Add an IObservable<int> ItemCount property to IReactiveList*

[clairernovotny]

Make it easy to subscribe to the list count.

[niik]

@onovotny I realize it's not in the interface but ReactiveList has CountChanged which is an IObservable<int> so if anything it should probably be named that.

Seeing how easy it is to get an observable from the Count property itself using ObservableFromProperty I have to admit I'm conflicted on having a dedicated property tbh. But I'll defer that to @paulcbetts

[anaisbetts]

I think that interfaces for collections are conceptually dumb, so I defer it to @haacked

[clairernovotny]

Silly question, but isn't it better to never expose concrete details? So in my VM (or anywhere else), it'd only expose an interface.

Not only that, but if I wanted better encapsulation, like using the IReadOnlyReactiveList interfaces, those might still want to subscribe to all of the observables in the concrete type.

[clairernovotny]

Since the IObservable's are already on the underlying collection, it should be as easy as adding them to the interfaces. I can do this and submit a PR if you want.

[haacked]

If this is already exposed on the underlying collection I see no harm in this. Send a PR. :)

[anaisbetts]

👍

[anaisbetts]

Silly question, but isn't it better to never expose concrete details?

For Classes That Do Stuff, I totally agree, but I look at ReactiveList the same way as I look as "Dictionary" or "string", it's more like a fundamental data structure than a class with a contract. While this is a bit of a stretch, I think that the interfaces generally just get in the way here.

The one useful semantic that the interfaces can describe wrt ReactiveList is, "Please don't change this list" (i.e. "read-only for you" semantics).

[clairernovotny]

I guess I've always been in the camp of never exposing concrete collection types, including List<T>, Dictionary or anything else. I'd rather not have a caller store it as a List<T> in case I want to change the type later...like maybe use a linked list or a ReactiveList where it was just a List before.