foreach(QWidget *win,qApp->topLevelWidgets())
    {
        if(MainWindow *mainWin=qobject_cast<MainWindow *>(win))
        {
            mainWin->test();
        }
    }

    QMdiSubWindow *sub=ui->mdiArea->activeSubWindow();
    QWidget *subwin=sub->widget();//這段容易出錯，必須呼叫widget()
    SubText *subWindow=(SubText*)subwin;
    if(subWindow==NULL)
    {
        return;
    }
    subWindow->print();