module AsyncPeriodicWork =

   let createCanceller () =
      new System.Threading.CancellationTokenSource()

   let start (canceller:System.Threading.CancellationTokenSource) (intervalMillis:int) (work:(unit->unit)) (onFault:(System.Exception->unit) option)=

      let loop = async {
         printfn "starting..."
         try
            while true do
               work()
               do! Async.Sleep intervalMillis
            with
               | ex -> match onFault with
                       | Some(handler) -> ex |> handler
                       | None -> ()
         printfn "exiting"
      }

      Async.Start (loop, canceller.Token)

   let startCancellable (intervalMillis:int) (work:(unit->unit)) (onFault:(System.Exception->unit) option) =

      let canceller = createCanceller()
      start canceller intervalMillis work onFault
      canceller



module examples =

    let run1 () =

        let printTimestamp x =
           fun () -> printfn "%s %A" x System.DateTime.Now

        let cts = AsyncPeriodicWork.createCanceller()
        AsyncPeriodicWork.start cts 500 (printTimestamp "A") None
        AsyncPeriodicWork.start cts 100 (printTimestamp "B") None
        //System.Threading.Thread.Sleep(2000)
        //cts.Cancel()
        cts

    let run2 () =

        let stopwatch = System.Diagnostics.Stopwatch()
        stopwatch.Start()
        let cts = AsyncPeriodicWork.startCancellable 100 (fun () -> printfn "%A" stopwatch.ElapsedMilliseconds) None
        //System.Threading.Thread.Sleep(1000)
        //cts.Cancel()
        cts

    let run3 () =

        let work = fun () ->
            match System.Random().Next() % 10 with
            | x when x < 8 -> printfn "random is %A" x
            | _ -> failwith "foobar"

        let cts = AsyncPeriodicWork.startCancellable 100 work (Some(fun ex -> printfn "%A!" ex.Message))
        cts

    let cancelAfterMillis (millis:int) (cts:System.Threading.CancellationTokenSource) =
        System.Threading.Thread.Sleep(millis)
        cts.Cancel()