Aaru/Aaru.Gui/App.xaml.cs

using System;
using Aaru.Gui.ViewModels;
using Aaru.Gui.Views;
using Avalonia;
using Avalonia.Controls;
using Avalonia.Controls.ApplicationLifetimes;
using Avalonia.Markup.Xaml;

namespace Aaru.Gui
{
    public class App : Application
    {
        public override void Initialize() => AvaloniaXamlLoader.Load(this);

        public override void OnFrameworkInitializationCompleted()
        {
            if(ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
            {
                var swvm = new SplashWindowViewModel();
                swvm.WorkFinished += OnSplashFinished;

                desktop.MainWindow = new SplashWindow
                {
                    DataContext = swvm
                };
            }

            base.OnFrameworkInitializationCompleted();
        }

        void OnSplashFinished(object sender, EventArgs e)
        {
            if(!(ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop))
                return;

            // Ensure not exit
            desktop.ShutdownMode = ShutdownMode.OnExplicitShutdown;

            // Close splash window
            desktop.MainWindow.Close();

            // Create and show main window
            desktop.MainWindow = new MainWindow
            {
                DataContext = new MainWindowViewModel()
            };

            desktop.MainWindow.Show();

            // Now can close when all windows are closed
            desktop.ShutdownMode = ShutdownMode.OnLastWindowClose;
        }
    }
}