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我的问题

我正在编写一个 PowerShell 脚本，该脚本需要从远程 Web 服务器下载几个大文件，然后才能继续执行其他任务。我的项目要求之一是显示每次下载的进度，以便最终用户知道发生了什么。

对另一个 SO 问题的响应包含使用已注册事件和 Net.WebClient.DownloadFileAsync 的解决方案。但是，当我在接受的答案中尝试解决方案时，在我的脚本处理所有 DownloadProgressChanged 事件之前很久就完成了下载。

我试过的

我创建了一个我在下面提供的测试脚本，以便重现问题并尝试隔离它发生的位置。起初我认为问题在于 Write-Progress cmdlet 速度很慢，因此我将其替换为 Write-Host 以显示完成的百分比。这产生了许多具有相同百分比值的重复行。考虑到 Write-Host 也可能很慢，我将事件处理程序的操作更改为仅在更改时输出百分比。这并没有什么不同。

我在输出中注意到，在事件处理程序指示下载完成超过百分之几之前，下载已经完成。在最初的几个百分比中有很大的延迟，然后它加速了。但是即使在下载完成后，百分比完成的输出仍然很慢。我对脚本进行了另一次修改，以显示每个百分比变化所经过的时间。

我的环境是使用 PowerShell 2.0 的 Windows 7 和 Server 2008。我已经在两个操作系统上尝试了该脚本，结果相同。我没有使用 PS 配置文件，并且 Server 2008 计算机是全新安装的。

这是我的测试脚本。

# based on https://stackoverflow.com/a/4927295/588006
$client = New-Object System.Net.WebClient
$url=[uri]"https://www.kernel.org/pub/linux/kernel/v2.6/linux-2.6.18.tar.bz2"
$file="$env:USERPROFILE\Downloads\linux-2.6.18.tar.bz2"

try {
    Register-ObjectEvent $client DownloadProgressChanged -action {     
        if ( $eventargs.ProgressPercentage -gt $percent ) {
            $percent = $eventargs.ProgressPercentage
            if ( $start_time -eq $null ) {
                $start_time = $(get-date)
            }
            # Get the elapsed time since we displayed the last percentage change
            $elapsed_time = new-timespan $start_time $(get-date)
            write-host "Percent complete:" $eventargs.ProgressPercentage "($elapsed_time)"
        }
    }

    Register-ObjectEvent $client DownloadFileCompleted -SourceIdentifier Finished

    $client.DownloadFileAsync($url, $file)

    # optionally wait, but you can break out and it will still write progress
    Wait-Event -SourceIdentifier Finished

} finally {
    write-host "File download completed"
    $client.dispose()
    Unregister-Event -SourceIdentifier Finished
    Remove-Event -SourceIdentifier Finished
}

该脚本产生以下输出。

PS C:\Users\devtest\Desktop> .\write-progress-speed.ps1

Id              Name            State      HasMoreData     Location             Command
--              ----            -----      -----------     --------             -------
1               7989b3fe-cce... NotStarted False                                     ...
Percent complete: 0 (00:00:00)
Percent complete: 1 (00:00:09.2435931)

ComputerName     :
RunspaceId       : 6c207bde-bb4a-442b-a7bd-05a9c12fae95
EventIdentifier  : 9978
Sender           : System.Net.WebClient
SourceEventArgs  : System.ComponentModel.AsyncCompletedEventArgs
SourceArgs       : {System.Net.WebClient, System.ComponentModel.AsyncCompletedEventArgs}
SourceIdentifier : Finished
TimeGenerated    : 8/9/2013 8:02:59 AM
MessageData      :

