How to dispose of AvaloniaUI Application

[matthewcrews]

I am using AvaloniuaFuncUI to create a add-in for Excel. I can open a window and close it using the "X" button in the top right of the window. When I go to open it again I get the following error:

Can not have multiple active ClassicDesktopStyleApplicationLifetime instances and the previously created one was not disposed

What is the correct way to handle this situation? I have searched the API for a way to dispose of the application and have not found one. I have also tried to open the main window again instead of launching the application again but have not found a way to do that either.

Is there any documentation or guides for a scenario where you want the application to stay open and just hide/show the main window?

[JordanMarr]

Hi Matthew!

In the world of add-ins, the AppDomain belongs to the parent app (Excel), so you have to be careful not to call your app init code more than once.

First of all, you have some control over the conditions that shutdown your app via the ShutdownMode property:

    override this.OnFrameworkInitializationCompleted() =
        match this.ApplicationLifetime with
        | :? IClassicDesktopStyleApplicationLifetime as desktop ->         
            let view = MainView()
            desktop.MainWindow <- view
            desktop.ShutdownMode <- ShutdownMode.OnMainWindowClose

Values are:

• ShutdownMode.OnMainWindowClose
• ShutdownMode.OnExplicitShutdown
• ShutdownMode.OnLastWindowClose

I've had to do this in the past with Revit add-ins:

if (Application.Current == null)
{
    var app = new Application();
    app.ShutdownMode = ShutdownMode.OnExplicitShutdown;
}

I know that's not a direct answer, but I hope that it's enough of a hint to put you on the right track!
Please share the final recipe if you get it working for future reference.

[matthewcrews]

Thank you. I already have something like this.

type App() =
    inherit Application()

    override this.Initialize() =
        this.Styles.Add (FluentTheme())
        this.RequestedThemeVariant <- Styling.ThemeVariant.Dark

    override this.OnFrameworkInitializationCompleted() =
        match this.ApplicationLifetime with
        | :? IClassicDesktopStyleApplicationLifetime as desktopLifetime ->
            let mainWindow = MainWindow()
            desktopLifetime.MainWindow <- mainWindow
            desktopLifetime.ShutdownMode <- Controls.ShutdownMode.OnMainWindowClose
        | _ -> ()

let appBuilder =
    AppBuilder
        .Configure<App>()
        .UsePlatformDetect()
        .UseSkia()
        .SetupWithoutStarting()

let showEditor () =
    let d (_: Application) (_: string[]) = ()
    let myDelegate = AppBuilder.AppMainDelegate d
    appBuilder.Start(myDelegate, [||])
    ()

The showEditor function is the function called from Excel to bring up the AvaloniaUI window.

I think I have two main issues:

1. I don't have a way to detect whether the AvaloniaUI app is already running
2. I don't know how to re-open the Main Window when it has been closed.

[JordanMarr]

Can you check the Application.Current static property for null?

[JordanMarr]

You might also try ShutdownMode.OnExplicitShutdown mode so that it doesn't shut down automatically when the window closes.

[matthewcrews]

With the following code I am now getting an error saying that I'm calling the Setup more than once.

type App() =
    inherit Application()

    override this.Initialize() =
        this.Styles.Add (FluentTheme())
        this.RequestedThemeVariant <- Styling.ThemeVariant.Dark

    override this.OnFrameworkInitializationCompleted() =
        match this.ApplicationLifetime with
        | :? IClassicDesktopStyleApplicationLifetime as desktopLifetime ->
            let mainWindow = MainWindow()
            desktopLifetime.MainWindow <- mainWindow
            desktopLifetime.ShutdownMode <- Controls.ShutdownMode.OnExplicitShutdown
        | _ -> ()

let appBuilder =
    AppBuilder
        .Configure<App>()
        .UsePlatformDetect()
        .UseSkia()
        .SetupWithoutStarting()

let showEditor () =
    match Application.Current with
    | null ->
        let d (_: Application) (_: string[]) = ()
        let myDelegate = AppBuilder.AppMainDelegate d
        appBuilder.Start(myDelegate, [||])

    | _ ->
        ()
    ()

[JordanMarr]

With WPF, I seem to recall that there were different ways that Application could be started.
The default templates would have an App.xaml file with its Build Action set in a certain way that caused it to automatically load. In Avalonia, it is set to Avalonia XAML. I'm guessing it probably uses reflection to see if you have an Application and then auto starts it.
I believe I removed that file altogether and just called my own class that manually created a new Application if Application.Current == null.

That might be a bit of a rabbit hole though. At the end of the day, a new Application was only created once within the lifetime of the host application (Revit, Excel, etc).