Add AutoSuspendHelper implementation

src/Avalonia.ReactiveUI/AutoSuspendHelper.cs

@@ -0,0 +1,44 @@
+// Copyright (c) The Avalonia Project. All rights reserved.
+// Licensed under the MIT license. See licence.md file in the project root for full license information.
+
+using Avalonia;
+using Avalonia.VisualTree;
+using Avalonia.Controls;
+using System.Threading;
+using System.Reactive.Disposables;
+using System.Reactive.Subjects;
+using System.Reactive.Linq;
+using System.Reactive;
+using ReactiveUI;
+using System;
+
+namespace Avalonia.ReactiveUI
+{
+    /// <summary>
+    /// A ReactiveUI AutoSuspendHelper which initializes suspension hooks for
+    /// Avalonia applications. Call its constructor in your app's composition root,
+    /// before calling the RxApp.SuspensionHost.SetupDefaultSuspendResume method.
+    /// </summary>
+    public sealed class AutoSuspendHelper
+    {
+        public AutoSuspendHelper(Application app)
+        {
+            RxApp.SuspensionHost.IsResuming = Observable.Never<Unit>();
+            RxApp.SuspensionHost.IsLaunchingNew = Observable.Return(Unit.Default);
+
+            var exiting = new Subject<IDisposable>();
+            app.Exit += (o, e) =>
+            {
+                // This is required to prevent the app from shutting down too early.
+                var manual = new ManualResetEvent(false);
+                exiting.OnNext(Disposable.Create(() => manual.Set()));
+                manual.WaitOne();
+            };
+            RxApp.SuspensionHost.ShouldPersistState = exiting;
+            
+            var errored = new Subject<Unit>();
+            AppDomain.CurrentDomain.UnhandledException += (o, e) => errored.OnNext(Unit.Default);
+            RxApp.SuspensionHost.ShouldInvalidateState = errored;
+        }
+    }
+}