fix: Replace default MainThreadScheduler on UAP (reactiveui#2100)

See reactiveui#2092, reactiveui#2032 for more info. This replaces the usage of
`WaitForDispatcherScheduler` and `CoreDispatcherScheduler` with a
scheduler that explicitly dispatches to the main app window regardless
of how many windows are created by the application.

src/ReactiveUI/Platforms/uap/PlatformRegistrations.cs

@@ -28,7 +28,7 @@ public void Register(Action<Func<object>, Type> registerFunction)
             registerFunction(() => new BooleanToVisibilityTypeConverter(), typeof(IBindingTypeConverter));
             registerFunction(() => new AutoDataTemplateBindingHook(), typeof(IPropertyBindingHook));
             RxApp.TaskpoolScheduler = TaskPoolScheduler.Default;
-            RxApp.MainThreadScheduler = new WaitForDispatcherScheduler(() => CoreDispatcherScheduler.Current);
+            RxApp.MainThreadScheduler = new SingleWindowDispatcherScheduler();
             registerFunction(() => new WinRTAppDataDriver(), typeof(ISuspensionDriver));
         }
     }

src/ReactiveUI/Platforms/uap/SingleWindowDispatcherScheduler.cs

@@ -0,0 +1,203 @@
+// Copyright (c) 2019 .NET Foundation and Contributors. All rights reserved.
+// 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 full license information.
+
+using System;
+using System.Reactive.Concurrency;
+using System.Reactive.Disposables;
+using System.Reactive.PlatformServices;
+using System.Runtime.ExceptionServices;
+using System.Threading;
+using Windows.ApplicationModel.Core;
+using Windows.System.Threading;
+using Windows.UI.Core;
+using Windows.UI.Xaml;
+
+namespace ReactiveUI
+{
+    /// <summary>
+    /// This scheduler forces all dispatching to go to the first window of the <see cref="CoreApplication.Views"/> enumeration.
+    /// This makes the intended behavior of only supporting single window apps on UWP explicit.
+    /// If your app creates multiple windows, you should explicitly supply a scheduler which marshals
+    /// back to that window's <see cref="CoreDispatcher"/>.
+    /// </summary>
+    /// <remarks>
+    /// This follows patterns set out in <see cref="CoreDispatcherScheduler"/> with some minor tweaks
+    /// for thread-safety and performance.
+    /// </remarks>
+    /// <seealso cref="System.Reactive.Concurrency.IScheduler" />
+    public class SingleWindowDispatcherScheduler : IScheduler
+    {
+        private static CoreDispatcher _dispatcher;
+        private readonly CoreDispatcherPriority _priority;
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="SingleWindowDispatcherScheduler"/> class.
+        /// </summary>
+        public SingleWindowDispatcherScheduler()
+        {
+            if (CoreApplication.Views.Count > 0)
+            {
+                Interlocked.CompareExchange(ref _dispatcher, CoreApplication.Views[0].Dispatcher, null);
+            }
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="SingleWindowDispatcherScheduler"/> class with an explicit dispatcher.
+        /// </summary>
+        /// <param name="dispatcher">
+        /// The explicit <see cref="CoreDispatcher"/> to use. If you supply a dispatcher here then all instances of
+        /// <see cref="SingleWindowDispatcherScheduler"/> will dispatch to that dispatcher from instantiation on.
+        /// </param>
+        /// <exception cref="System.ArgumentNullException">
+        /// dispatcher - To override the scheduler you must supply a non-null instance of CoreDispatcher.
+        /// </exception>
+        public SingleWindowDispatcherScheduler(CoreDispatcher dispatcher)
+        {
+            _dispatcher = dispatcher ?? throw new ArgumentNullException(nameof(dispatcher), "To override the scheduler you must supply a non-null instance of CoreDispatcher.");
+        }
+
+        /// <inheritdoc/>
+        public DateTimeOffset Now => SystemClock.UtcNow;
+
+        /// <inheritdoc/>
+        public IDisposable Schedule<TState>(TState state, Func<IScheduler, TState, IDisposable> action)
+        {
+            if (action is null)
+            {
+                throw new ArgumentNullException(nameof(action));
+            }
+
+            if (CoreApplication.Views.Count == 0)
+            {
+                return CurrentThreadScheduler.Instance.Schedule(state, action);
+            }
+
+            return ScheduleOnDispatcherNow(state, action);
+        }
+
