常用接口使用：

    ui->comboBox->addItem(QObject::tr("new item"));
    ui->comboBox->setMaxVisibleItems(10);
    ui->comboBox->setEditable(false);
    ui->comboBox->setInsertPolicy(QComboBox::InsertAtTop);
    
    qDebug()<<"changed index : "<<index;
    qDebug()<<"currentIndex(): "<<ui->comboBox->currentIndex();
    qDebug()<<"currentText(): "<<ui->comboBox->currentText();
    qDebug()<<"555 index is :" <<ui->comboBox->findText("555");