20;
         painter->setPen(myPen);
         painter->setBrush(myColor);

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

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

         double angle = ::acos(line().dx() / line().length());
         if (line().dy() >= 0)
             angle = (Pi * 2) - angle;

             QPointF arrowP1 = line().p1() + QPointF(sin(angle + Pi / 3) * arrowSize,
                                             cos(angle + Pi / 3) * arrowSize);
             QPointF arrowP2 = line().p1() + QPointF(sin(angle + Pi - Pi / 3) * arrowSize,
                                             cos(angle + Pi - Pi / 3) * arrowSize);

             arrowHead.clear();
             arrowHead << line().p1() << arrowP1 << arrowP2;
             painter->drawLine(line());
             painter->drawPolygon(arrowHead);
             if (isSelected()) {
                 painter->setPen(QPen(myColor, 1, Qt::DashLine));
             QLineF myLine = line();
             myLine.translate(0, 4.0);
             painter->drawLine(myLine);
             myLine.translate(0,-8.0);
             painter->drawLine(myLine);
         }
     }