QT 在信號草裏面刷新控件立即生效的辦法：qApp->processEvents();

void setdid::on_lbt_start_set_did_clicked()

{	
     ui->lln_set_did_result->setText( "-" );

    ui->lln_apiLicence->setText( "-" );

    qApp->processEvents();//加上這條語句後達到預期目的，解決問題

}