XAML Island doesn't respond to Windows app themes changes (Dark, Light) at run-time

[Palatis]

Describe the bug

I have no idea if this issue should go here (or to Microsoft.Toolkit.Win32), but since the source code is here, I post it here.

I'm using XamlIsland in WPF to host my UWP custom control.
However it occasionally crash with InvalidOperationException says something like "Cannot create new view because main window is not created." (the actual message is "無法建立新的檢視，因為尚未建立主視窗", this is just a translation.)
with a stacktrace:

   at Windows.ApplicationModel.Core.CoreApplication.get_MainView()
   at Microsoft.Toolkit.Uwp.UI.Helpers.ThemeListener.Settings_ColorValuesChanged(UISettings sender, Object args) in D:\a\1\s\Microsoft.Toolkit.Uwp.UI\Helpers\ThemeListener.cs:line 81
   at System.Threading.Tasks.Task.<>c.<ThrowAsync>b__139_1(Object state)
   at System.Threading.QueueUserWorkItemCallbackDefaultContext.Execute()
   at System.Threading.ThreadPoolWorkQueue.Dispatch()
   at System.Threading._ThreadPoolWaitCallback.PerformWaitCallback()

I haven't switch the theme, tho... I just leave it there running for several minute and it crashes.

• Is this bug a regression in the toolkit? If so, what toolkit version did you last see it work:

Steps to Reproduce

Steps to reproduce the behavior:

1. follow the WPF XamlIsland tutorial on Microsoft website
2. run the program and wait

Expected behavior

no problem

Screenshots

probably don't need one...

Environment

NuGet Package(s):

<ItemGroup>
  <PackageReference Include="Microsoft.Toolkit.UI.XamlHost" Version="6.0.0" />
  <PackageReference Include="Microsoft.Toolkit.Wpf.UI.XamlHost" Version="6.0.0" />
</ItemGroup>

Package Version(s): 6.0.0

Windows 10 Build Number:

• Fall Creators Update (16299)
• April 2018 Update (17134)
• October 2018 Update (17763)
• May 2019 Update (18362)
• Insider Build (build number: 18363.535)

App min and target version:

• Fall Creators Update (16299)
• April 2018 Update (17134)
• October 2018 Update (17763)
• May 2019 Update (18362)
• Insider Build (xxxxx)

Device form factor:

• Desktop
• Xbox
• Surface Hub
• IoT

Visual Studio

• 2017 (version: )
• 2019 (version: )
• 2019 Preview (version: 16.5.0)

Additional context

currently no

[michael-hawker]

Thanks @Palatis for the report, moving it to the Win32 for initial triage as it's occurring with XAML Islands, though see now that the stack does have a toolkit component. Not sure where that reference is from.

@marb2000 @ocalvo @azchohfi thoughts?

[ncarrillo]

I'm also running into this. I don't think we can call into CoreView in the context of a XAML Island.

For me, this was occurring because RadialGauge has a static ThemeResource field which sets up the event subscription to ColorValuesChanged.

I think the effect of this is that any XAML Island application using the Windows Community Toolkit will crash on a color change.

I'll leave the fix up to you all, but I do have a workaround that I came up with to get my own app to stop crashing :-):

I run this in my app's main page Loaded event:

`

        var radialGaugeType = typeof(RadialGauge);
        var themeListener = radialGaugeType
            .GetField("ThemeListener", BindingFlags.NonPublic | BindingFlags.Static)
            .GetValue(null) as ThemeListener;
        themeListener.Dispose();

`

Calling Dispose unregisters the event subscription to ColorValuesChanged.

It won't help anyone who wishes to use RadialGauge or MarkdownTextBlock directly though.

[marb2000]

Doing the repro steps and I can't repro the bug...

[Palatis]

Doing the repro steps and I can't repro the bug...

maybe try switch system theme in the control panel? this is really theme related.
it crashes on me everytime i switch theme.

sometimes my Windows 10 switch theme from dark to light and immediately back to dark randomly...
i dunno why... but it does......

[michael-hawker]

@azchohfi can you look at the ThemeListener helper here in the toolkit when you have time? Is there a different model we should be using that's more XAML Islands friendly due to it's use of CoreWindow?

[marb2000]

In UWP, the Application's RequestedTheme only changes in the constructor. So no sure what this helper is listening...

[Palatis]

@marb2000 so what actually happens if the user switch theme from the control panel?
my other UWP apps actually switch theme (like from light to dark or vise versa).

[marb2000]

Gotcha! I get the scenario. now. You are listening the Window activation event to figure out whether the Theme has changed and then raise the event, am I right?

[marb2000]

@Palatis can you attach/link a repro project where we can see the "Cannot create new view because main window is not created." exception, please?