feat(wpf): add DispatcherSequencer.Main for the WPF UI thread

src/Directory.Packages.props

@@ -5,8 +5,8 @@
     <CentralPackageTransitivePinningEnabled>true</CentralPackageTransitivePinningEnabled>
     <!-- StyleSharp.Analyzers, PerformanceSharp.Analyzers and SecuritySharp.Analyzers ship from the
          same release pipeline and always share a version. -->
-    <RoslynCommonAnalyzersVersion>5.1.1</RoslynCommonAnalyzersVersion>
-    <AvaloniaVersion>12.1.2</AvaloniaVersion>
+    <RoslynCommonAnalyzersVersion>5.1.2</RoslynCommonAnalyzersVersion>
+    <AvaloniaVersion>12.1.3</AvaloniaVersion>
   </PropertyGroup>
   <ItemGroup>
     <PackageVersion Include="Microsoft.SourceLink.GitHub" Version="10.0.401"/>
@@ -21,8 +21,8 @@
   </ItemGroup>
   <!-- Benchmarks + tests -->
   <ItemGroup>
-    <PackageVersion Include="BenchmarkDotNet" Version="0.16.0-preview.1"/>
-    <PackageVersion Include="BenchmarkDotNet.Diagnostics.Windows" Version="0.16.0-preview.1"/>
+    <PackageVersion Include="BenchmarkDotNet" Version="0.16.0-preview.2"/>
+    <PackageVersion Include="BenchmarkDotNet.Diagnostics.Windows" Version="0.16.0-preview.2"/>
     <PackageVersion Include="Microsoft.Extensions.TimeProvider.Testing" Version="10.10.0"/>
     <PackageVersion Include="System.Reactive" Version="7.0.0"/>
     <PackageVersion Include="Microsoft.Reactive.Testing" Version="7.0.0"/>
@@ -68,7 +68,7 @@
     <!-- BenchmarkDotNet needs Roslyn 5.3+; this also reaches the job projects it generates under the benchmark bin folders. -->
     <PackageVersion Include="Microsoft.CodeAnalysis.CSharp" Version="5.9.0" Condition="$(MSBuildProjectDirectory.Replace('\', '/').Contains('/benchmarks/'))"/>
     <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="18.10.1"/>
-    <PackageVersion Include="TUnit" Version="1.68.4"/>
+    <PackageVersion Include="TUnit" Version="1.69.0"/>
     <PackageVersion Include="Microsoft.Testing.Extensions.CodeCoverage" Version="18.11.2"/>
   </ItemGroup>
   <Import Project="$(MSBuildThisFileDirectory)/directory.packages.support.props" Condition="Exists('$(MSBuildThisFileDirectory)/directory.packages.support.props')"/>

src/ReactiveUI.Primitives.Wpf.Reactive/Concurrency/DispatcherSequencer.cs

@@ -15,6 +15,13 @@ namespace ReactiveUI.Primitives.Reactive.Concurrency;
 [System.Diagnostics.DebuggerDisplay("DispatcherSequencer: Dispatcher = {Dispatcher}, Priority = {Priority}")]
 public sealed class DispatcherSequencer : LocalScheduler
 {
+    /// <summary>The shared main-thread scheduler, set once the application's dispatcher is available.</summary>
+    private static DispatcherSequencer? _main;
+
+    /// <summary>The scheduler for the calling thread's dispatcher, cached per thread.</summary>
+    [ThreadStatic]
+    private static DispatcherSequencer? _current;
+
     /// <summary>Optional callback for posting ready work.</summary>
     private readonly Func<Action, bool>? _post;
 
@@ -60,6 +67,23 @@ internal DispatcherSequencer(
         _dispatch = new(RunDrain, DefaultScheduler.Instance);
     }
 
