Rx Observable only works once? - vb.net

Question

I have the following code which is being called from a Web Api. As you can see I want to return as soon as I can and shift the work onto the threadpool. (The client polls to see when the job is complete. But the polling is nothing to do with this. The purpose of these routines is simply to extract data and write a file away in background whilst maintaining the progress of the job in a table. The client will interrogates this to determine whether the file is ready so I'm not trying to push progress messages to the client.)
Public Function Extract(filepath as string, ...) as ExtractResult
dim source = ExtractInternal(filepath, ...)
' works first time it is called only!
using source.SubscribeOn(ThreadPoolScheduler.Instance)
.SubScribe()
end using
' works every time it is called ...
dim subscription = source.SubscribeOn(ThreadPoolScheduler.Instance)
.SubScribe()
Return New ExtractResult()
End Function
Public Function ExtractInternal(filepath as string, ...) as IObservable(of Unit)
return Observable.Create(of Unit)
Function()
....
uses filepath here
Return Disposable.Empty
End Function
End Function
As you can see in my comments, if I use auto-disposing of Using ..., I am finding that the observable gets called on the first occasion but not thereafter. Whereas if I assign the subscription to a local var it works every time the web call invokes the routine but I'm concerned that I'm actually leaving stuff hanging around.
Could someone explain why the observable doesn't get re-instantiated on subsequent calls and perhaps explain how I can get it to work every time and tidy up afterwards properly.
EDIT:
So I ended up using Observable.Defer which seems tom give me what I am after ...
Public Function Extract(filepath as string, ...) As ExtractResult
Observable.Defer(Function() ExtractInternal(filepath, ...) _
.SubscribeOn(NewThreadScheduler.Default) _
.Subscribe()
Return New ExtractResult()
End Function
I'm wondering if this is perhaps the correct way to do it to give me proper disposal whilst also using the current parameter values.
Could anyone confirm or correct?
EDIT 2
That was wrong! In fact if I rewrite it as
Public Function Extract(filepath as string, ...) As ExtractResult
Using Observable.Defer(Function() ExtractInternal(filepath, ...)
.SubscribeOn(NewThreadScheduler.Default) _
.Subscribe()
End Using
Return New ExtractResult()
End Function
I get the same behaviour as I originally was getting when I wrote the post.
One thing (amongst many) I don't understand is why if the observable is local var, when a second call is made to the Extract method, another observable is not created and subscribed to? It seems to go against scoping logic if I am actually referencing the same observable under the hood? I've obviously misunderstood.
Many thx
S

Answer 1

Yes, when you dispose the subscription, it stops receiving notifications.
You should keep it in an instance field and have the class implement disposable. Consumers of this class can then dispose it at their convenience.
In your Dispose implementation, you call subscription.Dispose().