Server Sent Events with Saturn and F#

const source = new EventSource("/sse");

source.addEventListener("open", function (event) {
  console.log("Connected");
});

// Let's skip ahead and set already our "reload" event
source.addEventListener("reload", function (event) {
  console.log("Reloading, file changed: ", event.data);
});
// Listen to any message sent not tied to a particular event
source.addEventListener("message", function (event) {
  console.log(event);
  console.log(event.data);
});

source.addEventListener("error", function (err) {
  console.error(err);
});

dotnet new console -lang F# -o SSESample
dotnet add package Saturn
code SSESample # or rider or visual studio, your choice

open System
open System.IO
open FSharp.Control.Tasks

open Giraffe
open Saturn

open Saturn.Endpoint.Router
open Microsoft.AspNetCore.Http

let sse next (ctx: HttpContext) =
    task {
        let res = ctx.Response
        ctx.SetStatusCode 200
        ctx.SetHttpHeader("Content-Type", "text/event-stream")
        ctx.SetHttpHeader("Cache-Control", "no-cache")

        while true do
            // VERY IMPORTANT
            // MESSAGES MUST START WITH `data:`
            // AND MUST END WITH `\n\n`
            // otherwise the client will keep buffering and no data will be consumed
            do! res.WriteAsync $"data:Hello, world!\n\n"
            do! res.Body.FlushAsync()
            // not required but prevent your CPU from running crazy :P
            do! Async.Sleep(TimeSpan.FromSeconds 2.5)

        // This won't be hit because we're expecting
        // the browser to break the connection
        // although you can indeed break from the loop above and get here
        return! text "" next ctx
    }

[<EntryPoint>]
let main args =
    let app =
        application {
            use_endpoint_router (router { get "/sse" sse })
            // serve an index file with the JS code from above
            use_static "wwwroot"
        }

    run app
    0

dotnet run

id:ABC-DFG-HIJ-LMN
event:signup
data:{ "id": 10500, "name": "Peter Parker", "email":"im-not@spiderman.com" }

source.addEventListener("signup", function (event) {
  const user = JSON.parse(event.data);
  const eventId = event.id;
  console.log(
    `Event: ${eventId}, User signed up: ${user.name} - ${user.email}`
  );
});

dotnet new console -lang F# -o SSEFileWatcher
dotnet add package Saturn
dotnet add package FSharp.Control.Reactive
code SSESample # or rider or visual studio, your choice

open System
open System.IO
open System.Text.Json

open FSharp.Control.Tasks
open FSharp.Control.Reactive

open Microsoft.AspNetCore.Http

open Giraffe
open Saturn
open Saturn.Endpoint.Router

// a simple inerface
type INotifierService =
    inherit IDisposable
    abstract OnFileChanged : IObservable<string>


// this function will build an INotifier service
// if you're using DI you can easily change this into
// a service factory and use services.AddSingleton<INotifierService>(getNotifier)
// you would need to read the path from the configuration
// and take IService collection as a paremeter though
let getNotifier (path: string) : INotifierService =
    let fsw = new FileSystemWatcher(path)

    // Filter events by filename and size
    fsw.NotifyFilter <- NotifyFilters.FileName ||| NotifyFilters.Size
    fsw.EnableRaisingEvents <- true

    // the next part is creating observables from .NET events and just getting
    // the Name of the file that changed.

    let changed =
        fsw.Changed
        |> Observable.map (fun args -> args.Name)

    let deleted =
        fsw.Deleted
        |> Observable.map (fun args -> args.Name)

    let renamed =
        fsw.Renamed
        |> Observable.map (fun args -> args.Name)

    let created =
        fsw.Created
        |> Observable.map (fun args -> args.Name)

    let obs =
        // merge all of the file events into a single stream of events
        Observable.mergeSeq [ changed
                              deleted
                              renamed
                              created ]

    // return a  INotifierService
    { new INotifierService with
        override _.Dispose() : unit = fsw.Dispose()
        override _.OnFileChanged: IObservable<string> = obs }

let sse next (ctx: HttpContext) =
    task {
        let res = ctx.Response
        ctx.SetStatusCode 200
        ctx.SetHttpHeader("Content-Type", "text/event-stream")
        ctx.SetHttpHeader("Cache-Control", "no-cache")
        // get our notifier
        let notifier = getNotifier @"C:\Users\scyth\Desktop"

        let onFileChanged =
            notifier.OnFileChanged
            // subscribe to events
            |> Observable.subscribe
                (fun filename ->
                    // Sync operations in the response are disallowed by default
                    // so we will wrap this in a task and run it immediately
                    task {
                        let data =
                            JsonSerializer.Serialize({| filename = filename |})
                        // write our event to the body
                        do! res.WriteAsync $"event:reload\ndata:{data}\n\n"
                        // Flush the contents so the client can read this information
                        do! res.Body.FlushAsync()
                    }
                    |> Async.AwaitTask
                    |> Async.StartImmediate)
        // Write an initial
        do! res.WriteAsync($"id:{ctx.Connection.Id}\nevent:start\ndata:{DateTime.Now}\n\n")
        do! res.Body.FlushAsync()

        // release resources when the client disconnects
        ctx.RequestAborted.Register
            (fun _ ->
                notifier.Dispose()
                onFileChanged.Dispose())
        |> ignore

        // keep the connection alive
        while true do
            do! Async.Sleep(TimeSpan.FromSeconds 1.)

        return! text "" next ctx
    }

[<EntryPoint>]
let main args =
    let app =
        application {
            use_endpoint_router (router { get "/sse" sse })
            // serve an index.html file with the first JS block above
            use_static "wwwroot"
        }

    run app
    0