VSCode and Powershell: How to debug and trace into code called by Register-ObjectEvent?

Question

Am I missing something?

I can start the debug process with F5, but I cannot end it, and I cannot step through code or do normal debugging.

I assume this is due to the fact that the code is hanging off Register-ObjectEvent ?

(Watching a file system event....)

What is the method to run this code and keep the debugger attached to what is going on?

The code:

$folder_to_watch = 'C:\Users\demouser\Downloads\'
$file_name_filter = '*.aac'
# to archive .aac files
$destination = 'c:\temp\test\arc\'  
$DestinationDirMP3 = 'C:\data\personal\hinative-mp3'
$Watcher = New-Object IO.FileSystemWatcher $folder_to_watch, $file_name_filter -Property @{ 
    IncludeSubdirectories = $false
    NotifyFilter = [IO.NotifyFilters]'FileName, LastWrite'
}
$VLCExe = 'C:\Program Files\VideoLAN\VLC\vlc.exe' 

$onCreated = Register-ObjectEvent $Watcher -EventName Created -SourceIdentifier FileCreated -Action {
   $path = $Event.SourceEventArgs.FullPath
   $name = $Event.SourceEventArgs.Name
   $changeType = $Event.SourceEventArgs.ChangeType
   $timeStamp = $Event.TimeGenerated
   Write-Host "The file '$name' was $changeType at $timeStamp"
   Write-Host $path

   # File Checks
    while (Test-LockedFile $path) {
      Start-Sleep -Seconds .2
    }
    # Move File
    Write-Host "moving $path to $destination"
    Move-Item $path -Destination $destination -Force -Verbose
    # build the path to the archived .aac file
    $SourceFileName = Split-Path $path -Leaf
    $DestinationAACwoQuotes = Join-Path $destination $SourceFileName
    $DestinationAAC = "`"$DestinationAACwoQuotes`""
    $MP3FileName = [System.IO.Path]::ChangeExtension($SourceFileName,".mp3")
    $DestinationMP3woQuotes = Join-Path $DestinationDirMP3 $MP3FileName
    $DestinationMP3 = "`"$DestinationMP3woQuotes`""
    $VLCArgs = "-I dummy -vvv $DestinationAAC --sout=#transcode{acodec=mp3,ab=48,channels=2,samplerate=32000}:standard{access=file,mux=ts,dst=$DestinationMP3} vlc://quit"
    Write-Host "args $VLCArgs"
    Start-Process -FilePath $VLCExe -ArgumentList $VLCArgs
    

}


function Test-LockedFile {
    param ([parameter(Mandatory=$true)][string]$Path)  
    $oFile = New-Object System.IO.FileInfo $Path
    if ((Test-Path -Path $Path) -eq $false)
    {
      return $false
    }
  
    try
    {
      $oStream = $oFile.Open([System.IO.FileMode]::Open, [System.IO.FileAccess]::ReadWrite, [System.IO.FileShare]::None)
        if ($oStream)
        {
          $oStream.Close()
        }
        $false
    }
    catch
    {
      # file is locked by a process.
      return $true
    }
  }

Answer 1

From the official documentation of Register-ObjectEvent (notes section)

Events, event subscriptions, and the event queue exist only in the current session. If you close the current session, the event queue is discarded and the event subscription is canceled.

Everything above belong to your session, which is terminated when the process exit. Even if it was somewhere else, your .net FileSystemWatcher is part of that process and is terminated as soon your session exit.

Now, when you debug through VSCode / ISE, your session is created beforehand and does not terminate after you exit the script, which allow you to evaluate variable of the last execution. It also mean your subscriptions and event callbacks and the associated .net objects remain in memory and active at this time.

That being said, your debugger also detached at the moment your script exited. It also mean that if you were not debugging and just running the script, your session would exit immediately after the script execution and thus, your listener, events callback and everything else would immediately exit without a chance to process anything.

To keep the debugger attached, be able to debug and also to have the script working in a normal context at all, you need to somewhat keep the process alive.

The usual way to ensure that is to add a loop at the end of the script.

# At the end of the script.
while ($true) {
    Start-Sleep -Seconds 1
}

Note that events are raised to the main thread, which mean that if your script is sleeping, it won't be processed immediately. Therefore, in the example above, if 10 events were to occurs within the 1 second period, they would all get processed at the same time, when the thread stop sleeping.

Debugging note: To deal with event already registered error during debugging Register-ObjectEvent : Cannot subscribe to the specified event. A subscriber with the source identifier 'FileCreated' already exists.., you can add cleanup code in the Finally part of a Try / Catch / Finally block.

try {
    # At the end of the script... 
    while ($true) {
        Start-Sleep -Seconds 1
    }    
}
catch {}
Finally {
    # Work with CTRL + C exit too !
    Unregister-Event -SourceIdentifier FileCreated 
}

References:

Register-ObjectEvent