ReactiveCommand and UIThread? (ThemeVariant toggle)

[houstonhaynes]

I'm working on a "new" app that's using an old method from a previous app - so I had pretty high confidence in it working. Basically it's looking at the Application level ThemeVariant and using that to determine the toggle from "Dark" to "Light" and back again.

<ToggleButton Grid.Column="5" Name="ThemeToggleSwitch"
                                Theme="{StaticResource MaterialFlatButton}"
                                Width="32" Height="32" Padding="4" 
                                Command="{Binding SwitchThemeCommand}" >
                      <icon:MaterialIcon Kind="ThemeLightDark" Width="24" Height="24" />
                  </ToggleButton>

member this.SwitchThemeCommand =
        ReactiveCommand.Create(fun () -> switchTheme())

let switchTheme() =
        Dispatcher.UIThread.InvokeAsync(fun () ->
            if Application.Current.ActualThemeVariant = ThemeVariant.Light then
                Application.Current.RequestedThemeVariant <- ThemeVariant.Dark
            else
                Application.Current.RequestedThemeVariant <- ThemeVariant.Light
        ) |> ignore

For some reason I'm getting a complaint that seems to point to the UIThread - at least that's my reflex when I see "call from invalid thread"

Unhandled exception. System.InvalidOperationException: Call from invalid thread
   at Avalonia.Threading.Dispatcher.<VerifyAccess>g__ThrowVerifyAccess|16_0()
   at Avalonia.AvaloniaObject.GetValue[T](StyledProperty`1 property)
   at Avalonia.Controls.Button.get_Command()
   at Avalonia.Controls.Button.CanExecuteChanged(Object sender, EventArgs e)
   at ReactiveUI.ReactiveCommandBase`2.OnCanExecuteChanged(Boolean newValue) in /_/src/ReactiveUI/ReactiveCommand/ReactiveCommandBase.cs:line 144
   at System.Reactive.AnonymousSafeObserver`1.OnNext(T value)
   at System.Reactive.Sink`1.ForwardOnNext(TTarget value)
   at System.Reactive.IdentitySink`1.OnNext(T value)
   at System.Reactive.Subjects.FastImmediateObserver`1.EnsureActive(Int32 count)
   at System.Reactive.Subjects.FastImmediateObserver`1.EnsureActive()
   at System.Reactive.Subjects.ReplaySubject`1.ReplayBase.OnNext(T value)
   at System.Reactive.Subjects.ReplaySubject`1.OnNext(T value)
   at System.Reactive.Sink`1.ForwardOnNext(TTarget value)
   at System.Reactive.IdentitySink`1.OnNext(T value)
   at System.Reactive.Sink`1.ForwardOnNext(TTarget value)
   at System.Reactive.Linq.ObservableImpl.DistinctUntilChanged`2._.OnNext(TSource value)
   at System.Reactive.Sink`1.ForwardOnNext(TTarget value)
   at System.Reactive.Linq.ObservableImpl.CombineLatest`3._.SecondObserver.OnNext(TSecond value)
   at System.Reactive.Sink`1.ForwardOnNext(TTarget value)
   at System.Reactive.IdentitySink`1.OnNext(T value)
   at System.Reactive.Subjects.FastImmediateObserver`1.EnsureActive(Int32 count)
   at System.Reactive.Subjects.FastImmediateObserver`1.EnsureActive()
   at System.Reactive.Subjects.ReplaySubject`1.ReplayBase.OnNext(T value)
   at System.Reactive.Subjects.ReplaySubject`1.OnNext(T value)
   at System.Reactive.Sink`1.ForwardOnNext(TTarget value)
   at System.Reactive.IdentitySink`1.OnNext(T value)
   at System.Reactive.Sink`1.ForwardOnNext(TTarget value)
   at System.Reactive.Linq.ObservableImpl.DistinctUntilChanged`2._.OnNext(TSource value)
   at System.Reactive.Sink`1.ForwardOnNext(TTarget value)
   at System.Reactive.IdentitySink`1.OnNext(T value)
   at System.Reactive.Sink`1.ForwardOnNext(TTarget value)
   at System.Reactive.Linq.ObservableImpl.Select`2.Selector._.OnNext(TSource value)
   at System.Reactive.Sink`1.ForwardOnNext(TTarget value)
   at System.Reactive.Linq.ObservableImpl.Scan`2._.OnNext(TSource value)
   at System.Reactive.SafeObserver`1.WrappingSafeObserver.OnNext(TSource value)
   at System.Reactive.Sink`1.ForwardOnNext(TTarget value)
   at System.Reactive.ObserveOnObserverLongRunning`1.Drain()
   at System.Reactive.ObserveOnObserverLongRunning`1.<>c.<.cctor>b__17_0(ObserveOnObserverLongRunning`1 self, ICancelable cancelable)
   at System.Reactive.Concurrency.DefaultScheduler.LongRunning.LongScheduledWorkItem`1.<>c.<.ctor>b__3_0(Object thisObject)
   at System.Reactive.Concurrency.ConcurrencyAbstractionLayerImpl.<>c.<StartThread>b__8_0(Object itemObject)

I'm at a loss as to what I might be doing wrong here. I'm hoping it might be something you've seen in your travels. Thanks!

Quick Addendum: I also tried changing the "BaseTheme" attribute in the MaterialTheme as they did in their demo app and I get the same error/issue.

let switchTheme() =
        Dispatcher.UIThread.InvokeAsync(fun () ->
            let materialTheme = Application.Current.LocateMaterialTheme<MaterialTheme>()
            materialTheme.BaseTheme <-
                if materialTheme.BaseTheme = BaseThemeMode.Light then
                    BaseThemeMode.Dark
                else
                    BaseThemeMode.Light
        ) |> ignore

[houstonhaynes]

Disregard I was making it too hard...

member this.SwitchTheme() = switchTheme()