Observable.Next bug?

Question

Observable.Next is a blocking but non buffering conversion from IObservable to IEnumerable.

However it seems to be naive in that is does not cache any notification. I think it should at least cache OnError and OnCompleted. As it doesn't, if the enumerator is busy when the sequence completes or error's, the enumerator will miss it and then be left hanging for ever. :(

 

Code to reproduce

 

Console.WriteLine("{0:o} Starting", DateTime.Now);
var source = Observable.Interval(TimeSpan.FromSeconds(1))
    .Take(4);
var result = source.Next();
foreach (var element in result)
{
    Console.WriteLine("{0:o} {1}", DateTime.Now, element);
    Thread.Sleep(2000);
    //Misses the OnCompleted
}
//Never gets called.
Console.WriteLine("{0:o} done", DateTime.Now);
/*
Output:
2012-01-01T12:00:00.0000000 Starting
2012-01-01T12:00:01.1000000 1
2012-01-01T12:00:03.1000000 3
*/

 

Lee

---

Lee Campbell http://LeeCampbell.blogspot.com

Answer 1

Hi Lee,

Have you seen "Latest"

 

Console.WriteLine("{0:o} Starting", DateTime.Now);
var source = Observable.Interval(TimeSpan.FromSeconds(1))
	.Take(4);
var result = source.Latest();
foreach (var element in result)
{
	Console.WriteLine("{0:o} {1}", DateTime.Now, element);
	Thread.Sleep(2000);
	//Misses the OnCompleted
}
//Never gets called.
Console.WriteLine("{0:o} done", DateTime.Now);

 

OUTPUT

2012-01-25T11:07:41.6905541+08:00 Starting
2012-01-25T11:07:42.7483202+08:00 0
2012-01-25T11:07:44.7486018+08:00 2
2012-01-25T11:07:46.7498601+08:00 done

---

James Miles http://enumeratethis.com

Answer 2

Yes! And MostRecent(T), but I still think Next has a bug. Also the symantic for Latest and Next are different. Latest will cache a value (ie give you the value 2 not 3) but Next will not cache that value and block until it gets the next value (ie 3).

 

---

Lee Campbell http://LeeCampbell.blogspot.com

Answer 3

Yes, there is a slight difference in behaviour. Note however that Latest will only give you a cached value if you have not previously observed that value.

Next's behaviour used to be consistent with Subject<T> however Subject<T> has been changed so that it caches OnError & OnComplete notifications. Perhaps they need to revaluate Next's behaviour as well!

PS. I'd recommend changing the Topic type to question... otherwise they will never look at it! ;)

 

---

James Miles http://enumeratethis.com

Answer 4

Infinite blocking is not desirable in this case, so I'm considering it to be a bug. We'll look into it for a future release. Thanks for letting us know!

---

using (Microsoft.Sql.Cloud.DataProgrammability.Rx) { Signature.Emit("Bart De Smet"); }