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?

Fidele

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);

Ideally

I didn't see that one coming Paul - I've been having trouble with WhenAnyObservable (unexpected) - but isn't that sort of counter-intuitive ? If it's called 'WhenAny'. I feel like I have to get down and check the RxUI source code every so often. And I'm not complaining, you did great work with it. And Observables can do this to a code – Retene

System.NotSupportedException: Index expressions are only supported with constants. – Gilreath

Answer 2

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)

Dryer