Question

I’m trying to figure out how to create a Reactive subscription for an event not followed by a different event within a specific time window.

To illustrate, here’s a use case:

A busy indicator triggered by two events. Busy and NotBusy. They may fire close together but the indicator should not flash on/off too often.

When NotBusy fires, there should be no indicator. When Busy fires and NotBusy hasn’t fired within 5 seconds then it should display.

Is there a way to do this entirely within Reactive without adding external state?

Answer 1

Ah, I can’t resist – how often do you get to counter-answer the author of the source material? (see Benjol’s comment on question) 🙂

Here’s my stab at it (LINQPad-ready):

Output looks like this:

At 1/1/0001 12:00:00 AM +00:00, Busy Signal Indicator is now:OFF
Sending BUSY at 1/1/0001 12:00:00 AM +00:00
Sending NOTBUSY at 1/1/0001 12:00:02 AM +00:00

Sending BUSY at 1/1/0001 12:00:03 AM +00:00
Sending BUSY at 1/1/0001 12:00:06 AM +00:00
At 1/1/0001 12:00:08 AM +00:00, Busy Signal Indicator is now:ON
Sending NOTBUSY at 1/1/0001 12:00:09 AM +00:00
At 1/1/0001 12:00:09 AM +00:00, Busy Signal Indicator is now:OFF

Here a basic definition of our events:

enum EventType 
{ 
    Busy, 
    NotBusy 
}
class StreamEvent 
{ 
     public EventType Type {get; set;} 
     public StreamEvent(EventType type) { Type = type;}
}

And here’s the query + test code:

void Main()
{
    // our simulated event stream
    var fakeSource = new System.Reactive.Subjects.Subject<StreamEvent>();

    // Let's use a scheduler we actually don't have to wait for
    var theTardis = new System.Reactive.Concurrency.HistoricalScheduler();

    var busySignal = fakeSource
        // Batch up events:
        .Window(
            // Starting batching on a busy signal
            fakeSource.Where(e => e.Type == EventType.Busy),
            // Stop batching on a not busy signal
            (open) => fakeSource.Where(e => e.Type == EventType.NotBusy)
                // but throw a timeout if we exceed 5 seconds per window
                .Timeout(TimeSpan.FromSeconds(5), theTardis))       
        // Unpack the windows
        .Switch()
        // Catch any timeout exception and inject a NULL into the stream        
        .Catch(fakeSource.StartWith((StreamEvent)null))
        // Bool-ify on "did a timeout happen?"
        .Select(e => e == null)
        // Start in an "unbusy" state
        .StartWith(false)
        // And only tell us about transitions
        .DistinctUntilChanged();    

    using(busySignal.Subscribe(signal => 
        Console.WriteLine("At {0}, Busy Signal Indicator is now:{1}",
            theTardis.Now,
            signal ? "ON" : "OFF")))
    {
        // should not generate a busy signal
        Console.WriteLine("Sending BUSY at {0}", theTardis.Now);
        fakeSource.OnNext(new StreamEvent(EventType.Busy));
        theTardis.AdvanceBy(TimeSpan.FromSeconds(2));
        Console.WriteLine("Sending NOTBUSY at {0}", theTardis.Now);
        fakeSource.OnNext(new StreamEvent(EventType.NotBusy));
        theTardis.AdvanceBy(TimeSpan.FromSeconds(1));
        Console.WriteLine();

        // should generate a busy signal
        Console.WriteLine("Sending BUSY at {0}", theTardis.Now);
        fakeSource.OnNext(new StreamEvent(EventType.Busy));
        theTardis.AdvanceBy(TimeSpan.FromSeconds(3));
        Console.WriteLine("Sending BUSY at {0}", theTardis.Now);
        fakeSource.OnNext(new StreamEvent(EventType.Busy));
        theTardis.AdvanceBy(TimeSpan.FromSeconds(3));

        // and this should clear it
        Console.WriteLine("Sending NOTBUSY at {0}", theTardis.Now);
        fakeSource.OnNext(new StreamEvent(EventType.NotBusy));      
        theTardis.AdvanceBy(TimeSpan.FromSeconds(1));
        Console.WriteLine();    
    }
}