StyleExtensions.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.

namespace CommunityToolkit.WinUI.Controls;

// Adapted from https://github.com/rudyhuyn/XamlPlus
public static partial class StyleExtensions
{
    // Used to distinct normal ResourceDictionary and the one we add.
    private sealed partial class StyleExtensionResourceDictionary : ResourceDictionary
    {
    }

    public static ResourceDictionary GetResources(Style obj)
    {
        return (ResourceDictionary)obj.GetValue(ResourcesProperty);
    }

    public static void SetResources(Style obj, ResourceDictionary value)
    {
        obj.SetValue(ResourcesProperty, value);
    }

    public static readonly DependencyProperty ResourcesProperty =
        DependencyProperty.RegisterAttached("Resources", typeof(ResourceDictionary), typeof(StyleExtensions), new PropertyMetadata(null, ResourcesChanged));

    private static void ResourcesChanged(DependencyObject sender, DependencyPropertyChangedEventArgs e)
    {
        if (!(sender is FrameworkElement frameworkElement))
        {
            return;
        }

        var mergedDictionaries = frameworkElement.Resources?.MergedDictionaries;
        if (mergedDictionaries == null)
        {
            return;
        }

        var existingResourceDictionary =
            mergedDictionaries.FirstOrDefault(c => c is StyleExtensionResourceDictionary);
        if (existingResourceDictionary != null)
        {
            // Remove the existing resource dictionary
            mergedDictionaries.Remove(existingResourceDictionary);
        }

        if (e.NewValue is ResourceDictionary resource)
        {
            var clonedResources = new StyleExtensionResourceDictionary();
            clonedResources.CopyFrom(resource);
            mergedDictionaries.Add(clonedResources);
        }

        if (frameworkElement.IsLoaded)
        {
            // Only force if the style was applied after the control was loaded
            ForceControlToReloadThemeResources(frameworkElement);
        }
    }

    private static void ForceControlToReloadThemeResources(FrameworkElement frameworkElement)
    {
        // To force the refresh of all resource references.
        // Note: Doesn't work when in high-contrast.
        var currentRequestedTheme = frameworkElement.RequestedTheme;
        frameworkElement.RequestedTheme = currentRequestedTheme == ElementTheme.Dark
            ? ElementTheme.Light
            : ElementTheme.Dark;
        frameworkElement.RequestedTheme = currentRequestedTheme;
    }
}