我正在编写一个PowerShell脚本，需要在继续执行其他任务之前从远程Web服务器下载几个大文件。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。
A response to another SO question contained a solution which uses registered events and Net.WebClient.DownloadFileAsync。但是，当我在接受的答案中尝试解决方案时，在我的脚本处理完所有DownloadProgressChanged事件之前很久就完成了下载。
我已经创建了一个测试脚本，我在下面提供了该脚本，以便重现问题并尝试隔离它发生的位置。起初我认为问题是Write-Progress cmdlet很慢，所以我用Write-Host替换它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线。考虑到Write-Host可能也很慢，我将事件处理程序的操作更改为仅在更改时输出百分比。这没有什么区别。
我在输出中注意到下载完成之前，事件处理程序表明它已经完成了几个百分点。在前几个百分点出现了很大的延迟，然后加速了。但即使下载完成，完成百分比的输出仍然很慢。我对脚本进行了另一次修改，以显示每次更改百分比所经过的时间。
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相同结果的脚本。我没有使用PS配置文件，Server 2008计算机是全新安装。
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
}
该脚本生成以下输出。
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)
是否有人就如何解决此问题或进一步调查导致问题的原因提出任何建议？
答案 0 :(得分：2)
Powershell支持异步事件。当事件被触发时，它们进入一些Powershell事件队列，然后在它们可用时将其提供给处理程序。我之前已经看到了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，请参阅this question。
据我所知，您的代码设置良好且“应该”正常工作。我觉得它很慢，因为PowerShell不能很好地支持这种模式。
这是一种解决方法，它恢复为纯.NET C＃代码来处理所有事件，这完全避免了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 }
}