protected override void SetItem(int index, SelectableItem <T> item)
{
this[index].PropertyChanged -= ItemOnPropertyChanged;
base.SetItem(index, item);
item.PropertyChanged += ItemOnPropertyChanged;
}
protected override void InsertItem(int index, SelectableItem <T> item)
{
base.InsertItem(index, item);
item.PropertyChanged += ItemOnPropertyChanged;
}