Qt5界面最小化后不刷新问题

https://stackoverflow.com/questions/14554903/widgets-freezing-after-minimise-window
解决办法：

void MainWindow::changeEvent(QEvent *event) {
  if(event->type() == QEvent::WindowStateChange) {
    if(!isMinimized()) {
      setAttribute(Qt::WA_Mapped);
    }
  }
}