arrow.cpp Example File

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  #include "arrow.h"

  #include <math.h>

  #include <QPen>
  #include <QPainter>

  const qreal Pi = 3.14;

  Arrow::Arrow(DiagramItem *startItem, DiagramItem *endItem, QGraphicsItem *parent)
      : QGraphicsLineItem(parent)
  {
      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);
      }
  }