Rx GroupBy: Remove item or Update exising item not trigger regroup - system.reactive

Question

private static void TestGroupBy()
{
var rxList = new ReactiveList<int>();
Observable.Merge(rxList.ItemsAdded, rxList.ItemChanged.Select(x => x.Sender) ).GroupBy(i => i%3)
.Subscribe(group =>
{
Console.WriteLine(string.Format("--> Group {0} is created", group.Key) );
int child = 0;
group.Subscribe(c =>
{
Console.WriteLine(string.Format(" ------> Adding child {0} to Group {1} - ChildCnt: {2}", c, group.Key, ++child) );
});
});
Console.WriteLine("Add 1 to 10... ");
rxList.AddRange(new[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 });
Console.WriteLine("Change item 0 to 11, expect to be moved from group 0, to Group 2 ");
rxList[0] = 11;
Console.WriteLine("Remove item at 0, expect the item to be removed from the group ...");
rxList.RemoveAt(0);
}
I have this piece of code here. I would like the regroup to trigger if item is either removed, or item is replaced with new one. How do I achieve this?

Answer 1

I'm not sure if it's possible to implement item removing using groupBy like you want to do. Take a look at this picture: it seems like you want to alter the elements that have already been pushed to one of the "grouped" streams.
In your case, I would simply create several derived collections (as many as you have groups), with different selectors:
public void Test()
{
var source = new ReactiveList<int>();
var derived1 = source.CreateDerivedCollection(x => x, filter: x => x%3 == 0);
var derived2 = source.CreateDerivedCollection(x => x, filter: x => x%3 == 1);
var derived3 = source.CreateDerivedCollection(x => x, filter: x => x%3 == 2);
}