F#:对第一个事件只执行一次操作，没有可变性/锁定

Question

我有这段代码，它下载了一个文件，并在控制台中告诉我文件有多大：

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()

有没有办法做同样的事情，但既不使用锁定也不使用可变变量？

Answer 1

以下是使用反应式扩展的一种方法：

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))