+    /// <summary>Gets the shared scheduler for the WPF main (UI) thread.</summary>
+    /// <exception cref="InvalidOperationException">No application exists yet and the calling thread has no dispatcher.</exception>
+    /// <remarks>
+    /// Bound once to the running <see cref="System.Windows.Application"/>'s dispatcher, from any thread. Until an
+    /// application exists nothing is cached, and the calling thread's existing dispatcher is used instead.
+    /// </remarks>
+    public static DispatcherSequencer Main =>
+        Volatile.Read(ref _main) ?? BindMain(ref _main, System.Windows.Application.Current?.Dispatcher) ?? Current;
+
+    /// <summary>Gets the scheduler for the calling thread's dispatcher.</summary>
+    /// <exception cref="InvalidOperationException">The calling thread has no dispatcher.</exception>
+    /// <remarks>
+    /// Cached per thread, for applications that run UI on more than one thread. Never creates a dispatcher: a thread
+    /// that has none cannot run the scheduled work.
+    /// </remarks>
+    public static DispatcherSequencer Current => _current ??= new(ResolveCurrentDispatcher());
+
     /// <summary>Gets the dispatcher whose thread runs the scheduled work.</summary>
     public Dispatcher Dispatcher { get; }
 
@@ -78,6 +102,29 @@ public override IDisposable Schedule<TState>(TState state, Func<IScheduler, TSta
     public override IDisposable Schedule<TState>(TState state, TimeSpan dueTime, Func<IScheduler, TState, IDisposable> action) =>
         _dispatch.Schedule(new DispatchHost(this), this, state, dueTime, action);
 
+    /// <summary>Caches the shared main-thread scheduler for the application's dispatcher, keeping the first one bound.</summary>
+    /// <param name="slot">The field that holds the shared scheduler.</param>
+    /// <param name="applicationDispatcher">The application's dispatcher, or <see langword="null"/> when no application exists.</param>
+    /// <returns>The shared scheduler, or <see langword="null"/> when <paramref name="applicationDispatcher"/> is <see langword="null"/>.</returns>
+    internal static DispatcherSequencer? BindMain(ref DispatcherSequencer? slot, Dispatcher? applicationDispatcher)
+    {
+        if (applicationDispatcher is null)
+        {
+            return null;
+        }
+
+        DispatcherSequencer created = new(applicationDispatcher);
+        return Interlocked.CompareExchange(ref slot, created, null) ?? created;
+    }
+
+    /// <summary>Returns the calling thread's existing dispatcher without creating one.</summary>
+    /// <returns>The calling thread's dispatcher.</returns>
+    /// <exception cref="InvalidOperationException">The calling thread has no dispatcher.</exception>
+    internal static Dispatcher ResolveCurrentDispatcher() =>
+        Dispatcher.FromThread(Thread.CurrentThread)
+        ?? throw new InvalidOperationException(
+            "The calling thread has no WPF dispatcher. Use DispatcherSequencer.Main or Current from a UI thread, or after the Application is created.");
+
     /// <summary>Posts the drain callback, through the test hook when one was supplied.</summary>
     /// <param name="drain">The drain callback.</param>
     /// <returns>Whether the dispatcher accepted the callback.</returns>

src/ReactiveUI.Primitives.Wpf.Reactive/PublicAPI/net10.0-windows/PublicAPI.txt

@@ -8,6 +8,8 @@ public sealed class DispatcherSequencer : System.Reactive.Concurrency.LocalSched
     public DispatcherSequencer(System.Windows.Threading.Dispatcher dispatcher) { }
     public DispatcherSequencer(System.Windows.Threading.Dispatcher dispatcher, System.Windows.Threading.DispatcherPriority priority) { }
     public System.Windows.Threading.Dispatcher Dispatcher { get; }
+    public static ReactiveUI.Primitives.Reactive.Concurrency.DispatcherSequencer Current { get; }
+    public static ReactiveUI.Primitives.Reactive.Concurrency.DispatcherSequencer Main { get; }
     public System.Windows.Threading.DispatcherPriority Priority { get; }
     [System.Runtime.CompilerServices.MethodImpl(System.Runtime.CompilerServices.MethodImplOptions.AggressiveInlining)]
     public override System.IDisposable Schedule<TState>(TState state, System.Func<System.Reactive.Concurrency.IScheduler, TState, System.IDisposable> action) { }

