Update a WPF GUI using Powershell Jobs

Question

I've been trying to create responsive GUIs for my personal Powershell scripts. I've come up with a problem that is highly discussed online: Freezing GUI (since Powershell is single threaded).

Similar to this problem, but my case is specific to Powershell. I successfully implemented a Powershell based solution for creating GUIs relying on XAML form. Now, let's consider this code:

#EVENT Handler
$Somebutton.add_Click({
    $SomeLabel.Content = "Calculating..." 

    Start-Job -ScriptBlock {
        #Computation that takes time
        #...
        $SomeLabel.Content = "Calculated value" 
    }
})

#Show XAML GUI
$xamlGUI.ShowDialog() | out-null

xamlGUI is the form itself and $Somebutton/$SomeLabel are controls I was able to read from xaml and transform to Powershell variables.

I'm trying to understand why the Job that I start is not updating my label when the computation is done. It actually does nothing.

Im new to Powershell jobs and I'm wondering if there is something I'm missing.

Answer 1

Here's a little boilerplate I use for reactive WPF forms in PowerShell:

# Hide yo console
$SW_HIDE, $SW_SHOW = 0, 5
$TypeDef = '[DllImport("User32.dll")]public static extern bool ShowWindow(IntPtr hWnd, int nCmdShow);'
Add-Type -MemberDefinition $TypeDef -Namespace Win32 -Name Functions
$hWnd = (Get-Process -Id $PID).MainWindowHandle
$Null = [Win32.Functions]::ShowWindow($hWnd,$SW_HIDE)

# Define your app + form
Add-Type -AssemblyName PresentationFramework
$App = [Windows.Application]::new() # or New-Object -TypeName Windows.Application
$Form = [Windows.Markup.XamlReader]::Load(
    [Xml.XmlNodeReader]::new([xml]@'
WPF form definition goes here
'@)
)
# or ::Load((New-Object -TypeName Xml.XmlNodeReader -ArgumentList ([xml]@'
#wpfdef
#'@))
#)

# Fixes the "freeze" problem
function Update-Gui {
    # Basically WinForms Application.DoEvents()
    $App.Dispatcher.Invoke([Windows.Threading.DispatcherPriority]::Background, [action]{})
}

# Event handlers go here
$Form.add_Closing({
    $Form.Close()
    $App.Shutdown()
    Stop-Process -Id $PID # or return your console: [Win32.Functions]::ShowWindow($hWnd,$SW_SHOW)
})

# Finally
$App.Run($Form)

Remember to clean up when your app is shutting down:

$Form.Close()
$App.Shutdown()
Stop-Process -Id $PID

Whenever you need your changes to the GUI to be reflected, call the Update-Gui function.