private void onCollectionCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
{
Contract.Requires(sender is ObservableCollection <T>);
Contract.Requires(this.collections.Contains(sender));
Contract.Requires(e != null);
switch (e.Action)
{
case NotifyCollectionChangedAction.Add:
this.Count += e.NewItems.Count;
break;
case NotifyCollectionChangedAction.Remove:
this.Count -= e.OldItems.Count;
break;
case NotifyCollectionChangedAction.Replace:
this.Count += e.NewItems.Count - e.NewItems.Count;
break;
case NotifyCollectionChangedAction.Move:
break;
case NotifyCollectionChangedAction.Reset:
recomputeCount(); // I don't think one can reason what the new count is going to be in this case. Just recomputing is the only solution
break;
default:
throw new DefaultSwitchCaseUnreachableException();
}
int startIndexOfSender = computeStartingIndex((ObservableCollection <T>)sender);
OnCollectionChanged(e.Shift(startIndexOfSender));
}