QTreeView and node's expanding

[aliks-os]

I have a custom model and a tree inherited from QTreeView. After a loading of data I need expand some nodes. I do it from mainWidget as

treeView.expand(index)

Work fine. But volume of data grows and iterate all tree take a time. Now I am looking for how insert function of expanding to a treeView or delegate. I mean when the treeView start loading a data from model, the node will expanded or not depends of property of item.
somethig like

some event(QModelIndex index) {
   Item *item = index.internalPointer();
   if (item.flag() == 2)
      this->expand(index);
}

Could anybody direct me what even/slot/function/solution I could use in QTreeView. This function I need just at the start of application. I mean when treeView->setModel(model) is occur