void MainWindow::on_mousedoubleclick(QPoint point)
{
    QPointF pointfScene = ui->gView->mapToScene(point);
    QGraphicsItem *itemScene = NULL;
    itemScene = scene->itemAt(pointfScene, ui->gView->transform());
    if(itemScene == NULL)
        return;

/*
 * QGraphicsItem *itemScene相当于基类
 * QGraphicsRectItem *item类似派生类
 * 进行下行转换
*/
    switch (itemScene->type())
    {
        case QGraphicsRectItem::Type:{
            QGraphicsRectItem *item;
            item = qgraphicsitem_cast<QGraphicsRectItem*>(itemScene);
            QColor color = item->brush().color();/*自动获取当前项目图形颜色*/
            color = QColorDialog::getColor(color, NULL, "choose a color ");
            if(color.isValid())/*如果颜色有效则返回true*/
                item->setBrush(QBrush(color));
        break;
        }
        case QGraphicsEllipseItem::Type:{
            QGraphicsEllipseItem *item;
            item = qgraphicsitem_cast<QGraphicsEllipseItem*>(itemScene);
            QColor color = item->brush().color();/*自动获取当前项目图形颜色*/
            color = QColorDialog::getColor(color, NULL, "choose a color ");
            if(color.isValid())
                item->setBrush(QBrush(color));
        break;
        }
        case QGraphicsPolygonItem::Type:{
            QGraphicsPolygonItem *item;
            item = qgraphicsitem_cast<QGraphicsPolygonItem*>(itemScene);
            QColor color = item->brush().color();/*自动获取当前项目图形颜色*/
            color = QColorDialog::getColor(color, NULL, "choose a color ");
            if(color.isValid())
                item->setBrush(QBrush(color));
        break;
        }
        case QGraphicsLineItem::Type:{
            QGraphicsLineItem *item;
            item = qgraphicsitem_cast<QGraphicsLineItem*>(itemScene);
            QColor color = item->pen().color();/*自动获取当前项目图形颜色*/
            color = QColorDialog::getColor(color, NULL, "choose a color ");
            if(color.isValid())
                item->setPen(QPen(color));
        break;
        }
        case QGraphicsTextItem::Type:{
            QGraphicsTextItem *item;
            item = qgraphicsitem_cast<QGraphicsTextItem*>(itemScene);
            QColor color = item->defaultTextColor();/*自动获取当前项目图形颜色*/
            color = QColorDialog::getColor(color, NULL, "choose a color ");
            if(color.isValid())
                item->setDefaultTextColor(color);
        break;
        }
        default:
            break;

    }
}