Update Textbox if var changes - powershell

Question

I want to update my WPF PowerShell GUI. I set up a small GUI with one button and one textbox to test my script. It looks like:
[void][System.Reflection.Assembly]::LoadWithPartialName('presentationframework')
Add-Type -AssemblyName System.Windows.Forms
[xml]$XAML = #"
<Window
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="test" Height="630" Width="500" ResizeMode="NoResize" WindowStartupLocation="CenterScreen">
<Grid>
<StackPanel>
<Button Name="test" Content="Button" HorizontalAlignment="Left" VerticalAlignment="Top" Width="75"/>
<TextBox Name="write" HorizontalAlignment="Left" Height="23" TextWrapping="Wrap" Text="TextBox" VerticalAlignment="Top" Width="120"/>
</StackPanel>
</Grid>
</Window>
"#
$reader = (New-Object System.Xml.XmlNodeReader $xaml)
try {
$Form = [Windows.Markup.XamlReader]::Load($reader)
} catch {
Write-Host "Something is wrong"; exit
}
$xaml.SelectNodes("//*[#Name]") | %{
Set-Variable -Name ($_.Name) -Value $Form.FindName($_.Name)
}
$test.Add_Click({
Write-Host $varvar
Start-Process powershell -Argument ".\varchecker.ps1"
})
$Form.ShowDialog() | Out-Null
By clicking the button I start another script where I read a variable. This looks like:
while ($true) {
$global:varvar = Get-Content var.txt
Write-Host $varvar
}
Now I want to print the var from the started script to the textbox in the other script. Is this possible? Or the better question: is that a good way to do that?

Answer 1

I don't think you can do it this way. You are trying to start a new PowerShell process, getting some data and passing it to the parent process. I think you can sort of do it in C with pipe() and fork() but havent found a way to do it in PowerShell yet.
You can however, use Start-Job, which creates a process in the background. Alternatively, you can look at Runspaces too for multithreading.
You can also use Invoke-Expression to call the script.
All you have to do from the script is return $Varvar.
If you can tell us what problem you are trying to solve with this method, maybe folks can be more helpful.

Answer 2

Getting values from a new PowerShell process inside your GUI is much too complicated if all you want is to read the contents of a file.
Why not do it all from the GUI code you have?
# [System.Reflection.Assembly]::LoadWithPartialName is deprecated.
Add-Type -AssemblyName PresentationFramework
[xml]$xaml = #"
<Window
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="test" Height="630" Width="500" ResizeMode="NoResize" WindowStartupLocation="CenterScreen">
<Grid>
<StackPanel>
<Button Name="test" Content="Button" HorizontalAlignment="Left" VerticalAlignment="Top" Width="75"/>
<TextBox Name="write" HorizontalAlignment="Left" Height="23" TextWrapping="Wrap" Text="TextBox" VerticalAlignment="Top" Width="120"/>
</StackPanel>
</Grid>
</Window>
"#
$reader = New-Object System.Xml.XmlNodeReader $xaml
try {
$Form = [Windows.Markup.XamlReader]::Load($reader)
}
catch {
Write-Host "Something is wrong"; exit
}
# get references for the button and textbox objects into PowerShell variables.
$button = $Form.FindName("test")
$textBox = $Form.FindName("write")
$button.Add_Click({
# don't start a new PowerShell process, but simply read the file here
$textBox.Text = $var = Get-Content -Path 'PATH TO var.txt' -Raw
Write-Host $var
})
$Form.ShowDialog() | Out-Null
# clean-up
$reader.Dispose()