Mica support in .NET MAUI

MauiProgram.cs

// ************************************************************************ //
// This implementation is inspired from the official WinUI3 backdrops examples
// https://github.com/microsoft/WinUI-Gallery/blob/winui3/XamlControlsGallery/ControlPagesSampleCode/SystemBackdrops/SystemBackdropsSample2.txt
// *********************************************************************** //

using Microsoft.Maui.LifecycleEvents;

#if WINDOWS10_0_17763_0_OR_GREATER
// The namespace of where the WindowsHelpers class resides
// In older versions of .NET MAUI, this was
//using YOUR_APP.Platforms.Windows
// In newer versions of .NET MAUI, it is now
using YOUR_APP.WinUI;
#endif

namespace YOUR_APP.Maui
{
    public static class MauiProgram
    {
        public static MauiApp CreateMauiApp()
        {
            var builder = MauiApp.CreateBuilder();
            builder
                .UseMauiApp<App>()
                .ConfigureFonts(fonts =>
                {
                    fonts.AddFont("OpenSans-Regular.ttf", "OpenSansRegular");
                    fonts.AddFont("OpenSans-Semibold.ttf", "OpenSansSemibold");
                });

        // **** THIS SECTION IS WHAT IS RELEVANT FOR YOU ************ //
        builder.ConfigureLifecycleEvents(events =>
        {
#if WINDOWS10_0_17763_0_OR_GREATER

            events.AddWindows(wndLifeCycleBuilder =>
            {
                wndLifeCycleBuilder.OnWindowCreated(window =>
                {
                    window.TryMicaOrAcrylic(); // requires 'using YOUR_APP.WinUI;'
                });
            });
#endif
        });
        // ************* END OF RELEVANT SECTION *********** //

            return builder.Build();
        }
    }
}

WindowHelpers.cs

// **************************************************** //
// Place this class file in your Platforms/Windows folder
// **************************************************** //

using Microsoft.UI.Composition;
using Microsoft.UI.Composition.SystemBackdrops;
using Microsoft.UI.Xaml;
using WinRT;

namespace YOUR_APP.Platforms.Windows
{
    public static class WindowHelpers
    {
        public static void TryMicaOrAcrylic(this Microsoft.UI.Xaml.Window window)
        {
            var dispatcherQueueHelper = new WindowsSystemDispatcherQueueHelper(); // in Platforms.Windows folder
            dispatcherQueueHelper.EnsureWindowsSystemDispatcherQueueController();

            // Hooking up the policy object
            var configurationSource = new SystemBackdropConfiguration();
            configurationSource.IsInputActive = true;

            switch (((FrameworkElement)window.Content).ActualTheme)
            {
                case ElementTheme.Dark:
                    configurationSource.Theme = SystemBackdropTheme.Dark; 
                    break;
                case ElementTheme.Light:
                    configurationSource.Theme = SystemBackdropTheme.Light;
                    break;
                case ElementTheme.Default:
                    configurationSource.Theme = SystemBackdropTheme.Default;
                    break;
            }
            
            // Let's try Mica first
            if (MicaController.IsSupported()) 
            {
                var micaController = new MicaController();
                micaController.AddSystemBackdropTarget(window.As<ICompositionSupportsSystemBackdrop>());
                micaController.SetSystemBackdropConfiguration(configurationSource);
                
                window.Activated += (object sender, WindowActivatedEventArgs args) =>
                {
                    if (args.WindowActivationState is WindowActivationState.CodeActivated or WindowActivationState.PointerActivated)
                    {
                        // Handle situation where a window is activated and placed on top of other active windows.
                        if (micaController == null)
                        {
                            micaController = new MicaController();
                            micaController.AddSystemBackdropTarget(window.As<ICompositionSupportsSystemBackdrop>()); // Requires 'using WinRT;'
                            micaController.SetSystemBackdropConfiguration(configurationSource);
                        }

                        if (configurationSource != null)
                            configurationSource.IsInputActive = args.WindowActivationState != WindowActivationState.Deactivated;
                    }
                };

                window.Closed += (object sender, WindowEventArgs args) =>
                {
                    if (micaController != null)
                    {
                        micaController.Dispose();
                        micaController = null;
                    }

                    configurationSource = null;
                };
            }
            // If no Mica, maybe we can use Acrylic instead
            else if (DesktopAcrylicController.IsSupported())
            {
                var acrylicController = new DesktopAcrylicController();
                acrylicController.AddSystemBackdropTarget(window.As<ICompositionSupportsSystemBackdrop>());
                acrylicController.SetSystemBackdropConfiguration(configurationSource);

                window.Activated += (object sender, WindowActivatedEventArgs args) =>
                {
                    if (args.WindowActivationState is WindowActivationState.CodeActivated or WindowActivationState.PointerActivated)
                    {
                        // Handle situation where a window is activated and placed on top of other active windows.
                        if (acrylicController == null)
                        {
                            acrylicController = new DesktopAcrylicController();
                            acrylicController.AddSystemBackdropTarget(window.As<ICompositionSupportsSystemBackdrop>()); // Requires 'using WinRT;'
                            acrylicController.SetSystemBackdropConfiguration(configurationSource);
                        }
                    }

                    if (configurationSource != null)
                        configurationSource.IsInputActive = args.WindowActivationState != WindowActivationState.Deactivated;
                };

                window.Closed += (object sender, WindowEventArgs args) =>
                {
                    if (acrylicController != null)
                    {
                        acrylicController.Dispose();
                        acrylicController = null;
                    }

                    configurationSource = null;
                };
            }
        }
    }
}

WindowsSystemDispatcherQueueHelper.cs

// **************************************************** //
// Place this class file in your Platforms/Windows folder
// **************************************************** //

using System.Runtime.InteropServices;
using Windows.System; // Required for DllImport attribute and DispatcherQueue

namespace YOUR_APP.Maui.Platforms.Windows;

public class WindowsSystemDispatcherQueueHelper
{
    [StructLayout(LayoutKind.Sequential)]
    struct DispatcherQueueOptions
    {
        internal int dwSize;
        internal int threadType;
        internal int apartmentType;
    }

    [DllImport("CoreMessaging.dll")]
    private static extern int CreateDispatcherQueueController([In] DispatcherQueueOptions options, [In, Out, MarshalAs(UnmanagedType.IUnknown)] ref object dispatcherQueueController);

    object m_dispatcherQueueController = null;

    public void EnsureWindowsSystemDispatcherQueueController()
    {
        if (DispatcherQueue.GetForCurrentThread() != null)
        {
            // one already exists, so we'll just use it.
            return;
        }

        if (m_dispatcherQueueController == null)
        {
            DispatcherQueueOptions options;

            options.dwSize = Marshal.SizeOf(typeof(DispatcherQueueOptions));
            options.threadType = 2;    // DQTYPE_THREAD_CURRENT
            options.apartmentType = 2; // DQTAT_COM_STA

            CreateDispatcherQueueController(options, ref m_dispatcherQueueController);
        }
    }
}

@kolkonos BOOM! Looks great.