如何在Xamarin.Forms中检查暗模式

Xamarin.iOS

using System;
using UIKit;
using Xamarin.Forms;
using MyNamespace;
using MyNamespace.iOS;

[assembly: Dependency(typeof(Environment_iOS))]
namespace MyNamespace.iOS
{
    public class Environment_iOS : IEnvironment
    {
        public async Task GetOperatingSystemTheme()
        {
            //Ensure the current device is running 12.0 or higher, because `TraitCollection.UserInterfaceStyle` was introduced in iOS 12.0
            if (UIDevice.CurrentDevice.CheckSystemVersion(12, 0))
            {
                var currentUIViewCOntroller= await GetVisibleViewController();

                var userInterfaceStyle = currentUIViewController.TraitCollection.UserInterfaceStyle;

                switch (userInterfaceStyle)
                {
                    case UIUserInterfaceStyle.Light:
                        return Theme.Light;
                    case UIUserInterfaceStyle.Dark:
                        return Theme.Dark;
                    default:
                        throw new NotSupportedException($"UIUserInterfaceStyle {userInterfaceStyle} not supported");
                }
            }
            else
            {
                return Theme.Light;
            }
        }

        static Task GetVisibleViewController()
        {
            // UIApplication.SharedApplication can only be referenced on by Main Thread, so we'll use Device.InvokeOnMainThreadAsync which was introduced in Xamarin.Forms v4.2.0
            return Device.InvokeOnMainThreadAsync(() =>
            {
                var rootCOntroller= UIApplication.SharedApplication.KeyWindow.RootViewController;

                switch (rootController.PresentedViewController)
                {
                    case UINavigationController navigationController:
                        return navigationController.TopViewController;

                    case UITabBarController tabBarController:
                        return tabBarController.SelectedViewController;

                    case null:
                        return rootController;

                    default:
                        return rootController.PresentedViewController;
                }
            }
        }
    }
}

using System;
using System.Threading.Tasks;
using Android.Content.Res;
using Plugin.CurrentActivity;
using Xamarin.Forms;

using MyNamespace;
using MyNamespace.Android;

[assembly: Dependency(typeof(Environment_Android))]
namespace MyNamespace.Android
{
    public class Environment_Android : IEnvironment
    {
        public Task GetOperatingSystemTheme()
        {
            //Ensure the device is running Android Froyo or higher because UIMode was added in Android Froyo, API 8.0
            if(Build.VERSION.SdkInt >= BuildVersionCodes.Froyo)
            {
                var uiModeFlags = CrossCurrentActivity.Current.AppContext.Resources.Configuration.UiMode & UiMode.NightMask;

                switch(uiModeFlags)
                {
                    case UiMode.NightYes:
                        return Task.FromResult(Theme.Dark);

                    case UiMode.NightNo:
                        return Task.FromResult(Theme.Light);

                    default:
                        throw new NotSupportedException($"UiMode {uiModeFlags} not supported");
                }
            }
            else
            {
                return Task.FromResult(Theme.Light);
            }
        }
    }
}