the check-state of checkbox of QTreeView (item with SetCheckable(true)) not update after setCheckState()

guys, I got one issue that the check-state of checkbox of QTreeView (item with SetCheckable(true)) not update after setCheckState(). my intention is: include a QTreeView in a QDialog. in this QTreeView, the items are with check-box enabled through QStandardItem::SetCheckable(), and set TriState enabled for items that has children. when clicked any item, I 'd like to update its ancestor items' check-state: 1. when all children checked, set Qt::Checked for the parent, 2. when all children unchecked, set Qt::Unchecked for the parent, 3. otherwise, set Qt::PartiallyChecked for the parent, below is some code in which I derived a class from QStandardDataModel:

void TriStateItemModel::setCheckStateNumForAllNodes() 
{
    connect(this, SIGNAL(itemChanged(QStandardItem*)), this, SLOT(itemCheckStateChanged(QStandardItem*))); 
}

void TriStateItemModel::itemCheckStateChanged(QStandardItem* pItem)
{
    if((NULL == pItem) || (!pItem->isCheckable()))
        return;
    QStandardItem* parent = pItem->parent();
    if(parent == NULL) return;

    if(NULL != parent)
    {
        int brotherCount = parent->rowCount();
        int checkedCount(0),unCheckedCount(0);
        Qt::CheckState state;
        for(int i=0; i<brotherCount; ++i)
        {
            QStandardItem* siblingItem = parent->child(i);
            state = siblingItem->checkState();
            if(Qt::Unchecked == state)
                ++unCheckedCount;
            else if(Qt::Checked == state)
                ++checkedCount;
        }

        if(checkedCount>0 && unCheckedCount>0)
            parent->setCheckState(Qt::PartiallyChecked);
        else if(checkedCount == brotherCount)
            parent->setCheckState(Qt::Checked);
        else if(unCheckedCount == brotherCount)
            parent->setCheckState(Qt::Unchecked);
    }
}

but it runs not expected:
    when click one item, its parent's check-box not update accordingly (but its check-state truly updated when I check it by:

if(pItem->parent()->checkState()==Qt::Checked)
{
    qDebug() << "parent checked-state";
}

could anyone help on this? enter image description herethanks ahead.