Add WithLatestFrom

[khellang]

I'd ❤️ to see the withLatestFrom method from RxJava and RxJS be ported over to .NET as well.

For reference: http://rxmarbles.com/#withLatestFrom

[james-world]

This is an implementation, if you want something quick:

public static IObservable<TResult> WithLatestFrom<TLeft, TRight, TResult>(
    this IObservable<TLeft> source,
    IObservable<TRight> other,
    Func<TLeft, TRight, TResult> resultSelector)
{
    return other.Publish(os =>
        source.SkipUntil(os)
              .Zip(os.MostRecent(default(TRight)), resultSelector));
}

[khellang]

I have something like this that's working (I think 😝). Would love some feedback on it:

public static IObservable<TResult> WithLatestFrom<TResult, TFirst, TSecond>(
    this IObservable<TFirst> first,
    IObservable<TSecond> second,
    Func<TFirst, TSecond, TResult> resultSelector)
{
    return Observable.Create<TResult>(observer =>
    {
        var published = first.Publish();

        var connection = published.Connect();

        var subscription = second
            .Select(a => published
                .Select(b => resultSelector(b, a)))
            .Switch()
            .Subscribe(observer);

        return new CompositeDisposable(connection, subscription);
    });
}

[james-world]

It passes my test, but it's got slightly more code in! ;) Looks fine though - nice approach. If you used the variant of Publish I used, you could get rid of the CompositeDisposable and the Create:

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());
}

[danyx23]

👍
This is a pretty useful operator and should come with rx .net by default

[bartdesmet]

This operator has been added for the next release of Rx.NET.

[khellang]

Thanks, @bartdesmet! 👍