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.

Once you start a job it is running in a completely different context. It is not aware of the objects that you have in memory in the context that the form is running in. In theory I suspect that you should be able to pass the button to the job. — EBGreen, May 07 '18 at 15:25
Thanks for answer, I would pass the label using: `-ArgumentList $SomeLabel` ? — scharette, May 07 '18 at 15:48
Most people who go for concurrency with PowerShell utilize runspaces over jobs since you can communicate with them more easily. Also, I'd suggest moving away from `ShowDialog()` in favor of `$App = [Windows.Application]::new(); $App.Run($Form)` — Maximilian Burszley, May 07 '18 at 15:49
You need to accept it as a parameter in the scriptblock too. — EBGreen, May 07 '18 at 15:49
Something I've used to manually update WPF forms in powershell: `$App.Dispatcher.Invoke([Windows.Threading.DispatcherPriority]::Background, [action]{})` — Maximilian Burszley, May 07 '18 at 15:50
Also, @TheIncorrigible1 has a better solution. I'm not even positive that my idea will work. It was just the first thing that came to mind. — EBGreen, May 07 '18 at 15:50
@TheIncorrigible1 Method invocation failed because [System.Windows.Application] doesn't contain a method named 'new'. Is there something I'm doing wrong ? — scharette, May 07 '18 at 15:57
@scharette That's the PowerShell 5 way of doing it. You have to do `New-Object -TypeName Windows.Application` — Maximilian Burszley, May 07 '18 at 15:59
@scharette I updated my answer with a boilerplate I use, alongside pre-v5 methods of accomplishing what I do. — Maximilian Burszley, May 07 '18 at 16:10

Maximilian Burszley · Accepted Answer · 2018-05-07T16:17:09.607

4

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.

edited May 07 '18 at 16:17

answered May 07 '18 at 15:58

Maximilian Burszley

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Thanks a lot for your answer! Can't believe this was not on SO yet... I will probably go for a c# based solution though. Working with multi-threading in c# seems so much easier and is in my opinion worth learning it for easier runspaces handling. I can then call my scripts in c# controls handlers like you proposed on another of my thread. If you have any comments on that too it would be appreciated. Thanks agian. – scharette May 07 '18 at 17:09
2

@scharette C# is definitely the way to go for anything more than the most basic forms. – Maximilian Burszley May 07 '18 at 17:10
Just because i'm curious, isn't it a comparable to `Application.DoEvents()` ? It is often seen as the devil by the community (see [this](https://blog.codinghorror.com/is-doevents-evil/)) – scharette May 08 '18 at 12:02
@scharette Yes, it is comparable (and I left a comment in my code as such). It's seen as the devil in C# apps because that's not how you're supposed to design a wpf app, but in PowerShell, it requires significantly more effort to do the same things as C# so I use it as a shortcut – Maximilian Burszley May 08 '18 at 13:59
I see. Thanks for the insight ! – scharette May 08 '18 at 14:10
Hi, since on SO there is no "private messaging" I have to ask here. You where talking you will work in C#. What do you mean exactly by that? I throught WPF is C#? I'm asking because I want to build a Tool that does many things with AD users using Powershell and WPF as Gui. – GeraltDieSocke Jul 05 '18 at 09:28
@GeraltDieSocke If you want to learn about creating GUIs in PowerShell with WPF, I recommend [reading this article](https://foxdeploy.com/2015/04/10/part-i-creating-powershell-guis-in-minutes-using-visual-studio-a-new-hope/). – Maximilian Burszley Jul 06 '18 at 15:08

Update a WPF GUI using Powershell Jobs

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