RX - Group/Batch bursts of elements in an observab

I have an observable sequence. When the first element is inserted, I would like to start a timer and batch subsequent inserted elements during the timespan of the timer. Then, the timer wouldn't start again until another element is inserted in the sequence.

So something like this:

--------|=====timespan====|---------------|=====timespan====|-------------->
        1  2 3 4    5                     6 7            8

would produce:

[1,2,3,4,5], [6,7,8]

I tried with Observable.Buffer() and a timespan but from my experimentation, I can see that the timer is started as soon as we subscribe to the observable sequence and is restarted as soon as the previous timer is completed.

So having the same sequence as the previous example and using the Buffer() with a timespan, I would have something like this:

|=====timespan====|=====timespan====|=====timespan====|=====timespan====|-->
        1  2 3 4    5                      6 7           8

which would produce this:

[1,2,3,4], [5], [6,7], [8]

Here is how I tested this behavior with the Buffer:

var source = Observable.Concat(Observable.Timer(TimeSpan.FromSeconds(6)).Select(o => 1),
                               Observable.Timer(TimeSpan.FromSeconds(1)).Select(o => 2),
                               Observable.Timer(TimeSpan.FromSeconds(3)).Select(o => 3),
                               Observable.Never<int>());

Console.WriteLine("{0} => Started", DateTime.Now);
source.Buffer(TimeSpan.FromSeconds(4))
      .Subscribe(i => Console.WriteLine("{0} => [{1}]", DateTime.Now, string.Join(",", i)));

With the output:

4/24/2015 7:01:09 PM => Started
4/24/2015 7:01:13 PM => []
4/24/2015 7:01:17 PM => [1,2]
4/24/2015 7:01:21 PM => [3]
4/24/2015 7:01:25 PM => []
4/24/2015 7:01:29 PM => []
4/24/2015 7:01:33 PM => []

Anyone has an idea on how to do this? Thanks in advance!

Give this a go:

var source = Observable.Concat(Observable.Timer(TimeSpan.FromSeconds(6)).Select(o => 1),
                           Observable.Timer(TimeSpan.FromSeconds(1)).Select(o => 2),
                           Observable.Timer(TimeSpan.FromSeconds(4)).Select(o => 3),
                           Observable.Never<int>());

Console.WriteLine("{0} => Started", DateTime.Now);
source
    .GroupByUntil(x => 1, g => Observable.Timer(TimeSpan.FromSeconds(4)))
    .Select(x => x.ToArray())
    .Switch()
    .Subscribe(i => Console.WriteLine("{0} => [{1}]", DateTime.Now, string.Join(",", i)));

I had to change your test code duration for the third timer to make sure the value was outside of the grouped timer.