public ObservableGroupedOrderedCollection(Func <TItem, IComparable> indexKey, Func <TItem, IComparable> orderingKey, Func <TItem, IComparable> groupingKey, bool descending = false)
{
this.indexKey = indexKey;
collection = new GroupedOrderedCollection <TItem>(indexKey, orderingKey, groupingKey, descending);
}