+        /// <inheritdoc/>
+        public IDisposable Schedule<TState>(TState state, TimeSpan dueTime, Func<IScheduler, TState, IDisposable> action)
+        {
+            if (action == null)
+            {
+                throw new ArgumentNullException(nameof(action));
+            }
+
+            if (CoreApplication.Views.Count == 0)
+            {
+                return CurrentThreadScheduler.Instance.Schedule(state, dueTime, action);
+            }
+
+            var dt = Scheduler.Normalize(dueTime);
+            if (dt.Ticks == 0)
+            {
+                return ScheduleOnDispatcherNow(state, action);
+            }
+
+            return ScheduleSlow(state, dt, action);
+        }
+
+        /// <inheritdoc/>
+        public IDisposable Schedule<TState>(TState state, DateTimeOffset dueTime, Func<IScheduler, TState, IDisposable> action)
+        {
+            if (action is null)
+            {
+                throw new ArgumentNullException(nameof(action));
+            }
+
+            if (CoreApplication.Views.Count == 0)
+            {
+                return CurrentThreadScheduler.Instance.Schedule(state, dueTime, action);
+            }
+
+            var dt = Scheduler.Normalize(dueTime - DateTimeOffset.Now);
+            if (dt.Ticks == 0)
+            {
+                return ScheduleOnDispatcherNow(state, action);
+            }
+
+            return ScheduleSlow(state, dt, action);
+        }
+
+        /// <summary>
+        /// Work-around for the behavior of throwing from "async void" or an <see cref="IAsyncResult"/> not propagating
+        /// the exception to the <see cref="Application.UnhandledException" /> event as users have come to expect from
+        /// previous XAML stacks using Rx.
+        /// </summary>
+        /// <param name="ex">The exception.</param>
+        private void RaiseUnhandledException(Exception ex)
+        {
+            var timer = new DispatcherTimer();
+            timer.Interval = TimeSpan.Zero;
+            timer.Tick += RaiseToDispatcher;
+
+            timer.Start();
+            void RaiseToDispatcher(object sender, object e)
+            {
+                timer.Stop();
+                timer.Tick -= RaiseToDispatcher;
+                timer = null;
+
+                ExceptionDispatchInfo.Capture(ex).Throw();
+            }
+        }
+
+        private IDisposable ScheduleOnDispatcherNow<TState>(TState state, Func<IScheduler, TState, IDisposable> action)
+        {
+            Interlocked.CompareExchange(ref _dispatcher, CoreApplication.Views[0].Dispatcher, null);
+
+            if (_dispatcher.HasThreadAccess)
+            {
+                return action(this, state);
+            }
+
+            var d = new SingleAssignmentDisposable();
+
+            var dispatchResult = _dispatcher.RunAsync(
+                _priority,
+                () =>
+                {
+                    if (!d.IsDisposed)
+                    {
+                        try
+                        {
+                            d.Disposable = action(this, state);
+                        }
+                        catch (Exception ex)
+                        {
+                            RaiseUnhandledException(ex);
+                        }
+                    }
+                });
+
+            return StableCompositeDisposable.Create(
+                d,
+                Disposable.Create(() => dispatchResult.Cancel()));
+        }
+
+        private IDisposable ScheduleSlow<TState>(TState state, TimeSpan dueTime, Func<IScheduler, TState, IDisposable> action)
+        {
+            var d = new MultipleAssignmentDisposable();
+
+            // Why ThreadPoolTimer?
+            // --
+            // Because, we can't guarantee that DispatcherTimer will dispatch to the correct CoreDispatcher if there are multiple
+            // so we dispatch explicitly from our own method.
+            var timer = ThreadPoolTimer.CreateTimer(_ => d.Disposable = ScheduleOnDispatcherNow(state, action), dueTime);
+
+            d.Disposable = Disposable.Create(() =>
+            {
+                var t = Interlocked.Exchange(ref timer, null);
+                if (t != null)
+                {
+                    t.Cancel();
+                    action = (_, __) => Disposable.Empty;
+                }
+            });
+
+            return d;
+        }
+    }
+}