F# propper remove of ElapsedEventHandler handler - f#

Question

Hi I have a question I have quick program that will do something in every 10 seconds and it looks simple:
[<EntryPoint>]
let main argv =
let timer = new Timer(float 10000)
let OnTimedEvent (frameId:uint32) : ElapsedEventHandler = new ElapsedEventHandler (fun obj args -> printfn "DATE: %A FRAME: %i" DateTime.Now frameId)
while(true) do
let keyStroke = Console.ReadKey()
if keyStroke.Key.Equals(ConsoleKey.Enter) then
let frameId = 1u
timer.AutoReset <- true
timer.Elapsed.RemoveHandler(OnTimedEvent frameId)
timer.Elapsed.AddHandler(OnTimedEvent frameId)
timer.Start();
else
printfn "%c pressed" keyStroke.KeyChar
0
Problem is I can not properly remove handler as it is, If i press enter once it starts and gives me one message every 10 sec so thi is what I am aiming for. But if i press enter 3 times it increments and gives me 3 messeges and so on, but i only want one.
Another thing is that if I remove parameter from it it works perfectly, so i suppose problem is with parameter. Any solutions for this?

Answer 1

The problem with your current implementation is that every call to OnTimedEvent returns a new instance of the ElapsedEventHandler. When you call it as follows:
timer.Elapsed.RemoveHandler(OnTimedEvent frameId)
you are removing a new handler that has not previously been registerd and so nothing actually happens. When you change your code to add/remove the same handler, then you are always using the same instance:
let frameId = 1u
let timedHandler = new ElapsedEventHandler (fun obj args ->
printfn "DATE: %A FRAME: %i" DateTime.Now frameId)
timer.Elapsed.RemoveHandler(timedHandler)
timer.Elapsed.AddHandler(timedHandler)
Now you do not have a good way of passing the frameId to your event handler. In your code, frameId is always 1u and so it's hard to see what you actually want, but you could make it mutable:
let mutable frameId = 1u
let timedHandler = new ElapsedEventHandler (fun obj args ->
printfn "DATE: %A FRAME: %i" DateTime.Now frameId)
frameId <- 2u
timer.Elapsed.RemoveHandler(timedHandler)
timer.Elapsed.AddHandler(timedHandler)
That said, it's not really clear what you are trying to do and perhaps there is an altogether different way of doing what you want.
A completely different approach would be to use MailboxProcessor that keeps the current frameId and handles two types of messages - one triggered every 10 seconds by a timer and one that can be used to change the frame ID:
type Message =
| Tick
| ChangeFrameId of uint32
let agent = MailboxProcessor.Start(fun inbox ->
let rec run frameId = async {
let! msg = inbox.Receive()
match msg with
| ChangeFrameId newId ->
return! run newId
| Tick ->
printfn "DATE: %A FRAME: %i" DateTime.Now frameId
return! run frameId }
run 1u)
let timer = new Timer(float 10000, AutoReset = true)
timer.Elapsed.Add(fun _ -> agent.Post(Tick))
timer.Start()
agent.Post(ChangeFrameId 2u)

Answer 2

This code refactors what you have to store the handler so that it can be removed.
open System
open System.Timers
[<EntryPoint>]
let main argv =
let timer = new Timer(float 10000, AutoReset = true)
let onTimedEvent (frameId: uint32) : ElapsedEventHandler = new ElapsedEventHandler (fun obj args -> printfn "DATE: %A FRAME: %i" DateTime.Now frameId)
let rec readKey frameId =
let handler = onTimedEvent frameId
timer.Elapsed.AddHandler(handler)
timer.Start()
let keyStroke = Console.ReadKey()
timer.Stop()
timer.Elapsed.RemoveHandler(handler)
printfn "%c pressed" keyStroke.KeyChar
let nextFrameId =
if keyStroke.Key.Equals(ConsoleKey.Enter) then
frameId + 1u
else
frameId
readKey(nextFrameId)
readKey(1u)
0
There may be better ways to accomplish what you are after, but this answers your question.