ReactiveUI How to use WhenAnyObservable properly

Question

I am attempting to use WhenAnyObservable for the first time.

When a ReactiveList Count == 0 and a tipText length is > 0 then I want to set a local value to true in

Answer 1

I get an index error when trying to use WhenAnyObservable. I ended up using

Observable.CombineLatest(
    SomeItems.Changed.Select(x => true),
    this.WhenAnyValue(y => y.SomeBoolProperty),
    (b,g) => b && g)

Answer 2

You've got the right idea, but WhenAnyObservable doesn't return items until it has an initial item for both "sides" if you use >1 Observables. So you probably want:

this.ViewModel.WhenAnyObservable(
    x => x.AutoCompleteItems.CountChanged.StartWith(0),
    x => x.WhenAnyValue(y => y.TipText),
    (countChanged, tipText) => countChanged == 0 && tipText.Length > 0);