"Real" streaming in PowerShell? - powershell

Question

Is there any way to achieve real streaming in PowerShell?
The following statement illustrates what I want:
$(Write-Output "one"; Start-Sleep -Seconds 3; Write-Output "two")
What I'd expect is:
one
<three seconds delay>
two
What actually happens is this:
<three seconds delay>
one
two
So what PowerShell calls "streams" are in fact just lists, but is there some easy way to pass real streams around in PowerShell?
Addition:
I just found out that this does what I'm looking for:
function test {
Write-Output "one";
Start-Sleep -Seconds 3;
Write-Output "two";
}
test | % { Write-Host $_ }
...but why does $( ... ) not behave the same way?

Answer 1

$() executes every statement inside it before passing it's output. You can use ScriptBlocks {} to have output streamed.
To invoke the ScriptBlock in the current scope, use the dot operator.
. {Write-Output "one"; Start-Sleep -Seconds 3; Write-Output "two"}
To invoke it in a child scope, use the & call operator.
& {Write-Output "one"; Start-Sleep -Seconds 3; Write-Output "two"}
You can assign a scriptblock to a variable to pass it around.
function Show-ExecutingScript ($ScriptBlock) {
Write-Host "Executing ScriptBlock: `n$ScriptBlock" -Fore Yellow
& $ScriptBlock
}
$MyCommands = {Write-Output "one"; Start-Sleep -Seconds 3; Write-Output "two"}
Show-ExecutingScript $MyCommands
To learn more about ScriptBlocks, check out about_Script_Blocks