open System
open System.Threading.Tasks
open Microsoft.Extensions.DependencyInjection
open Microsoft.Extensions.Hosting
open Microsoft.Extensions.Logging

type Worker(logger : ILogger<Worker>) =
    inherit BackgroundService()
    let _logger = logger
    override bs.ExecuteAsync stoppingToken =
        let f : Async<unit> = async {
            while not stoppingToken.IsCancellationRequested do
                _logger.LogInformation("Worker running at: {time}", DateTimeOffset.Now)
                do! Async.Sleep(1000)
        }
        // ExecuteAsync needs to return a task, hence up-cast from Task<unit> to plain Task.
        // pass stoppingToken into async launch so that it's passed down automatically to other async CE's
        //     per https://medium.com/@eulerfx/f-async-guide-eb3c8a2d180a
        Async.StartAsTask (computation = f, cancellationToken = stoppingToken) :> Task

let CreateHostBuilder argv : IHostBuilder =
    let builder = Host.CreateDefaultBuilder(argv)
    builder
        .UseWindowsService()
        .ConfigureServices(fun hostContext services -> services.AddHostedService<Worker>() |> ignore<IServiceCollection>)

[<EntryPoint>]
let main argv =
    let hostBuilder = CreateHostBuilder argv
    hostBuilder.Build().Run()
    0 // return an integer exit code