ParallelModule.psm1

Function CallbackWrapper {
    param (
        [string]$callbackScript,
        [object]$param,
        [int]$order
    )
    $callback = [ScriptBlock]::Create($callbackScript)
    return [PSCustomObject]@{
        Result = $callback.InvokeReturnAsIs($param)
        Order = $order
    }
}

Function Invoke-Parallel {
    param(
        [Parameter(Mandatory = $true)][Collections.IEnumerable]$WorkQueue,
        [Parameter(Mandatory = $true)][ScriptBlock]$ScriptBlock,
        [uint16]$MaxThreads = [uint16]$env:NUMBER_OF_PROCESSORS + 1
    )
    $t0 = [DateTime]::UtcNow
    Write-Verbose "Initialize pool of $MaxThreads"
    $pool = [RunspaceFactory]::CreateRunspacePool(1, $MaxThreads)
    $pool.ApartmentState = "STA"
    $pool.Open()
    Write-Verbose "Queue up work"
    try {
        $i = 0
        $state = $WorkQueue | ForEach-Object {
            $runspace = [PowerShell]::Create()
            $runspace.RunspacePool = $pool   
            $runspace.AddScript(${function:CallbackWrapper}) | Out-Null
            $runspace.AddParameters(@("${ScriptBlock}", $_, $i++)) | Out-Null
            $asyncResult = $runspace.BeginInvoke() # here we go
            return [PSCustomObject]@{ 
                Runspace = $runspace
                AsyncResult = $asyncResult
            }
        }
        Write-Verbose "Wait for $($state.Length) results"
        $results = $state | ForEach-Object {
            $result = $_.RunSpace.EndInvoke($_.AsyncResult) # inherently awaits completion
            $_.RunSpace.Dispose()
            return $result
        }
    }
    finally {
        Write-Verbose "Clean up"
        $pool.Close()
        $pool.Dispose()
    }
    Write-Verbose "Sort and unwrap"
    $results `
    | Sort-Object -Property Order `
    | ForEach-Object { $_.Result }
    Write-Verbose "Done in $(([DateTime]::UtcNow - $t0).TotalSeconds)s"
}

# Invoke-Parallel @(1,1,1,1,2,2,4,6,8) { param ($x) Start-Sleep $x }

Export-ModuleMember Invoke-Parallel