How to call a powershell function within the script from Start-Job?

Background jobs are run in a seperate process, so the jobs you create with Start-Job can not interact with functions unless you include them in the $scriptblock.

Even if you included the function in the $scripblock, Write-Host would not output it's content to the console until you used Get-Job | Receive-Job to recieve the jobs result.

EDIT The problem is that your DisplayMessage function is in a local script-scope while your eventhandler runs in a different parent scope(like global which is the session scope), so it can't find your function. If you create the function in the global scope and call it from the global scope, it will work.

I've modified your script to do this now. I've also modified to scriptblock and unregistered the events when the script is done so you won't get 10x messages when you run the script multiple times :-)

Untitled1.ps1

function DoWork
{
  $event = Register-EngineEvent -SourceIdentifier NewMessage -Action {
      global:DisplayMessage $event.MessageData
  }

  $scriptBlock =  {

    Register-EngineEvent -SourceIdentifier NewMessage -Forward

    $message = "Starting work $args"
    $null = New-Event -SourceIdentifier NewMessage -MessageData $message

    ### DO SOME WORK HERE ###

    $message = "Ending work $args"
    $null = New-Event -SourceIdentifier NewMessage -MessageData $message

    Unregister-Event -SourceIdentifier NewMessage
  }

  DisplayMessage("Processing Starts")

  $array = @(1,2,3)
  foreach ($a in $array)
  {
      Start-Job -Name "DoActualWork" $ScriptBlock -ArgumentList $a | Out-Null
  }

  #$jobs = Get-Job -Name "DoActualWork"
  While (Get-Job -Name "DoActualWork" | where { $_.State -eq "Running" } )
  {
      Start-Sleep 1
  }

  DisplayMessage("Processing Ends")

  #Get-Job -Name "DoActualWork" | Receive-Job
}

function global:DisplayMessage([string]$message)
{
    Write-Host $message -ForegroundColor Red
}

DoWork

Get-EventSubscriber | Unregister-Event

Test

PS > .\Untitled1.ps1
Processing Starts
Starting work 1
Starting work 2
Ending work 1
Ending work 2
Starting work 3
Ending work 3
Processing Ends

An easy way to include local functions in a background job:

$init=[scriptblock]::create(@"
function DoWork {$function:DoWork}
"@)

Start-Job -Name "DoActualWork" $ScriptBlock -ArgumentList $array -InitializationScript $init | Out-Null