App.xaml.cs

using System;
using Avalonia;
using Avalonia.Controls;
using Avalonia.Controls.ApplicationLifetimes;
using Avalonia.Markup.Xaml;
using Avalonia.Platform;
using Avalonia.Styling;
using Avalonia.Themes.Simple;
using Avalonia.Themes.Fluent;
using ControlCatalog.Models;
using ControlCatalog.ViewModels;

namespace ControlCatalog
{
    public class App : Application
    {
        private readonly Styles _themeStylesContainer = new();
        private FluentTheme? _fluentTheme;
        private Avalonia.Themes.Fluent2.Fluent2Theme? _fluent2Theme;
        private SimpleTheme? _simpleTheme;
        private IStyle? _colorPickerFluent, _colorPickerSimple;

        public App()
        {
            DataContext = new ApplicationViewModel();
        }

        public override void Initialize()
        {
            Styles.Add(_themeStylesContainer);

            AvaloniaXamlLoader.Load(this);

            _fluentTheme = (FluentTheme)Resources["FluentTheme"]!;
            _fluent2Theme = (Avalonia.Themes.Fluent2.Fluent2Theme)Resources["Fluent2Theme"]!;
            _simpleTheme = (SimpleTheme)Resources["SimpleTheme"]!;
            _colorPickerFluent = (IStyle)Resources["ColorPickerFluent"]!;
            _colorPickerSimple = (IStyle)Resources["ColorPickerSimple"]!;

            SetCatalogThemes(CatalogTheme.Fluent2);
        }

        public override void OnFrameworkInitializationCompleted()
        {
            if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktopLifetime)
            {
                desktopLifetime.MainWindow = new MainWindow { DataContext = new MainWindowViewModel() };
            }
            else if (ApplicationLifetime is IActivityApplicationLifetime singleViewFactoryApplicationLifetime)
            {
                singleViewFactoryApplicationLifetime.MainViewFactory = () => new PageNavigationHost()
                {
                    Page = new MainView { DataContext = new MainWindowViewModel() }
                };
            }
            else if (ApplicationLifetime is ISingleViewApplicationLifetime singleViewLifetime)
            {
                singleViewLifetime.MainView = new PageNavigationHost()
                {
                    Page = new MainView { DataContext = new MainWindowViewModel() }
                };
            }

            if (this.TryGetFeature<IActivatableLifetime>() is { } activatableApplicationLifetime)
            {
                activatableApplicationLifetime.Activated += (sender, args) =>
                    Console.WriteLine($"App activated: {args.Kind}");
                activatableApplicationLifetime.Deactivated += (sender, args) =>
                    Console.WriteLine($"App deactivated: {args.Kind}");
            }

            base.OnFrameworkInitializationCompleted();
        }

        public void OnDockNewWindowClicked(object? sender, EventArgs e)
        {
            if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime)
            {
                var window = new MainWindow();
                window.Show();
            }
        }

        public void OnDockShowMainWindowClicked(object? sender, EventArgs e)
        {
            if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktopLifetime)
            {
                desktopLifetime.MainWindow?.Activate();
            }
        }

        private int _dockMenuItemCount;

        public void OnDockAddItemClicked(object? sender, EventArgs e)
        {
            var dockMenu = NativeDock.GetMenu(this);
            if (dockMenu is not null)
            {
                _dockMenuItemCount++;
                var item = new NativeMenuItem($"New item {_dockMenuItemCount}");
                item.Click += (_, _) =>
                {
                    dockMenu.Items.Remove(item);
                };
                dockMenu.Items.Insert(0, item);
            }
        }

        private CatalogTheme _prevTheme;
        public static CatalogTheme CurrentTheme => ((App)Current!)._prevTheme;
        public static void SetCatalogThemes(CatalogTheme theme)
        {
            var app = (App)Current!;
            var prevTheme = app._prevTheme;
            app._prevTheme = theme;
            var shouldReopenWindow = prevTheme != theme;

            if (app._themeStylesContainer.Count == 0)
            {
                app._themeStylesContainer.Add(new Style());
                app._themeStylesContainer.Add(new Style());
                app._themeStylesContainer.Add(new Style());
            }

            if (theme == CatalogTheme.Fluent)
            {
                app._themeStylesContainer[0] = app._fluentTheme!;
                app._themeStylesContainer[1] = app._colorPickerFluent!;
            }
            else if (theme == CatalogTheme.Fluent2)
            {
                app._themeStylesContainer[0] = app._fluent2Theme!;
                app._themeStylesContainer[1] = app._colorPickerFluent!;
            }
            else if (theme == CatalogTheme.Simple)
            {
                app._themeStylesContainer[0] = app._simpleTheme!;
                app._themeStylesContainer[1] = app._colorPickerSimple!;
            }

            if (shouldReopenWindow)
            {
                if (app.ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktopLifetime)
                {
                    var oldWindow = desktopLifetime.MainWindow;
                    var newWindow = new MainWindow()
                    {
                        DataContext = new MainWindowViewModel()
                    };
                    desktopLifetime.MainWindow = newWindow;
                    newWindow.Show();
                    oldWindow?.Close();
                }
                else if (app.ApplicationLifetime is IActivityApplicationLifetime singleViewFactoryApplicationLifetime)
                {
                    singleViewFactoryApplicationLifetime.MainViewFactory = () => new PageNavigationHost()
                    {
                        Page = new MainView { DataContext = new MainWindowViewModel() }
                    };
                }
                else if (app.ApplicationLifetime is ISingleViewApplicationLifetime singleViewLifetime)
                {
                    singleViewLifetime.MainView = new PageNavigationHost()
                    {
                        Page = new MainView { DataContext = new MainWindowViewModel() }
                    };
                }
            }
        }
    }
}