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

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     #include <QGraphicsScene>
     #include <QGraphicsSceneMouseEvent>
     #include <QPainter>
     #include <QStyleOption>

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

     Node::Node(GraphWidget *graphWidget)
         : graph(graphWidget)
     {
         setFlag(ItemIsMovable);
         setFlag(ItemSendsGeometryChanges);
         setCacheMode(DeviceCoordinateCache);
         setZValue(-1);
     }

     void Node::addEdge(Edge *edge)
     {
         edgeList << edge;
         edge->adjust();
     }

     QList<Edge *> Node::edges() const
     {
         return edgeList;
     }

     void Node::calculateForces()
     {
         if (!scene() || scene()->mouseGrabberItem() == this) {
             newPos = pos();
             return;
         }

         // Sum up all forces pushing this item away
         qreal xvel = 0;
         qreal yvel = 0;
         foreach (QGraphicsItem *item, scene()->items()) {
             Node *node = qgraphicsitem_cast<Node *>(item);
             if (!node)
                 continue;

             QLineF line(mapFromItem(node, 0, 0), QPointF(0, 0));
             qreal dx = line.dx();
             qreal dy = line.dy();
             double l = 2.0 * (dx * dx + dy * dy);
             if (l > 0) {
                 xvel += (dx * 150.0) / l;
                 yvel += (dy * 150.0) / l;
             }
         }

         // Now subtract all forces pulling items together
         double weight = (edgeList.size() + 1) * 10;
         foreach (Edge *edge, edgeList) {
             QPointF pos;
             if (edge->sourceNode() == this)
                 pos = mapFromItem(edge->destNode(), 0, 0);
             else
                 pos = mapFromItem(edge->sourceNode(), 0, 0);
             xvel += pos.x() / weight;
             yvel += pos.y() / weight;
         }

         if (qAbs(xvel) < 0.1 && qAbs(yvel) < 0.1)
             xvel = yvel = 0;

         QRectF sceneRect = scene()->sceneRect();
         newPos = pos() + QPointF(xvel, yvel);
         newPos.setX(qMin(qMax(newPos.x(), sceneRect.left() + 10), sceneRect.right() - 10));
         newPos.setY(qMin(qMax(newPos.y(), sceneRect.top() + 10), sceneRect.bottom() - 10));
     }

     bool Node::advance()
     {
         if (newPos == pos())
             return false;

         setPos(newPos);
         return true;
     }

     QRectF Node::boundingRect() const
     {
         qreal adjust = 2;
         return QRectF(-10 - adjust, -10 - adjust,
                       23 + adjust, 23 + adjust);
     }

     QPainterPath Node::shape() const
     {
         QPainterPath path;
         path.addEllipse(-10, -10, 20, 20);
         return path;
     }

     void Node::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *)
     {
         painter->setPen(Qt::NoPen);
         painter->setBrush(Qt::darkGray);
         painter->drawEllipse(-7, -7, 20, 20);

         QRadialGradient gradient(-3, -3, 10);
         if (option->state & QStyle::State_Sunken) {
             gradient.setCenter(3, 3);
             gradient.setFocalPoint(3, 3);
             gradient.setColorAt(1, QColor(Qt::yellow).light(120));
             gradient.setColorAt(0, QColor(Qt::darkYellow).light(120));
         } else {
             gradient.setColorAt(0, Qt::yellow);
             gradient.setColorAt(1, Qt::darkYellow);
         }
         painter->setBrush(gradient);
         painter->setPen(QPen(Qt::black, 0));
         painter->drawEllipse(-10, -10, 20, 20);
     }

     QVariant Node::itemChange(GraphicsItemChange change, const QVariant &value)
     {
         switch (change) {
         case ItemPositionHasChanged:
             foreach (Edge *edge, edgeList)
                 edge->adjust();
             graph->itemMoved();
             break;
         default:
             break;
         };

         return QGraphicsItem::itemChange(change, value);
     }

     void Node::mousePressEvent(QGraphicsSceneMouseEvent *event)
     {
         update();
         QGraphicsItem::mousePressEvent(event);
     }

     void Node::mouseReleaseEvent(QGraphicsSceneMouseEvent *event)
     {
         update();
         QGraphicsItem::mouseReleaseEvent(event);
     }