QGraphicsItem leaves junk on screen

[Gopala Krishna]

Hello,
I am developing an electric cad software with qt4.2 and obviously using QGraphicsView framework
The problem I am facing is the Wire item - subclass of QGraphicsItem leaves junk on screen when its length or path of wire change rapidly. (Junk ,in the sense, it leaves behind trails of lines on screen). The screenshot attached tells it better.
Actually this happens when component is moved madly fast. But since my system is pretty high end and sincethere is every possibility of lots of items to be on view, I don't wan't to take a chance.

The length/path of wire changes when any component connected to wire moves which makes it necessary for the wire to regrow.
Currently I am achieving this by calling Wire::rebuild in component's itemChange()
I guess QGraphicsItem:: prepareGeometryChange() is the culprit behind this. Or may be I am doing something wrong in boundingRect() or shape().
How can I avoid this ?

void Wire::rebuild(const QPointF& s, const QPointF& e)
{
   //QList<QLineF*> m_lines is member variable
   if(!m_lines.isEmpty())
   {
      qDeleteAll(m_lines);
      m_lines.clear();
   }
 
 
   QPointF st = mapFromScene(s);
   QPointF en = mapFromScene(e);
 
   if(st.x() == en.x() || st.y() == en.y())
   {
      m_lines.append(new QLineF(st,en));
      prepareGeometryChange();
      return;
   }
 
   QPointF inter = QPointF(st.x(),en.y());
   m_lines.append(new QLineF(st,inter));
   m_lines.append(new QLineF(inter,en));
   prepareGeometryChange();
}
 
QRectF Wire::rectForLine(const QLineF& line) const
{
   qreal x = qMin(line.p1().x() , line.p2().x());
   qreal y = qMin(line.p1().y() , line.p2().y());
   qreal w = qAbs(line.p1().x() - line.p2().x());
   if(w  < 1.0)
      w = 1.0;
   qreal h = qAbs(line.p1().y() - line.p2().y());
   if(h < 1.0)
      h = 1.0;
   return QRectF(x,y,w,h);
}
 
QRectF Wire::boundingRect() const
{
   QRectF rect(0.0,0.0,0.0,0.0);
   foreach(QLineF* line, m_lines)
      rect |= rectForLine(*line);
   return rect.adjusted(-1.0,-1.0,1.0,1.0);
}
 
QPainterPath Wire::shape() const
{
   QPainterPath path;
   if(m_lines.isEmpty())
      return path;
   foreach(QLineF *line, m_lines)
      path.addRect(rectForLine(*line));
   return path;
}

void Wire::rebuild(const QPointF& s, const QPointF& e)
{
   //QList<QLineF*> m_lines is member variable
   if(!m_lines.isEmpty())
   {
      qDeleteAll(m_lines);
      m_lines.clear();
   }
   
   
   QPointF st = mapFromScene(s);
   QPointF en = mapFromScene(e);

   if(st.x() == en.x() || st.y() == en.y())
   {
      m_lines.append(new QLineF(st,en));
      prepareGeometryChange();
      return;
   }

   QPointF inter = QPointF(st.x(),en.y());
   m_lines.append(new QLineF(st,inter));
   m_lines.append(new QLineF(inter,en));
   prepareGeometryChange();
}

QRectF Wire::rectForLine(const QLineF& line) const
{
   qreal x = qMin(line.p1().x() , line.p2().x());
   qreal y = qMin(line.p1().y() , line.p2().y());
   qreal w = qAbs(line.p1().x() - line.p2().x());
   if(w  < 1.0)
      w = 1.0;
   qreal h = qAbs(line.p1().y() - line.p2().y());
   if(h < 1.0)
      h = 1.0;
   return QRectF(x,y,w,h);
}

QRectF Wire::boundingRect() const
{
   QRectF rect(0.0,0.0,0.0,0.0);
   foreach(QLineF* line, m_lines)
      rect |= rectForLine(*line);
   return rect.adjusted(-1.0,-1.0,1.0,1.0);
}

QPainterPath Wire::shape() const
{
   QPainterPath path;
   if(m_lines.isEmpty())
      return path;
   foreach(QLineF *line, m_lines)
      path.addRect(rectForLine(*line));
   return path;
}

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