Observable, Scan and FirstAsync are from Rx. (FirstAsync is roughly equivalent to LINQ First(), except that it doesn't block.) This particular Subscribe metho

Observable, Scan and FirstAsync are from Rx. (FirstAsync is roughly equivalent to LINQ First(), except that it doesn't block.) This particular Subscribe method is also part of Rx, but it's basically a helper method around the BCL System.IObservable<T>.Subscribe(IObserver<T>) that saves you having to implement IObserver.