ThemeListener.cs

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

using Windows.Foundation.Metadata;
using Windows.UI.ViewManagement;

#if !WINAPPSDK
using Windows.System;
#else
using Microsoft.UI.Dispatching;
#endif

// TODO: Fix tests for WinUI3
// [assembly: InternalsVisibleTo("UnitTests.XamlIslands.UWPApp")]
namespace CommunityToolkit.WinUI.Helpers;

/// <summary>
/// The Delegate for a ThemeChanged Event.
/// </summary>
/// <param name="sender">Sender ThemeListener</param>
public delegate void ThemeChangedEvent(ThemeListener sender);

/// <summary>
/// Class which listens for changes to Application Theme or High Contrast Modes
/// and Signals an Event when they occur.
/// </summary>
[AllowForWeb]
public sealed class ThemeListener : IDisposable
{
    /// <summary>
    /// Gets the Name of the Current Theme.
    /// </summary>
    public string CurrentThemeName
    {
        get { return this.CurrentTheme.ToString(); }
    }

    /// <summary>
    /// Gets or sets the Current Theme.
    /// </summary>
    public ApplicationTheme CurrentTheme { get; set; }

    /// <summary>
    /// Gets or sets a value indicating whether the current theme is high contrast.
    /// </summary>
    public bool IsHighContrast { get; set; }

    /// <summary>
    /// Gets or sets which DispatcherQueue is used to dispatch UI updates.
    /// </summary>
    public DispatcherQueue DispatcherQueue { get; set; }

    /// <summary>
    /// An event that fires if the Theme changes.
    /// </summary>
    public event ThemeChangedEvent ThemeChanged;

    private AccessibilitySettings _accessible = new AccessibilitySettings();
    private UISettings _settings = new UISettings();

    /// <summary>
    /// Initializes a new instance of the <see cref="ThemeListener"/> class.
    /// </summary>
    /// <param name="dispatcherQueue">The DispatcherQueue that should be used to dispatch UI updates, or null if this is being called from the UI thread.</param>
#pragma warning disable CS8618 // Non-nullable field must contain a non-null value when exiting constructor. Consider declaring as nullable.
    public ThemeListener(DispatcherQueue? dispatcherQueue = null)
#pragma warning restore CS8618 // Non-nullable field must contain a non-null value when exiting constructor. Consider declaring as nullable.
    {
        CurrentTheme = Application.Current.RequestedTheme;

        if (ApiInformation.IsPropertyPresent("Windows.UI.ViewManagement.AccessibilitySettings", "HighContrast"))
        {
            IsHighContrast = _accessible.HighContrast;
        }

        DispatcherQueue = dispatcherQueue ?? DispatcherQueue.GetForCurrentThread();

        if (Window.Current != null)
        {
            _accessible.HighContrastChanged += Accessible_HighContrastChanged;
            _settings.ColorValuesChanged += Settings_ColorValuesChanged;

            Window.Current.CoreWindow.Activated += CoreWindow_Activated;
        }
    }

    private async void Accessible_HighContrastChanged(AccessibilitySettings sender, object args)
    {
#if DEBUG
        global::System.Diagnostics.Debug.WriteLine("HighContrast Changed");
#endif

        await OnThemePropertyChangedAsync();
    }

    // Note: This can get called multiple times during HighContrast switch, do we care?
    private async void Settings_ColorValuesChanged(UISettings sender, object args)
    {
        await OnThemePropertyChangedAsync();
    }

    /// <summary>
    /// Dispatches an update for the public properties and the firing of <see cref="ThemeChanged"/> on <see cref="DispatcherQueue"/>.
    /// </summary>
    /// <returns>A <see cref="Task"/> that indicates when the dispatching has completed.</returns>
    internal Task OnThemePropertyChangedAsync()
    {
        // Getting called off thread, so we need to dispatch to request value.
        return DispatcherQueue.EnqueueAsync(
            () =>
            {
                // TODO: This doesn't stop the multiple calls if we're in our faked 'White' HighContrast Mode below.
                if (CurrentTheme != Application.Current.RequestedTheme ||
                    IsHighContrast != _accessible.HighContrast)
                {
#if DEBUG
                    global::System.Diagnostics.Debug.WriteLine("Color Values Changed");
#endif

                    UpdateProperties();
                }
            }, DispatcherQueuePriority.Normal);
    }

    private void CoreWindow_Activated(Windows.UI.Core.CoreWindow sender, Windows.UI.Core.WindowActivatedEventArgs args)
    {
        if (CurrentTheme != Application.Current.RequestedTheme ||
            IsHighContrast != _accessible.HighContrast)
        {
#if DEBUG
            global::System.Diagnostics.Debug.WriteLine("CoreWindow Activated Changed");
#endif

            UpdateProperties();
        }
    }

    /// <summary>
    /// Set our current properties and fire a change notification.
    /// </summary>
    private void UpdateProperties()
    {
        // TODO: Not sure if HighContrastScheme names are localized?
        if (_accessible.HighContrast && _accessible.HighContrastScheme.IndexOf("white", StringComparison.OrdinalIgnoreCase) != -1)
        {
            // If our HighContrastScheme is ON & a lighter one, then we should remain in 'Light' theme mode for Monaco Themes Perspective
            IsHighContrast = false;
            CurrentTheme = ApplicationTheme.Light;
        }
        else
        {
            // Otherwise, we just set to what's in the system as we'd expect.
            IsHighContrast = _accessible.HighContrast;
            CurrentTheme = Application.Current.RequestedTheme;
        }

        ThemeChanged?.Invoke(this);
    }

    /// <inheritdoc/>
    public void Dispose()
    {
        _accessible.HighContrastChanged -= Accessible_HighContrastChanged;
        _settings.ColorValuesChanged -= Settings_ColorValuesChanged;
        if (Window.Current != null)
        {
            Window.Current.CoreWindow.Activated -= CoreWindow_Activated;
        }
    }
}