cursor-lang/CursorLang/Services/ThemeService.cs

using System.ComponentModel;
using System.Windows;
using System.Windows.Interop;
using CursorLang.Interop;
using CursorLang.Models;
using Microsoft.Win32;

namespace CursorLang.Services;

/// <summary>
/// Keeps the palette of the chosen theme in the application resources and swaps it
/// when the setting changes — the windows are recoloured without a restart.
/// </summary>
public sealed class ThemeService : IThemeService, IDisposable
{
    private const string PersonalizeKey =
        @"Software\Microsoft\Windows\CurrentVersion\Themes\Personalize";

    private readonly AppSettings _settings;
    private readonly Func<AppTheme> _detectSystemTheme;
    private readonly List<Window> _windows = [];
    private ResourceDictionary? _palette;
    private AppTheme _current;

    public ThemeService(AppSettings settings)
        : this(settings, DetectSystemTheme)
    {
    }

    /// <summary>
    /// Takes the source of the system theme explicitly: in tests it is not the registry
    /// that provides it.
    /// </summary>
    internal ThemeService(AppSettings settings, Func<AppTheme> detectSystemTheme)
    {
        _settings = settings;
        _detectSystemTheme = detectSystemTheme;
        _settings.PropertyChanged += OnSettingsChanged;
        SystemEvents.UserPreferenceChanged += OnUserPreferenceChanged;
        Apply();
    }

    /// <summary>The theme the user sees: <c>System</c> is already resolved here.</summary>
    public AppTheme CurrentTheme => _current;

    public void Register(Window window)
    {
        _windows.Add(window);
        window.Closed += OnWindowClosed;

        if (new WindowInteropHelper(window).Handle == IntPtr.Zero)
        {
            window.SourceInitialized += OnWindowSourceInitialized;
            return;
        }

        ApplyTitleBar(window);
    }

    public void Dispose()
    {
        _settings.PropertyChanged -= OnSettingsChanged;
        SystemEvents.UserPreferenceChanged -= OnUserPreferenceChanged;

        foreach (Window window in _windows)
        {
            window.Closed -= OnWindowClosed;
            window.SourceInitialized -= OnWindowSourceInitialized;
        }

        _windows.Clear();
    }

    /// <summary>The app theme from the Windows settings.</summary>
    internal static AppTheme DetectSystemTheme()
    {
        try
        {
            using RegistryKey? key = Registry.CurrentUser.OpenSubKey(PersonalizeKey);
            return key?.GetValue("AppsUseLightTheme") is int light && light == 0
                ? AppTheme.Dark
                : AppTheme.Light;
        }
        catch (Exception e) when (e is System.Security.SecurityException or UnauthorizedAccessException)
        {
            return AppTheme.Light;
        }
    }

    private void OnSettingsChanged(object? sender, PropertyChangedEventArgs e)
    {
        if (e.PropertyName == nameof(AppSettings.Theme))
        {
            Apply();
        }
    }

    // Windows reports a theme change from outside the interface thread
    private void OnUserPreferenceChanged(object? sender, UserPreferenceChangedEventArgs e) =>
        Application.Current?.Dispatcher.InvokeAsync(Apply);

    private void Apply()
    {
        AppTheme theme = _settings.Theme == AppTheme.System ? _detectSystemTheme() : _settings.Theme;
        if (_palette is not null && theme == _current)
        {
            return;
        }

        _current = theme;

        var next = new ResourceDictionary
        {
            Source = PaletteUri(theme),
        };

        ICollection<ResourceDictionary> dictionaries = Application.Current.Resources.MergedDictionaries;
        dictionaries.Add(next);

        if (_palette is not null)
        {
            dictionaries.Remove(_palette);
        }

        _palette = next;

        foreach (Window window in _windows)
        {
            ApplyTitleBar(window);
        }
    }

    // The assembly name in the address is there on purpose: without it the dictionary
    // is looked up in the assembly the process started from, which is not always the
    // application itself
    internal static Uri PaletteUri(AppTheme theme) =>
        new($"pack://application:,,,/CursorLang;component/Themes/{theme}.xaml", UriKind.Absolute);

    private void ApplyTitleBar(Window window) =>
        WindowThemeNative.SetDarkTitleBar(
            new WindowInteropHelper(window).Handle,
            _current == AppTheme.Dark);

    private void OnWindowSourceInitialized(object? sender, EventArgs e)
    {
        if (sender is Window window)
        {
            window.SourceInitialized -= OnWindowSourceInitialized;
            ApplyTitleBar(window);
        }
    }

    private void OnWindowClosed(object? sender, EventArgs e)
    {
        if (sender is Window window)
        {
            window.Closed -= OnWindowClosed;
            _windows.Remove(window);
        }
    }
}