AsyncTimer.fs

type AsyncTimer(duration) =
    let tickEvent = new Event<_>()
    let endEvent = new Event<_>()

    [<CLIEvent>]
    member this.Tick = tickEvent.Publish

    [<CLIEvent>]
    member this.End = endEvent.Publish

    member private this.CancellationToken interval = 
        let cts = new CancellationTokenSource()
        let task = async {
            while true do
                do! Async.Sleep interval                
                tickEvent.Trigger(this)
        }

        Async.StartImmediate(task, cts.Token)

        async {
            do! Async.Sleep (duration * 1000)

            cts.Cancel()        
            tickEvent.Trigger(this)
            endEvent.Trigger(this) 
        } |> Async.StartImmediate
    
    member private this.MutableLoop interval =
        let startingTime = DateTime.Now
        let mutable isLooping = true
        async {
            while isLooping do
                do! Async.Sleep interval

                let timeExceeded = DateTime.Now - startingTime >= TimeSpan.FromSeconds(float duration)
                if  timeExceeded then isLooping <- false
                
                tickEvent.Trigger(this)
            
            if not isLooping then endEvent.Trigger(this)
        } |> Async.StartImmediate

    member this.Start = this.CancellationToken
    
// Usage
let timer = AsyncTimer(5)
timer.Tick.Add (fun _ -> printfn "%A" DateTime.Now)
timer.End.Add (fun _ -> printfn "Finished!")
timer.Start 1000