How run in background a blocking call in F#?

Question

I need to call in the background a API that call a webservice. I don't wish to turn the (very complex) method to async, just say "do all this on the background".

But I'm lost in how do this with F#. This is what I have:

            let task = async {
                let result = SyncApi.syncData(login.url, login.zone, login.user, login.pwd) <-- THIS MUST RUN IN BACKGROUND...
                match result with
                |Some(msg) -> failwith msg
                | None -> ()
            }

            task
            |> Async.Catch
            |> Async.RunSynchronously
            |> fun x -> 
                  match x with
                  | Choice1Of2 x -> rootPage.Navigation.PopToRootAsync(true) |> ignore
                  | Choice2Of2 ex -> showMsgError(ex.Message)

Answer 1

If you're looking for simple fire and forget style to start the API call an don't use the result on the current thread, Async.Start(task) might be what you're looking for. It takes the Async workflow, starts it on a thread pool and immediately returns so that your method can continue.

But seeing that you need the result to either change navigation or show an error message, you might need to call the SyncApi synchronously on the current thread and wait for its result.

Alternatively, if your application model allows it, you can do something like this:

(* Define the task including error handling. *)
let task = async {
    let result = SyncApi.syncData(login.url, login.zone, login.user, login.pwd)
    match result with
    | Some msg ->
        (* This may have to be posted back to the UI context.
           Correct way depends on technology (Xamarin vs. WPF vs. MVC...) *)
        showMsgError msg
    | None -> ()
}

(* Fire and forget the async API call. *)
Async.Start(task)

(* Optimistically navigate away immediately,
   while `task` may still be in progress. *)
rootPage.Navigation.PopToRootAsync(true) |> ignore

This will start the task on a thread pool, navigate away, but in case the async task failed, it will trigger the error message. However it assumes that your application can show the error message asynchronously for example as a popup, not only in the context of the page that started the task.