Can't change the text display of a QTreeview item.

[wysota]

What is wrong with the default flags()?

It depends which model you use. If you use a direct subclass of QAbstractItemModel, then it doesn't allow items to be modified. You should make sure it returns ItemIsEditable as part of the flags.

For that matter, shouldn't I be using something like setFlags?

No. The model itself should know whether a particular item is to be editable (or otherwise interactable) and should return proper flags for the item. If you want something like "setFlags", you should somehow set/change the internal state for an item and tell the model to act accordingly upon this. For example you might implement the model in such a way that it won't allow items to be editable if they contain children. It would look more or less like so:

Qt::ItemFlags MyModel::flags ( const QModelIndex & index ) const {
  if(hasChildren(index)){
    return Qt::ItemIsEnabled|Qt::ItemIsSelectable;
  } else {
    return Qt::ItemIsEnabled|Qt::ItemIsSelectable|Qt::ItemIsEditable;
  }
  return Qt::ItemIsEnabled;
}

Qt::ItemFlags MyModel::flags ( const QModelIndex & index ) const {
  if(hasChildren(index)){
    return Qt::ItemIsEnabled|Qt::ItemIsSelectable;
  } else {
    return Qt::ItemIsEnabled|Qt::ItemIsSelectable|Qt::ItemIsEditable;
  }
  return Qt::ItemIsEnabled;
}

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Allowing a user to directly modify the state of any item could ruin the model.