/// Convert async observable to async sequence, non-blocking.
/// Producer will be awaited until item is consumed by the async
/// enumerator.
let toAsyncSeq (source: IAsyncObservable<'a>) : AsyncSeq<'a> =
    let ping = new AutoResetEvent false
    let pong = new AutoResetEvent false
    let mutable latest : Notification<'a> = OnCompleted

    let _obv n =
        async {
            latest <- n
            ping.Set () |> ignore
            do! Async.AwaitWaitHandle pong |> Async.Ignore
        }

    asyncSeq {
        let! dispose = AsyncObserver _obv |> source.SubscribeAsync
        let mutable running = true

        while running do
            do! Async.AwaitWaitHandle ping |> Async.Ignore
            match latest with
            | OnNext x ->
                yield x
            | OnError ex ->
                running <- false
                raise ex
            | OnCompleted ->
                running <- false
            pong.Set () |> ignore

        do! dispose.DisposeAsync ()
    }