Avalonia/src/Android/Avalonia.Android/AvaloniaMainActivity.cs

using System;
using Android.App;
using Android.Content;
using Android.OS;
using Android.Runtime;
using Android.Views;
using AndroidX.AppCompat.App;

namespace Avalonia.Android
{
    public abstract class AvaloniaMainActivity : AppCompatActivity, IActivityResultHandler, IActivityNavigationService
    {
        internal static object ViewContent;

        public Action<int, Result, Intent> ActivityResult { get; set; }
        internal AvaloniaView View;
        private GlobalLayoutListener _listener;

        protected override void OnCreate(Bundle savedInstanceState)
        {
            View = new AvaloniaView(this);
            if (ViewContent != null)
            {
                View.Content = ViewContent;
            }

            if (Avalonia.Application.Current.ApplicationLifetime is SingleViewLifetime lifetime)
            {
                lifetime.View = View;
            }

            base.OnCreate(savedInstanceState);

            SetContentView(View);

            _listener = new GlobalLayoutListener(View);

            View.ViewTreeObserver?.AddOnGlobalLayoutListener(_listener);
        }

        public object Content
        {
            get
            {
                return ViewContent;
            }
            set
            {
                ViewContent = value;
                if (View != null)
                    View.Content = value;
            }
        }

        public event EventHandler<AndroidBackRequestedEventArgs> BackRequested;

        public override void OnBackPressed()
        {
            var eventArgs = new AndroidBackRequestedEventArgs();

            BackRequested?.Invoke(this, eventArgs);

            if (!eventArgs.Handled)
            {
                base.OnBackPressed();
            }
        }

        protected override void OnDestroy()
        {
            View.Content = null;

            View.ViewTreeObserver?.RemoveOnGlobalLayoutListener(_listener);

            base.OnDestroy();
        }

        protected override void OnActivityResult(int requestCode, [GeneratedEnum] Result resultCode, Intent data)
        {
            base.OnActivityResult(requestCode, resultCode, data);

            ActivityResult?.Invoke(requestCode, resultCode, data);
        }

        class GlobalLayoutListener : Java.Lang.Object, ViewTreeObserver.IOnGlobalLayoutListener
        {
            private AvaloniaView _view;

            public GlobalLayoutListener(AvaloniaView view)
            {
                _view = view;
            }

            public void OnGlobalLayout()
            {
                _view.TopLevelImpl?.Resize(_view.TopLevelImpl.ClientSize);
            }
        }
    }
}