“Exclusive” and “Default” Subscription Modes in Rx

Question

I have an observable sequence of event objects and a number of observers handling specific types of events. I need to accomplish the following scenarios:

1. Some event types need to be handled by the first observer matching a condition (e.g. observable.SubscribeExclusively(x=>{}) and become "unobservable" to the others.
2. If there are no subscriptions, set some default handler (e.g. observable.SubscribeIfNoSubscriptions(x=>{})) so that no items get lost (this handler may for example save the item to a database so that it is processed later).

Is there a way to handle these cases with Rx?

Answer 1

I'm not sure I quite grok your scenario, but how does this strike you:

IObservable<Event> streamOfEvents.SelectMany(x => {
    if (matchesExclusiveItem1(x)) {
        x += exclusiveItem1Handler;
        return Observable.Empty<Event>();
    }

    // Default case
    return Observable.Return(x);
}).Subscribe(x => {
    // Default case
    x += defaultHandler;
});

I'm using "Event objects" because that's what you specified, but it'd probably be better to use IObservable<IObservable<T>> - this selector has side effects (connecting the event), which is less than good.

Answer 2

The "Exclusivity" is easier - you just have everybody else subscribe to the filtered output of the exclusive observer.

"Default" is harder - RX programming is a functional programming and subscribers do not know of each other, while by definition having a "Default" subscriber means having some state shared between observers. One way to have shared state is to create a producer/consumer queue using ConcurrentBag or BufferBlock from TPL DataFlow. Another way is to attach the "processed" state to the event itself using a class like this:

public class Handled<T>
{
    public bool IsHandled { get; set; }
    public T Data { get; set; }
}

In any case, you would have to give the observers some time to react before you employ the "default" handler. The code below illustrates both "Exclusive" and "Default" concepts:

        var source = new[] {0, 1, 2, 3, 4}.ToObservable();
        var afterExclusive = source
            .Where(x =>
                       {
                           if (x == 0)
                           {
                               Console.WriteLine("exclusive");
                               return false;
                           }
                           return true;
                       })
            .Select(x => new Handled<int> {Data = x})
            .Publish(); // publish is a must otherwise 
        afterExclusive  // we'll get non shared objects
            .Do(x => { x.IsHandled = true; })
            .Subscribe();
        afterExclusive
            .Delay(TimeSpan.FromSeconds(1))
            .Where(x => !x.IsHandled)
            .Subscribe(x => Console.WriteLine("missed by all {0}", x));
        afterExclusive.Connect();