void CDropableMdiArea::dragEnterEvent(QDragEnterEvent *event)
{
	QList<QMdiSubWindow *> list = subWindowList() ;
	for ( int i = 0 ; i < list.size() ; i ++ ) {
		QMdiSubWindow *p = list.at(i) ;
		if ( p->rect().contains(event->pos()-p->pos()) ) {
			event->ignore();
			return ;
		}
	}

	event->accept();
}