Is there any counter observable through Rx? - system.reactive

Question

I am looking for something I would call CounterObservable One side would count the numbers on it and other side would be the observer side that will receive notification every time total count changes.
In other words I will have something like this
public CounterObservable totalMailsReceived = new CounterObservable(0);
public void OnNewMail(Mail mail)
{
totalMailsReceived++;
///Rest of the code goes here
}
on the Observer side I will have
mailManager.totalMailsReceived.Subscribe(count => labelCount.Text = count.ToString());
Or if I want to go real classy, I would use Paul Betts' ReactiveUI like the following
mailManager.totalMailsReceived.ToProperty(x => x.TotalMailsReceived);
I have so far found nothing in Rx that could help me. But I figured if I create my own class that implements IObservable<int>. I am thinking of leveraging the Sample MSDN Code for IObservable implementation for that.
My questions are
1. Is that MSDN Sample thread-safe ?
2. Is there really nothing in Rx already that does what I am trying to do ?

Answer 1

Just use a BehaviorSubject:
public class MailServer
{
private BehaviorSubject<int> _count = new BehaviorSubject<int>(0);
public IObservable<int> TotalMailsReceived
{
get { return _count; }
}
public void OnNewMail(Mail mail)
{
_count.OnNext(_count.Value + 1);
}
}
Or, if you decide to go deeper into Rx, so that you are just observing a Mail stream, then you can use Scan operator which is good for that and Publish to remember the most recent value and multicast it to all subscribers.
You can write this new extension method:
public IObservable<T> RunningTotal<T>(this IObservable<T> source)
{
return source.Scan(0, sum => sum + 1);
}
And use it like so:
public class MailServer
{
private IConnectableObservable<int> _total;
private IDisposable _subscription;
public MailServer(IObservable<Mail> incomingMail)
{
_total = incomingMail.RunningTotal().Publish(0);
_subscription = _total.Connect();
}
public IObservable<int> TotalMailsReceived
{
get { return _total; }
}
}