ReactiveX C#: Do something when subscription starts - system.reactive

Question

I've created an IObservable<> combination using CombineLatest():
IObservable<IList<List<TileItem>>> combination = Observable.CombineLatest(combinations);
I need:
1. to do something `doSomethingOnFirst()` when first element is reached.
2. to do something `doSomethingOnComplete()` when subscription has completed.
Up to now, I've been able to do something when combination is completed. Nevertheless, I don't quite figure out what do I need to do in order to do something when subscription starts...
combination
.Subscribe(
_ => { },
_ => { },
() => this.doSomethingOnCompleted()
);
Any ideas?

Answer 1

It's probably best to use two subscriptions to accomplish this. When subscribing twice to an observable, it's also best to share a subscription to the source, which can be accomplished with .Publish().RefCount().
IEnumerable<IObservable<List<TileItem>>> combinations = null /* using null so I can compile /;
IObservable<IList<List<TileItem>>> combination = Observable.CombineLatest(combinations)
.Publish()
.RefCount();
combination.Take(1)
.Subscribe(_ => doSomethingOnFirst());
combination.Subscribe(_ => { }, _ => {}, () => doSomethingOnCompleted);
You can also accomplish the functionality by filtering the observable by index:
IObservable<IList<List<TileItem>>> combination = Observable.CombineLatest(combinations)
.Where((item, i) => i == 0)
combination.Subscribe(_ => doSomethingOnFirst(), _ => {}, () => doSomethingOnCompleted);

Answer 2

I would avoid .Publish().RefCount() (as this combination can prevent multiple future subscriptions after the initial subscription completes) and just do it this way:
combination
.Select((x, n) => new { x, n })
.Subscribe(xn =>
{
if (xn.n == 0)
{
doSomethingOnFirst();
}
}, () => doSomethingOnCompleted());