src/ReactiveUI.Primitives.Wpf.Reactive/PublicAPI/net11.0-windows/PublicAPI.txt

@@ -8,6 +8,8 @@ public sealed class DispatcherSequencer : System.Reactive.Concurrency.LocalSched
     public DispatcherSequencer(System.Windows.Threading.Dispatcher dispatcher) { }
     public DispatcherSequencer(System.Windows.Threading.Dispatcher dispatcher, System.Windows.Threading.DispatcherPriority priority) { }
     public System.Windows.Threading.Dispatcher Dispatcher { get; }
+    public static ReactiveUI.Primitives.Reactive.Concurrency.DispatcherSequencer Current { get; }
+    public static ReactiveUI.Primitives.Reactive.Concurrency.DispatcherSequencer Main { get; }
     public System.Windows.Threading.DispatcherPriority Priority { get; }
     [System.Runtime.CompilerServices.MethodImpl(System.Runtime.CompilerServices.MethodImplOptions.AggressiveInlining)]
     public override System.IDisposable Schedule<TState>(TState state, System.Func<System.Reactive.Concurrency.IScheduler, TState, System.IDisposable> action) { }

src/ReactiveUI.Primitives.Wpf.Reactive/PublicAPI/net462/PublicAPI.txt

@@ -7,6 +7,8 @@ public sealed class DispatcherSequencer : System.Reactive.Concurrency.LocalSched
     public DispatcherSequencer(System.Windows.Threading.Dispatcher dispatcher) { }
     public DispatcherSequencer(System.Windows.Threading.Dispatcher dispatcher, System.Windows.Threading.DispatcherPriority priority) { }
     public System.Windows.Threading.Dispatcher Dispatcher { get; }
+    public static ReactiveUI.Primitives.Reactive.Concurrency.DispatcherSequencer Current { get; }
+    public static ReactiveUI.Primitives.Reactive.Concurrency.DispatcherSequencer Main { get; }
     public System.Windows.Threading.DispatcherPriority Priority { get; }
     [System.Runtime.CompilerServices.MethodImpl(System.Runtime.CompilerServices.MethodImplOptions.AggressiveInlining)]
     public override System.IDisposable Schedule<TState>(TState state, System.Func<System.Reactive.Concurrency.IScheduler, TState, System.IDisposable> action) { }

src/ReactiveUI.Primitives.Wpf.Reactive/PublicAPI/net472/PublicAPI.txt

@@ -7,6 +7,8 @@ public sealed class DispatcherSequencer : System.Reactive.Concurrency.LocalSched
     public DispatcherSequencer(System.Windows.Threading.Dispatcher dispatcher) { }
     public DispatcherSequencer(System.Windows.Threading.Dispatcher dispatcher, System.Windows.Threading.DispatcherPriority priority) { }
     public System.Windows.Threading.Dispatcher Dispatcher { get; }
+    public static ReactiveUI.Primitives.Reactive.Concurrency.DispatcherSequencer Current { get; }
+    public static ReactiveUI.Primitives.Reactive.Concurrency.DispatcherSequencer Main { get; }
     public System.Windows.Threading.DispatcherPriority Priority { get; }
     [System.Runtime.CompilerServices.MethodImpl(System.Runtime.CompilerServices.MethodImplOptions.AggressiveInlining)]
     public override System.IDisposable Schedule<TState>(TState state, System.Func<System.Reactive.Concurrency.IScheduler, TState, System.IDisposable> action) { }

src/ReactiveUI.Primitives.Wpf.Reactive/PublicAPI/net48/PublicAPI.txt

@@ -7,6 +7,8 @@ public sealed class DispatcherSequencer : System.Reactive.Concurrency.LocalSched
     public DispatcherSequencer(System.Windows.Threading.Dispatcher dispatcher) { }
     public DispatcherSequencer(System.Windows.Threading.Dispatcher dispatcher, System.Windows.Threading.DispatcherPriority priority) { }
     public System.Windows.Threading.Dispatcher Dispatcher { get; }
+    public static ReactiveUI.Primitives.Reactive.Concurrency.DispatcherSequencer Current { get; }
+    public static ReactiveUI.Primitives.Reactive.Concurrency.DispatcherSequencer Main { get; }
     public System.Windows.Threading.DispatcherPriority Priority { get; }
     [System.Runtime.CompilerServices.MethodImpl(System.Runtime.CompilerServices.MethodImplOptions.AggressiveInlining)]
     public override System.IDisposable Schedule<TState>(TState state, System.Func<System.Reactive.Concurrency.IScheduler, TState, System.IDisposable> action) { }

