Hi,
Actually this is continuation of this thread but the question is quite different.
I am running into performance problems when multiple items are moved. The peformance is better when I disable the children of resistor(nodes and text item). But I want those features in my app. Also the grid look corrupted when items are moved. Any suggestion for that ?
Here is the profile output
Flat profile:

Each sample counts as 0.01 seconds.
% cumulative self self total
time seconds seconds calls Ts/call Ts/call name
75.08 0.03 0.03 GridScene::drawBackground(QPainter*, QRectF const&)
25.03 0.04 0.01 Resistor::boundingRect() const
0.00 0.04 0.00 119168 0.00 0.00 QVarLengthArray<QLineF, 100>::realloc(int, int)
Here is the shorter version of the code. Select all items and move so that you can see what I mean. Please help me.

Qt Code: 
  1. #include <QtGui>
  2.  
  3. class GridScene : public QGraphicsScene
  4. {
  5.    public:
  6.       GridScene(qreal x, qreal y, qreal w, qreal h)
  7.          : QGraphicsScene(x, y, w, h)
  8.       { }
  9.  
  10.    protected:
  11.       void drawBackground(QPainter *painter, const QRectF &rect)
  12.       {
  13.          const int gridSize = 25;
  14.          painter->setPen(QPen(Qt::darkGreen,0));
  15.          qreal left = int(rect.left()) - (int(rect.left()) % gridSize);
  16.          qreal top = int(rect.top()) - (int(rect.top()) % gridSize);
  17.  
  18.          QVarLengthArray<QLineF, 100> lines;
  19.  
  20.          for (qreal x = left; x < rect.right(); x += gridSize)
  21.             lines.append(QLineF(x, rect.top(), x, rect.bottom()));
  22.          for (qreal y = top; y < rect.bottom(); y += gridSize)
  23.             lines.append(QLineF(rect.left(), y, rect.right(), y));
  24.  
  25.          painter->drawLines(lines.data(), lines.size());
  26.       }
  27. };
  28.  
  29. class Node : public QGraphicsItem
  30. {
  31.    public:
  32.       Node(QPointF pos ,QGraphicsItem *par = 0, QGraphicsScene *sc = 0) : QGraphicsItem(par,sc)
  33.       {
  34.          setPos(pos);
  35.          setFlags(0);
  36.          setAcceptedMouseButtons(0);
  37.       }
  38.  
  39.       void paint(QPainter* p,const QStyleOptionGraphicsItem *, QWidget *)
  40.       {
  41.          p->setPen(Qt::darkRed);
  42.          p->drawEllipse(QRectF(-3, -3, 2*3, 2*3));
  43.       }
  44.  
  45.       QPainterPath shape() const
  46.       {
  48.          path.addEllipse(QRectF(-3, -3, 2*3, 2*3));
  49.          return path;
  50.       }
  51.  
  52.       QRectF boundingRect() const
  53.       {
  54.          return QRectF(-3, -3, 2*3, 2*3);
  55.       }
  56.  
  57. };
  58.  
  59. class Resistor : public QGraphicsItem
  60. {
  61.    public:
  62.       Resistor(QGraphicsItem *par = 0, QGraphicsScene *scene = 0) : QGraphicsItem(par,scene)
  63.       {
  64.          setFlags(ItemIsMovable | ItemIsSelectable | ItemIsFocusable);
  65.          // comment the following  4 lines to see the performance difference
  66.          QGraphicsTextItem *t = new QGraphicsTextItem("R1 = 100k",this,scene);
  67.          t->setPos(0,-35);
  68.          new Node(QPointF(-30,0),this,scene);
  69.          new Node(QPointF(30,0),this,scene);
  70.       }
  71.  
  72.       void paint(QPainter *p, const QStyleOptionGraphicsItem *o, QWidget *)
  73.       {
  74.          if(!(o->state & QStyle::State_Open))
  75.             p->setPen(Qt::darkBlue);
  76.          if(o->state & QStyle::State_Selected)
  77.             p->setPen(QPen(Qt::darkGray,1));
  78.  
  79.          p->drawLine(-18, -9, 18, -9);
  80.          p->drawLine( 18, -9, 18,  9);
  81.          p->drawLine( 18,  9,-18,  9);
  82.          p->drawLine(-18,  9,-18, -9);
  83.          p->drawLine(-27,  0,-18,  0);
  84.          p->drawLine( 18,  0, 27,  0);
  85.       }
  86.  
  87.       QRectF boundingRect() const
  88.       {
  89.          qreal pw = 0.5;
  90.          return QRectF(-27,-9,54,18).adjusted(-pw,-pw,pw,pw);
  91.       }
  92. };
  93.  
  94. int main(int argc,char *argv[])
  95. {
  96.    QApplication app(argc,argv);
  97.    GridScene scene(0,0,1024,768);
  98.    for(int j = 2; j < 4; ++j)
  99.       for(int i = 1; i <11; ++i)
  100.       {
  101.          Resistor *r = new Resistor(0,&scene);
  102.          r->setPos(j*100, i * 50);
  103.       }
  104.    QGraphicsView *view = new QGraphicsView(&scene);
  105.    view->setRenderHints(QPainter::Antialiasing | QPainter::SmoothPixmapTransform);
  106.    view->setDragMode(QGraphicsView::RubberBandDrag);
  107.    view->show();
  108.    return app.exec();
  109. }
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