Make a Powershell respond to Register-ObjectEvent events in script mode - powershell

Question

I have a simple Powershell script that I wrote in the Powershell ISE. The gist of it is that it watches a named pipe for a write as a signal to perform an action, while at the same time monitoring its boss process. When the boss-process exits, the script exits as well. Simple.
After struggling to get the named pipe working in Powershell without crashing, I managed to get working code, which is shown below. However, while this functions great in the Powershell ISE and interactive terminals, I've been hopeless in getting this to work as a standalone script.
$bosspid = 16320
# Create the named pipe
$pipe = new-object System.IO.Pipes.NamedPipeServerStream(
-join('named-pipe-',$bosspid),
[System.IO.Pipes.PipeDirection]::InOut,
1,
[System.IO.Pipes.PipeTransmissionMode]::Byte,
[System.IO.Pipes.PipeOptions]::Asynchronous
)
# If we don't do it this way, Powershell crashes
# Brazenly stolen from github.com/Tadas/PSNamedPipes
Add-Type #"
using System;
public sealed class CallbackEventBridge
{
public event AsyncCallback CallbackComplete = delegate {};
private void CallbackInternal(IAsyncResult result)
{
CallbackComplete(result);
}
public AsyncCallback Callback
{
get { return new AsyncCallback(CallbackInternal); }
}
}
"#
$cbbridge = New-Object CallBackEventBridge
Register-ObjectEvent -InputObject $cbbridge -EventName CallBackComplete -Action {
param($asyncResult)
$pipe.EndWaitForConnection($asyncResult)
$pipe.Disconnect()
$pipe.BeginWaitForConnection($cbbridge.Callback, 1)
Host-Write('The named pipe has been written to!')
}
# Make sure to close when boss closes
$bossproc = Get-Process -pid $bosspid -ErrorAction SilentlyContinue
$exitsequence = {
$pipe.Dispose()
[Environment]::Exit(0)
}
if (-Not $bossproc) {$exitsequence.Invoke()}
Register-ObjectEvent $bossproc -EventName Exited -Action {$exitsequence.Invoke()}
# Begin watching for events until boss closes
$pipe.BeginWaitForConnection($cbbridge.Callback, 1)
The first problem is that the script terminates before doing anything meaningful. But delaying end of execution with such tricks like while($true) loops, the -NoExit flag, pause command, or even specific commands which seem made for the purpose, like Wait-Event, will cause the process to stay open, but still won't make it respond to the events.

Answer 1

I gave up on doing it the "proper" way and have instead reverted to using synchronous code wrapped in while-true blocks and Job control.
$bosspid = (get-process -name notepad).id
# Construct the named pipe's name
$pipename = -join('named-pipe-',$bosspid)
$fullpipename = -join("\\.\pipe\", $pipename) # fix SO highlighting: "
# This will run in a separate thread
$asyncloop = {
param($pipename, $bosspid)
# Create the named pipe
$pipe = new-object System.IO.Pipes.NamedPipeServerStream($pipename)
# The core loop
while($true) {
$pipe.WaitForConnection()
# The specific signal I'm using to let the loop continue is
# echo m > %pipename%
# in CMD. Powershell's echo will *not* work. Anything other than m
# will trigger the exit condition.
if ($pipe.ReadByte() -ne 109) {
break
}
$pipe.Disconnect()
# (The action this loop is supposed to perform on trigger goes here)
}
$pipe.Dispose()
}
# Set up the exit sequence
$bossproc = Get-Process -pid $bosspid -ErrorAction SilentlyContinue
$exitsequence = {
# While PS's echo doesn't work for passing messages, it does
# open and close the pipe which is enough to trigger the exit condition.
&{echo q > $fullpipename} 2> $null
[Environment]::Exit(0)
}
if ((-Not $bossproc) -or $bossproc.HasExited) { $exitsequence.Invoke() }
# Begin watching for events until boss closes
Start-Job -ScriptBlock $asyncloop -Name "miniloop" -ArgumentList $pipename,$bosspid
while($true) {
Start-Sleep 1
if ($bossproc.HasExited) { $exitsequence.Invoke() }
}
This code works just fine now and does the job I need.