QPainter如何自适应大小画图

函数声明：void QPainter::drawImage ( const QRectF & target, const QImage & image, const QRectF & source, Qt::ImageConversionFlags flags = Qt::AutoColor )
举例：
  QRectF target(10.0, 20.0, 80.0, 60.0); //建立目标矩形
  QRectF source(0.0, 0.0, 70.0, 40.0); //建立源矩形，用来框定源图像文件中要显示的部分
  QImage image(":/images/myImage.png"); //建立QImage类对象image

  QPainter painter(this);
  painter.drawImage(target, image, source); //将源图像文件中(0.0, 0.0, 70.0, 40.0)位置的图像画到painter device中的

                                                                       //(10.0, 20.0, 80.0, 60.0)位置上，大小自适应

   1: void ImageWidget::paintEvent(QPaintEvent *event) {

   2:     QPainter painter(this);

   3:     QRectF target(0, 0, this->width(), this->height());

   4:     if(!img.isNull())

   5:         painter.drawImage(target,img);

   6:         //painter.drawImage(0,0,img);

   7:     //img.save("test.bmp");

   8: }