Scan 2.linq

<Query Kind="Statements">
  <Reference>&lt;ApplicationData&gt;\LINQPad\Samples\Programming Reactive Extensions and LINQ\System.Reactive.dll</Reference>
  <Namespace>System.Reactive</Namespace>
  <Namespace>System.Reactive.Linq</Namespace>
</Query>

/* Scan 2:
 *
 * Scan can also be used in more clever ways, such as this example that
 * implements a reference count using two Subjects.
 *
 * (Select 'C# Program' here since we use a function)
 */

IObservable<int> ThrowIfBelowZero(int refCount)
{
	if (refCount >= 0) {
		return Observable.Return(refCount);
	}
	return Observable.Throw<int>(new Exception("Refcount dropped below Zero!"));
}

void Main()
{	
	var AddRef = new Subject<Unit>();
	var Release = new Subject<Unit>();
	
	var referenceCount = Observable.Merge(
			AddRef.Select(_ => 1),
			Release.Select(_ => -1))
		.Scan(0, (acc, x) => acc + x)
		.SelectMany(x => ThrowIfBelowZero(x));
	
	referenceCount.Subscribe(x => Console.WriteLine("Current RefCount is {0}", x));
	
	AddRef.OnNext(Unit.Default);
	AddRef.OnNext(Unit.Default);
	Release.OnNext(Unit.Default);
	AddRef.OnNext(Unit.Default);
	Release.OnNext(Unit.Default);
	Release.OnNext(Unit.Default);
	Release.OnNext(Unit.Default);
}