Question on checkable QStandardItem in QTreeView

[ttvo]

I have a QTreeView with QStandardItemModel model and QStandardItem nodes in which
1) top level folder nodes are not checkable unless it contains one of more child node(s)
2) child node is always checkable
I try to connect signal/slot to the select/deselect user interaction on checkable node

connect(model, SIGNAL(itemChanged(QStandardItem * item)), this, SLOT(myItemChanged(QStandardItem *item)));

connect(model, SIGNAL(itemChanged(QStandardItem * item)), this, SLOT(myItemChanged(QStandardItem *item)));

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where model is QStandardItemModel , this is MainWindow. My MainWindow::myItemChanged(...) is never called. Please help.