src/ReactiveUI.Primitives.Wpf.Reactive/PublicAPI/net481/PublicAPI.txt

@@ -7,6 +7,8 @@ public sealed class DispatcherSequencer : System.Reactive.Concurrency.LocalSched
     public DispatcherSequencer(System.Windows.Threading.Dispatcher dispatcher) { }
     public DispatcherSequencer(System.Windows.Threading.Dispatcher dispatcher, System.Windows.Threading.DispatcherPriority priority) { }
     public System.Windows.Threading.Dispatcher Dispatcher { get; }
+    public static ReactiveUI.Primitives.Reactive.Concurrency.DispatcherSequencer Current { get; }
+    public static ReactiveUI.Primitives.Reactive.Concurrency.DispatcherSequencer Main { get; }
     public System.Windows.Threading.DispatcherPriority Priority { get; }
     [System.Runtime.CompilerServices.MethodImpl(System.Runtime.CompilerServices.MethodImplOptions.AggressiveInlining)]
     public override System.IDisposable Schedule<TState>(TState state, System.Func<System.Reactive.Concurrency.IScheduler, TState, System.IDisposable> action) { }

src/ReactiveUI.Primitives.Wpf.Reactive/PublicAPI/net8.0-windows/PublicAPI.txt

@@ -8,6 +8,8 @@ public sealed class DispatcherSequencer : System.Reactive.Concurrency.LocalSched
     public DispatcherSequencer(System.Windows.Threading.Dispatcher dispatcher) { }
     public DispatcherSequencer(System.Windows.Threading.Dispatcher dispatcher, System.Windows.Threading.DispatcherPriority priority) { }
     public System.Windows.Threading.Dispatcher Dispatcher { get; }
+    public static ReactiveUI.Primitives.Reactive.Concurrency.DispatcherSequencer Current { get; }
+    public static ReactiveUI.Primitives.Reactive.Concurrency.DispatcherSequencer Main { get; }
     public System.Windows.Threading.DispatcherPriority Priority { get; }
     [System.Runtime.CompilerServices.MethodImpl(System.Runtime.CompilerServices.MethodImplOptions.AggressiveInlining)]
     public override System.IDisposable Schedule<TState>(TState state, System.Func<System.Reactive.Concurrency.IScheduler, TState, System.IDisposable> action) { }

src/ReactiveUI.Primitives.Wpf.Reactive/PublicAPI/net9.0-windows/PublicAPI.txt

@@ -8,6 +8,8 @@ public sealed class DispatcherSequencer : System.Reactive.Concurrency.LocalSched
     public DispatcherSequencer(System.Windows.Threading.Dispatcher dispatcher) { }
     public DispatcherSequencer(System.Windows.Threading.Dispatcher dispatcher, System.Windows.Threading.DispatcherPriority priority) { }
     public System.Windows.Threading.Dispatcher Dispatcher { get; }
+    public static ReactiveUI.Primitives.Reactive.Concurrency.DispatcherSequencer Current { get; }
+    public static ReactiveUI.Primitives.Reactive.Concurrency.DispatcherSequencer Main { get; }
     public System.Windows.Threading.DispatcherPriority Priority { get; }
     [System.Runtime.CompilerServices.MethodImpl(System.Runtime.CompilerServices.MethodImplOptions.AggressiveInlining)]
     public override System.IDisposable Schedule<TState>(TState state, System.Func<System.Reactive.Concurrency.IScheduler, TState, System.IDisposable> action) { }

src/ReactiveUI.Primitives.Wpf/Concurrency/DispatcherSequencer.cs

@@ -14,6 +14,13 @@ namespace ReactiveUI.Primitives.Concurrency;
 [System.Diagnostics.DebuggerDisplay("{DebuggerDisplay,nq}")]
 public sealed class DispatcherSequencer : ISequencer
 {
+    /// <summary>The shared main-thread sequencer, set once the application's dispatcher is available.</summary>
+    private static DispatcherSequencer? _main;
+
+    /// <summary>The sequencer for the calling thread's dispatcher, cached per thread.</summary>
+    [ThreadStatic]
+    private static DispatcherSequencer? _current;
+
     /// <summary>Coalescing dispatch engine.</summary>
     private DispatchSequencerState _state;
 
