F#: only do one action once for the first event, without mutability/locking?

问题

I have this code that downloads a file and tells me in the console how big is the file:

use webClient = new WebClient()

let lockObj = new Object()
let mutable firstProgressEvent = true
let onProgress (progressEventArgs: DownloadProgressChangedEventArgs) =
    lock lockObj (fun _->
        if (firstProgressEvent) then
            let totalSizeInMB = progressEventArgs.TotalBytesToReceive / 1000000L
            Console.WriteLine ("Starting download of {0}MB...", totalSizeInMB)
        firstProgressEvent <- false
    )

webClient.DownloadProgressChanged.Subscribe onProgress |> ignore
let task = webClient.DownloadFileTaskAsync (uri, Path.GetFileName(uri.LocalPath))
task.Wait()

Is there a way to do the same, but using neither locking nor mutable vars?

回答1:

Here is one way using the Reactive Extensions:

open System.Net
open FSharp.Control

let webClient = new WebClient()
let disposable = 
  Observable.take 1 webClient.DownloadProgressChanged 
  |> Observable.subscribe (fun progress -> printfn "%A" (progress.TotalBytesToReceive))

来源：https://stackoverflow.com/questions/41439213/f-only-do-one-action-once-for-the-first-event-without-mutability-locking