RX如何将命令与另一个可观察到的结合

Question

I've got a number of reactive commands as well as some observables holding some information, and I'm trying to do something like:

_navigate = ReactiveCommand.Create(CanNavigate);
_navigate.CombineLatest(navigationTarget, (_, tgt) => tgt)
    .Subscribe(tgt => Navigation.NavigateTo(tgt));

I've tried a couple of different approaches:

1. SelectMany
2. Zip

I either end up with:

1. Subscribe stops invoking after the first time (if I use Zip)
2. Subscribe invokes even when the command hasn't been executed after it was executed once

Essentially I want:

An observable that triggers every time (and only ) when the command has been executed, along with pulling in the most recent value of the second observable.

Can't quite get my head around how best to achieve this...

Answer 1

If you are able to use a pre-release version the latest (2.3.0-beta2) has the method WithLatestFrom which does exactly this.

_navigate.WithLatestFrom(navigationTarget, (_, tgt) => tgt)
  .Subscribe(tgt => Navigation.NavigateTo(tgt));

If not you can create your own by doing:

public static IObservable<TResult> WithLatestFrom<TLeft, TRight, TResult>(
    this IObservable<TLeft> source,
    IObservable<TRight> other,
    Func<TLeft, TRight, TResult> resultSelector)
{
    return source.Publish(os =>
        other.Select(a => os
            .Select(b => resultSelector(b,a)))
            .Switch());
}

Source

Answer 2

we use Join to achive this behavoir.

the Idea is that at one moment you have one window for navigtion target and no window for _navigate command. When command appears, it takes the value from current open window of another sequence. The window for navigationTarget value closes, when new navigationTarget arrives.

_navigate.Join(
    navigationTarget,
    _ => Observable.Empty<Unit>(),
    _ => navigationTarget,
    (_, tgt) => tgt).Subscribe(tgt => Navigation.NavigateTo(tgt));