Reactive approach to simple imperative task

Question

Application requirements:

• subscribe to two event streams A and B
• for each A event there should be corresponding B event some time later
• the a

Answer 1

I think this does what you want, albeit it is C# rather than Java - I'm sure you can convert easily enough.

private IObservable MonitorAB(
    IObservable a, IObservable b, TimeSpan threshold)
{
    return Observable.Create(o =>
        b.Publish(pb =>
            a.Select(ax =>
                pb.Where(pbx => pbx == ax)
                    .Take(1)
                    .Timeout(threshold, Observable.Return(-1L))
                    .Select(pbx => String.Format("{0},{1}", ax, pbx)))
            .Merge())
            .Subscribe(o));
}

So this simply uses the inline .Publish to make sure b is hot within the query. The outer .Select filters the published pb observable for those that match the value from a (matching a & b), takes only one cause we only want one, and then does a .Timeout for the threshold time and returns a -1L (long) if the timeout is reached. The inner .Select just turns the two long values into a single string. At this point the query is an IObservable> so the .Merge flattens it out.