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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);
     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(0.8, 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(1.2);
         break;
     case Qt::Key_Minus:
         scaleView(1 / 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);
 }