void MainWindow::paintEvent(QPaintEvent *event)
{
QPainter painter(this);
painter.setRenderHint(QPainter::Antialiasing,true);
painter.save();
//线性渐变
QLinearGradient linearGradient(100,150,150,100);//渐变起点终点坐标表示方向
linearGradient.setColorAt(0.0,Qt::red);
linearGradient.setColorAt(0.2,