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

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

     #include <QDebug>
     #include <QGraphicsScene>
     #include <QWheelEvent>

     #include <math.h>

     GraphWidget::GraphWidget()
         : timerId(0)
     {
         QGraphicsScene *scene = new QGraphicsScene(this);
         scene->setItemIndexMethod(QGraphicsScene::NoIndex);
         scene->setSceneRect(-200, -200, 400, 400);
         setScene(scene);
         setCacheMode(CacheBackground);
         setViewportUpdateMode(BoundingRectViewportUpdate);
         setRenderHint(QPainter::Antialiasing);
         setTransformationAnchor(AnchorUnderMouse);
         setResizeAnchor(AnchorViewCenter);

         Node *node1 = new Node(this);
         Node *node2 = new Node(this);
         Node *node3 = new Node(this);
         Node *node4 = new Node(this);
         centerNode = new Node(this);
         Node *node6 = new Node(this);
         Node *node7 = new Node(this);
         Node *node8 = new Node(this);
         Node *node9 = new Node(this);
         scene->addItem(node1);
         scene->addItem(node2);
         scene->addItem(node3);
         scene->addItem(node4);
         scene->addItem(centerNode);
         scene->addItem(node6);
         scene->addItem(node7);
         scene->addItem(node8);
         scene->addItem(node9);
         scene->addItem(new Edge(node1, node2));
         scene->addItem(new Edge(node2, node3));
         scene->addItem(new Edge(node2, centerNode));
         scene->addItem(new Edge(node3, node6));
         scene->addItem(new Edge(node4, node1));
         scene->addItem(new Edge(node4, centerNode));
         scene->addItem(new Edge(centerNode, node6));
         scene->addItem(new Edge(centerNode, node8));
         scene->addItem(new Edge(node6, node9));
         scene->addItem(new Edge(node7, node4));
         scene->addItem(new Edge(node8, node7));
         scene->addItem(new Edge(node9, node8));

         node1->setPos(-50, -50);
         node2->setPos(0, -50);
         node3->setPos(50, -50);
         node4->setPos(-50, 0);
         centerNode->setPos(0, 0);
         node6->setPos(50, 0);
         node7->setPos(-50, 50);
         node8->setPos(0, 50);
         node9->setPos(50, 50);

         scale(qreal(0.8), qreal(0.8));
         setMinimumSize(400, 400);
         setWindowTitle(tr("Elastic Nodes"));
     }

     void GraphWidget::itemMoved()
     {
         if (!timerId)
             timerId = startTimer(1000 / 25);
     }

     void GraphWidget::keyPressEvent(QKeyEvent *event)
     {
         switch (event->key()) {
         case Qt::Key_Up:
             centerNode->moveBy(0, -20);
             break;
         case Qt::Key_Down:
             centerNode->moveBy(0, 20);
             break;
         case Qt::Key_Left:
             centerNode->moveBy(-20, 0);
             break;
         case Qt::Key_Right:
             centerNode->moveBy(20, 0);
             break;
         case Qt::Key_Plus:
             scaleView(qreal(1.2));
             break;
         case Qt::Key_Minus:
             scaleView(1 / qreal(1.2));
             break;
         case Qt::Key_Space:
         case Qt::Key_Enter:
             foreach (QGraphicsItem *item, scene()->items()) {
                 if (qgraphicsitem_cast<Node *>(item))
                     item->setPos(-150 + qrand() % 300, -150 + qrand() % 300);
             }
             break;
         default:
             QGraphicsView::keyPressEvent(event);
         }
     }

     void GraphWidget::timerEvent(QTimerEvent *event)
     {
         Q_UNUSED(event);

         QList<Node *> nodes;
         foreach (QGraphicsItem *item, scene()->items()) {
             if (Node *node = qgraphicsitem_cast<Node *>(item))
                 nodes << node;
         }

         foreach (Node *node, nodes)
             node->calculateForces();

         bool itemsMoved = false;
         foreach (Node *node, nodes) {
             if (node->advance())
                 itemsMoved = true;
         }

         if (!itemsMoved) {
             killTimer(timerId);
             timerId = 0;
         }
     }

     void GraphWidget::wheelEvent(QWheelEvent *event)
     {
         scaleView(pow((double)2, -event->delta() / 240.0));
     }

     void GraphWidget::drawBackground(QPainter *painter, const QRectF &rect)
     {
         Q_UNUSED(rect);

         // Shadow
         QRectF sceneRect = this->sceneRect();
         QRectF rightShadow(sceneRect.right(), sceneRect.top() + 5, 5, sceneRect.height());
         QRectF bottomShadow(sceneRect.left() + 5, sceneRect.bottom(), sceneRect.width(), 5);
         if (rightShadow.intersects(rect) || rightShadow.contains(rect))
             painter->fillRect(rightShadow, Qt::darkGray);
         if (bottomShadow.intersects(rect) || bottomShadow.contains(rect))
             painter->fillRect(bottomShadow, Qt::darkGray);

         // Fill
         QLinearGradient gradient(sceneRect.topLeft(), sceneRect.bottomRight());
         gradient.setColorAt(0, Qt::white);
         gradient.setColorAt(1, Qt::lightGray);
         painter->fillRect(rect.intersect(sceneRect), gradient);
         painter->setBrush(Qt::NoBrush);
         painter->drawRect(sceneRect);

         // Text
         QRectF textRect(sceneRect.left() + 4, sceneRect.top() + 4,
                         sceneRect.width() - 4, sceneRect.height() - 4);
         QString message(tr("Click and drag the nodes around, and zoom with the mouse "
                            "wheel or the '+' and '-' keys"));

         QFont font = painter->font();
         font.setBold(true);
         font.setPointSize(14);
         painter->setFont(font);
         painter->setPen(Qt::lightGray);
         painter->drawText(textRect.translated(2, 2), message);
         painter->setPen(Qt::black);
         painter->drawText(textRect, message);
     }

     void GraphWidget::scaleView(qreal scaleFactor)
     {
         qreal factor = matrix().scale(scaleFactor, scaleFactor).mapRect(QRectF(0, 0, 1, 1)).width();
         if (factor < 0.07 || factor > 100)
             return;

         scale(scaleFactor, scaleFactor);
     }