ReactiveX Self-Cancelling Timer - reactive-programming

Question

I want to create an extension method of the form:
IObservable<bool> CancellableTimer( this IObservable source, TimeSpan delay )
{
...
}
which produces a sequence which is always false when the source is, but will go true when the source sequence has stayed true for a period defined by a delay, t:
source: 0---1---------0--1-0-1-0-1-0-1----------0
t------> t------>
result: 0----------1--0---------------------1---0
I'm sure there must be a way to do this using Rx primitives but I'm new to Rx and having trouble getting my head round it. Any ideas please?

Answer 1

Okay so this is what I came up with. I also renamed the method to AsymetricDelay() as it seems like a more appropriate name:
static public IObservable<bool> AsymetricDelay(this IObservable<bool> source, TimeSpan delay, IScheduler scheduler)
{
var distinct = source.DistinctUntilChanged();
return distinct.
Throttle(delay, scheduler) // Delay both trues and falses
.Where(x => x) // But we only want trues to be delayed
.Merge( // Merge the trues with...
distinct.Where(x=>!x) // non delayed falses
)
.DistinctUntilChanged(); // Get rid of any repeated values
}
And here is a unit test to confirm its operation:
[Fact]
public static void Test_AsymetricDelay()
{
var scheduler = new TestScheduler();
var xs = scheduler.CreateHotObservable(
new Recorded<Notification<bool>>(10000000, Notification.CreateOnNext(true)),
new Recorded<Notification<bool>>(60000000, Notification.CreateOnNext(false)),
new Recorded<Notification<bool>>(70000000, Notification.CreateOnNext(true)),
new Recorded<Notification<bool>>(80000000, Notification.CreateOnNext(false)),
new Recorded<Notification<bool>>(100000000, Notification.CreateOnCompleted<bool>())
);
var dest = xs.DelayOn( TimeSpan.FromSeconds(2), scheduler);
var testObserver = scheduler.Start(
() => dest,
0,
0,
TimeSpan.FromSeconds(10).Ticks);
testObserver.Messages.AssertEqual(
new Recorded<Notification<bool>>(30000000, Notification.CreateOnNext(true)),
new Recorded<Notification<bool>>(60000000, Notification.CreateOnNext(false)),
new Recorded<Notification<bool>>(100000000, Notification.CreateOnCompleted<bool>())
);
}