File download completed


Percent complete: 2 (00:00:12.2896000)
PS C:\Users\devtest\Desktop> Percent complete: 3 (00:00:12.6756120)
Percent complete: 4 (00:00:13.0646281)
Percent complete: 5 (00:00:13.2796284)
Percent complete: 6 (00:00:13.4656313)
Percent complete: 7 (00:00:13.6106315)
Percent complete: 8 (00:00:13.7756318)
Percent complete: 9 (00:00:13.9656320)
Percent complete: 10 (00:00:14.1306323)
Percent complete: 11 (00:00:14.2406324)
Percent complete: 12 (00:00:14.3706326)
Percent complete: 13 (00:00:14.5006328)
Percent complete: 14 (00:00:14.6556330)
Percent complete: 15 (00:00:14.7806332)
Percent complete: 16 (00:00:14.9006333)
Percent complete: 17 (00:00:15.0156335)
Percent complete: 18 (00:00:15.1406337)
Percent complete: 19 (00:00:15.2556338)
Percent complete: 20 (00:00:15.3656340)
Percent complete: 21 (00:00:15.4756342)
Percent complete: 22 (00:00:15.5856343)
Percent complete: 23 (00:00:15.6706344)
Percent complete: 24 (00:00:15.7906346)
Percent complete: 25 (00:00:15.9056348)
Percent complete: 26 (00:00:16.0156349)
Percent complete: 27 (00:00:16.1206351)
Percent complete: 28 (00:00:16.2056352)
Percent complete: 29 (00:00:16.3006353)
Percent complete: 30 (00:00:16.4006354)
Percent complete: 31 (00:00:16.5106356)
Percent complete: 32 (00:00:16.6206358)
Percent complete: 33 (00:00:16.7356359)
Percent complete: 34 (00:00:16.8256360)
Percent complete: 35 (00:00:16.9156362)
Percent complete: 36 (00:00:17.0306363)
Percent complete: 37 (00:00:17.1506365)
Percent complete: 38 (00:00:17.2606367)
Percent complete: 39 (00:00:17.3756368)
Percent complete: 40 (00:00:17.5856371)
Percent complete: 41 (00:00:17.7356373)
Percent complete: 42 (00:00:17.9056376)
Percent complete: 43 (00:00:18.0256377)
Percent complete: 44 (00:00:18.1366405)
Percent complete: 45 (00:00:18.2216406)
Percent complete: 46 (00:00:18.3216408)
Percent complete: 47 (00:00:18.4166409)
Percent complete: 48 (00:00:18.5066410)
Percent complete: 49 (00:00:18.6116412)
Percent complete: 50 (00:00:18.7166413)
Percent complete: 51 (00:00:18.8266415)
Percent complete: 52 (00:00:18.9316416)
Percent complete: 53 (00:00:19.0716418)
Percent complete: 54 (00:00:19.1966420)
Percent complete: 55 (00:00:19.2966421)
Percent complete: 56 (00:00:19.3766423)
Percent complete: 57 (00:00:19.4616424)
Percent complete: 58 (00:00:19.5441441)
Percent complete: 59 (00:00:19.6426453)
Percent complete: 60 (00:00:19.7526454)
Percent complete: 61 (00:00:19.8476455)
Percent complete: 62 (00:00:19.9226457)
Percent complete: 63 (00:00:20.0026458)
Percent complete: 64 (00:00:20.0676459)
Percent complete: 65 (00:00:20.1626460)
Percent complete: 66 (00:00:20.2626461)
Percent complete: 67 (00:00:20.3626463)
Percent complete: 68 (00:00:20.4576464)
Percent complete: 69 (00:00:20.5676466)
Percent complete: 70 (00:00:20.6826467)
Percent complete: 71 (00:00:20.7776468)
Percent complete: 72 (00:00:20.8626470)
Percent complete: 73 (00:00:20.9526471)
Percent complete: 74 (00:00:21.0326472)
Percent complete: 75 (00:00:21.1076473)
Percent complete: 76 (00:00:21.1976474)
Percent complete: 77 (00:00:21.2776475)
Percent complete: 78 (00:00:21.3626477)
Percent complete: 79 (00:00:21.4476478)
Percent complete: 80 (00:00:21.5276479)
Percent complete: 81 (00:00:21.6076480)
Percent complete: 82 (00:00:21.6876481)
Percent complete: 83 (00:00:21.7726482)
Percent complete: 84 (00:00:21.8226483)
Percent complete: 85 (00:00:21.8876484)
Percent complete: 86 (00:00:21.9876485)
Percent complete: 87 (00:00:22.0626486)
Percent complete: 88 (00:00:22.1226487)
Percent complete: 89 (00:00:22.1876488)
Percent complete: 90 (00:00:22.2626489)
Percent complete: 91 (00:00:22.3026490)
Percent complete: 92 (00:00:22.3726491)
Percent complete: 93 (00:00:22.4376492)
Percent complete: 94 (00:00:22.4926493)
Percent complete: 95 (00:00:22.5676494)
Percent complete: 96 (00:00:22.6126494)
Percent complete: 97 (00:00:22.6926495)
Percent complete: 98 (00:00:22.7776496)
Percent complete: 99 (00:00:22.8426497)
Percent complete: 100 (00:00:22.9176498)

有没有人对我如何解决这个问题或进一步调查导致它的原因有任何建议？

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1 回答 1

4

Powershell 对异步事件的支持相当差。当事件被触发时，它们会进入一些 Powershell 事件队列，然后在它们可用时将其提供给处理程序。我之前已经看到过各种性能或功能问题，处理程序似乎需要等待控制台在执行之前变为空闲状态。有关示例，请参见此问题。

据我所知，您的代码设置得很好并且“应该”工作。我认为它很慢只是因为 powershell 不能很好地支持这种模式。

这是一种变通方法，它恢复为纯 .NET C# 代码来处理所有事件内容，从而完全避免了 Powershell 事件队列。

# helper for handling events
Add-Type -TypeDef @"
  using System;
  using System.Text;
  using System.Net;
  using System.IO;

  public class Downloader
  {
      private Uri source;
      private string destination;
      private string log;
      private object syncRoot = new object();
      private int percent = 0;

      public Downloader(string source, string destination, string log)
      {
          this.source = new Uri(source);
          this.destination = destination;
          this.log = log;
      }

      public void Download()
      {
          WebClient wc = new WebClient();
          wc.DownloadProgressChanged += new DownloadProgressChangedEventHandler(OnProgressChanged);
          wc.DownloadFileAsync(source, destination);
      }

      private void OnProgressChanged(object sender, DownloadProgressChangedEventArgs e)
      {
          lock (this.syncRoot)
          {
              if (e.ProgressPercentage > this.percent)
              {
                  this.percent = e.ProgressPercentage;
                  string message = String.Format("{0}: {1} percent", DateTime.Now, this.percent);
                  File.AppendAllLines(this.log, new string[1] { message }, Encoding.ASCII);
              }
          }
      }
  }
"@


$source = 'https://www.kernel.org/pub/linux/kernel/v2.6/linux-2.6.18.tar.bz2'
$dest = "$env:USERPROFILE\Downloads\linux-2.6.18.tar.bz2"
$log = [io.path]::GetTempFileName()

$downloader = new-object Downloader $source,$dest,$log
$downloader.Download();

gc $log -tail 1 -wait `
 |?{ $_ -match ': (\d+) percent' } `
 |%{ 
     $percent = [int]$matches[1]
     if($percent -lt 100)
     {
         Write-Progress -Activity "Downloading $source" -Status "${percent}% complete" -PercentComplete $percent
     }
     else{ break }
 }
于 2013-08-09T19:41:57.027 回答