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 the subscribe, or the opposite.

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

I am having trouble getting this to work.

Is there any trick I should be doing, or should I be using one of the other WhenAny commands?

Answer 1

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);