Weak Observer Subscription - https://stackoverflow.com/questions/45899576/how-to-weakly-subscribe-to-events-observables

WeakObserver.fs

module Observable =
    open System

    // ('a -> 'b -> unit) -> 'a -> IObservable<'b>
    let subscribeWeakly callback target source = 

        let mutable sub:IDisposable = null
        let mutable disposed = false
        let wr = new WeakReference<_>(target)

        let dispose() =
            lock (sub) (fun () -> 
                if not disposed then sub.Dispose(); disposed <- true)

        let callback' x =
            let isAlive, target = wr.TryGetTarget()
            if isAlive then callback target x else dispose()

        sub <- Observable.subscribe callback' source
        sub