PowerShell. Can't get variable from {}

Question

In the code below there is a parameter named "$action" = {...}. Inside the brackets there are some variables ($path, $logPath, etc.) How can I access variables from this parameter outside of the braces "{}"? I need result of one of my functions("RunFunctions") for a while loop. Or if it's some stupid question please can you point me to the place where I can find some information about this {} notation? Thanks.

try {
    ### SET FOLDER TO WATCH + FILES TO WATCH + SUBFOLDERS YES/NO
    $watcher = New-Object System.IO.FileSystemWatcher
    $watcher.Path = "C:\Testers\ResetOptimisation\tracking"
    $watcher.Filter = "user_name*.*"
    $watcher.IncludeSubdirectories = $true
    $watcher.EnableRaisingEvents = $true

    ### DEFINE ACTIONS AFTER AN EVENT IS DETECTED
    $action = { $path = $Event.SourceEventArgs.FullPath
                $logPath = "C:\Testers\ResetOptimisation\output\log.txt"
                $changeType = $Event.SourceEventArgs.ChangeType
                $logline = "$(Get-Date), $changeType, $path"
                Add-content $logPath -value $logline
                $terminateFlag = RunFunctions $path $changeType $logPath
            }

    ### DECIDE WHICH EVENTS SHOULD BE WATCHED
    Register-ObjectEvent $watcher "Created" -Action $action
    Register-ObjectEvent $watcher "Changed" -Action $action
    Register-ObjectEvent $watcher "Deleted" -Action $action
    Register-ObjectEvent $watcher "Renamed" -Action $action
    while ($true) {
        Write-Host $teminateFlag
        if ($teminateFlag -eq 1) {
            Exit
        }
        Start-Sleep 3
    }
}
catch {
    Write-Host $_
}

Answer 1

{ ... } is a script block (literal) - a reusable piece of PowerShell code that you can execute on demand (like a function pointer or delegate in other languages).

By passing such a block, stored in variable $action, to Register-ObjectEvent -Action, PowerShell invokes it whenever the event of interest fires, and does so in a dynamic module, whose scope is entirely separate from the caller's.

Therefore, your calling code doesn't see the variables created inside the block, as they are local to that block.

^{For more information about scopes in PowerShell, see the bottom section of this answer.}

While PowerShell generally lets you create and modify variables in other scopes as well, if you take explicit action, this is not an option with Register-ObjectEvent -Action, because a dynamic module's scope fundamentally doesn't have access to the caller's scope, only to the global scope.

# !! This works, but is ill-advised.
$global:terminateFlag = RunFunctions $path $changeType $logPath

However, using the global scope is best avoided, because global variables linger even after the script exits (they are session-global).

The better solution is to:

• make the action script block output a value for the caller.

• have the caller receive that output via Receive-Job, using the event job that
  Register-ObjectEvent -Action returns.

Here's a simplified, self-contained example that demonstrates the technique:

It sets up a watcher, attaches an event handler, and creates a file that triggers a watcher event.

try {

  # Specify the target folder: the system's temp folder in this example.
  $dir = (Get-Item -EA Ignore temp:).FullName; if (-not $dir) { $dir = $env:TEMP }

  # Create and initialize the watcher.
  $watcher = [System.IO.FileSystemWatcher] @{
    Filter                = '*.tmp'
    Path                  = $dir
  }

  # Define the action script block (event handler).
  $action = {
    # Print the event data to the host.
    Write-Host "Event raised:`n$($EventArgs | Format-List | Out-String)"
    $terminateFlag = $true
    # *Return* a value that indicates whether watching should be stopped.
    # This value is later retrieved via Receive-Job.
    return $terminateFlag
  }

  # Subscribe to the watcher's Created events, which returns an event job.
  # This indefinitely running job receives the output from the -Action script
  # block whenever the latter is called after an event fires.
  $eventJob = Register-ObjectEvent $watcher Created -Action $action

  # Start watching:
  # Note: Not strictly necessary, because, curiously, 
  #       Register-ObjectEvent has aleady done this for us.
  $watcher.EnableRaisingEvents = $true 

  # Using an aux. background job, create a sample file that will trigger the 
  # watcher, after a delay.
  $tempFile = Join-Path $dir "$PID.tmp"
  $auxJob = Start-Job { Start-Sleep 3; 'hi' > $using:tempFile }

  Write-Host "Watching $dir for creation of $($watcher.Filter) files..."

  # Wait in a loop until the action block is run in response to an event and
  # produces $true as output to signal the intent to terminate, via Receive-Job.
  while ($true -ne (Receive-Job $eventJob)) {
    write-host . -NoNewline
    Start-Sleep -Milliseconds 500  # sleep a little
  }

}
finally {
  # Clean up.
  # Dispose of the watcher.
  $watcher.Dispose() 
  # Remove the event job (and with it the event subscription).
  $eventJob | Remove-Job -Force 
  # Clean up the helper job.
  Remove-Job -ea Ignore -Force $auxJob 
  # Remove the temp. file
  Remove-Item -ea Ignore $tempFile
}

Answer 2

In this code, my script block "$action" is capable to overwrite my global parameter "terminateFlag" initialized in the main script. Many thanks for @mklement0 and link which he provided Click

try {
    ### SET FOLDER TO WATCH + FILES TO WATCH + SUBFOLDERS YES/NO
    $watcher = New-Object System.IO.FileSystemWatcher
    $watcher.Path = "C:\Testers\ResetOptimisation\tracking"
    $watcher.Filter = "user_name*.*"
    $watcher.IncludeSubdirectories = $true
    $watcher.EnableRaisingEvents = $true
    $global:terminateFlag = 0

    ### DEFINE ACTIONS AFTER AN EVENT IS DETECTED
    $action = { $path = $Event.SourceEventArgs.FullPath
                $logPath = "C:\Testers\ResetOptimisation\output\log.txt"
                $changeType = $Event.SourceEventArgs.ChangeType
                $logline = "$(Get-Date), $changeType, $path"
                Add-content $logPath -value $logline
                $global:terminateFlag = RunFunctions $path $changeType $logPath
            }

    ### DECIDE WHICH EVENTS SHOULD BE WATCHED
    Register-ObjectEvent $watcher "Created" -Action $action

    while ($true) {
        Write-Host "waiting for file..."
        if ($terminateFlag -eq 1) {
            Exit
        }
        Start-Sleep 10
    }
}
catch {
    Write-Host $_
}