Semi.Avalonia/demo/Semi.Avalonia.Demo/Views/MainView.axaml.cs

using System;
using System.Collections.Generic;
using System.Threading.Tasks;
using System.Windows.Input;
using Avalonia;
using Avalonia.Controls;
using Avalonia.Controls.ApplicationLifetimes;
using Avalonia.Styling;
using CommunityToolkit.Mvvm.ComponentModel;
using CommunityToolkit.Mvvm.Input;

namespace Semi.Avalonia.Demo.Views;

public partial class MainView : UserControl
{
    public MainView()
    {
        InitializeComponent();
        this.DataContext = new MainViewModel();
    }
}

public partial class MainViewModel : ObservableObject
{
    public string DocumentationUrl => "https://docs.irihi.tech/semi";
    public string RepoUrl => "https://github.com/irihitech/Semi.Avalonia";
    public IReadOnlyList<MenuItemViewModel> MenuItems { get; }

    public MainViewModel()
    {
        MenuItems = [];
    }

    [RelayCommand]
    private void ToggleTheme()
    {
        var app = Application.Current;
        if (app is null) return;
        var theme = app.ActualThemeVariant;
        app.RequestedThemeVariant = theme == ThemeVariant.Dark ? ThemeVariant.Light : ThemeVariant.Dark;
    }

    [RelayCommand]
    private void SelectTheme(object? obj)
    {
        var app = Application.Current;
        if (app is not null)
        {
            app.RequestedThemeVariant = obj as ThemeVariant;
        }
    }

    [RelayCommand]
    private static async Task OpenUrl(string url)
    {
        var launcher = ResolveDefaultTopLevel()?.Launcher;
        if (launcher is not null)
        {
            await launcher.LaunchUriAsync(new Uri(url));
        }
    }

    private static TopLevel? ResolveDefaultTopLevel()
    {
        return Application.Current?.ApplicationLifetime switch
        {
            IClassicDesktopStyleApplicationLifetime desktopLifetime => desktopLifetime.MainWindow,
            ISingleViewApplicationLifetime singleView => TopLevel.GetTopLevel(singleView.MainView),
            _ => null
        };
    }
}

public class MenuItemViewModel
{
    public string? Header { get; set; }
    public ICommand? Command { get; set; }
    public object? CommandParameter { get; set; }
    public IList<MenuItemViewModel>? Items { get; set; }
}