@@ -51,6 +58,23 @@ internal DispatcherSequencer(
         _state = new(this, post ?? Post, RunDrain, scheduleDelayed ?? ScheduleDelayed);
     }
 
+    /// <summary>Gets the shared sequencer for the WPF main (UI) thread.</summary>
+    /// <exception cref="InvalidOperationException">No application exists yet and the calling thread has no dispatcher.</exception>
+    /// <remarks>
+    /// Bound once to the running <see cref="System.Windows.Application"/>'s dispatcher, from any thread. Until an
+    /// application exists nothing is cached, and the calling thread's existing dispatcher is used instead.
+    /// </remarks>
+    public static DispatcherSequencer Main =>
+        Volatile.Read(ref _main) ?? BindMain(ref _main, System.Windows.Application.Current?.Dispatcher) ?? Current;
+
+    /// <summary>Gets the sequencer for the calling thread's dispatcher.</summary>
+    /// <exception cref="InvalidOperationException">The calling thread has no dispatcher.</exception>
+    /// <remarks>
+    /// Cached per thread, for applications that run UI on more than one thread. Never creates a dispatcher: a thread
+    /// that has none cannot run the scheduled work.
+    /// </remarks>
+    public static DispatcherSequencer Current => _current ??= new(ResolveCurrentDispatcher());
+
     /// <summary>Gets the dispatcher whose thread runs the scheduled work.</summary>
     public Dispatcher Dispatcher { get; }
 
@@ -75,6 +99,29 @@ internal DispatcherSequencer(
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public void Schedule(IWorkItem item, long dueTimestamp) => _state.Schedule(item, dueTimestamp);
 
+    /// <summary>Caches the shared main-thread sequencer for the application's dispatcher, keeping the first one bound.</summary>
+    /// <param name="slot">The field that holds the shared sequencer.</param>
+    /// <param name="applicationDispatcher">The application's dispatcher, or <see langword="null"/> when no application exists.</param>
+    /// <returns>The shared sequencer, or <see langword="null"/> when <paramref name="applicationDispatcher"/> is <see langword="null"/>.</returns>
+    internal static DispatcherSequencer? BindMain(ref DispatcherSequencer? slot, Dispatcher? applicationDispatcher)
+    {
+        if (applicationDispatcher is null)
+        {
+            return null;
+        }
+
+        DispatcherSequencer created = new(applicationDispatcher);
+        return Interlocked.CompareExchange(ref slot, created, null) ?? created;
+    }
+
+    /// <summary>Returns the calling thread's existing dispatcher without creating one.</summary>
+    /// <returns>The calling thread's dispatcher.</returns>
+    /// <exception cref="InvalidOperationException">The calling thread has no dispatcher.</exception>
+    internal static Dispatcher ResolveCurrentDispatcher() =>
+        Dispatcher.FromThread(Thread.CurrentThread)
+        ?? throw new InvalidOperationException(
+            "The calling thread has no WPF dispatcher. Use DispatcherSequencer.Main or Current from a UI thread, or after the Application is created.");
+
     /// <summary>Marshals the cached drain callback onto the dispatcher.</summary>
     /// <param name="drain">The drain callback.</param>
     /// <returns><see langword="true"/>, since the dispatcher always accepts the work.</returns>

src/ReactiveUI.Primitives.Wpf/PublicAPI/net10.0-windows/PublicAPI.txt

@@ -8,6 +8,8 @@ public sealed class DispatcherSequencer : ReactiveUI.Primitives.Concurrency.ISeq
     public DispatcherSequencer(System.Windows.Threading.Dispatcher dispatcher) { }
     public DispatcherSequencer(System.Windows.Threading.Dispatcher dispatcher, System.Windows.Threading.DispatcherPriority priority) { }
     public System.Windows.Threading.Dispatcher Dispatcher { get; }
+    public static ReactiveUI.Primitives.Concurrency.DispatcherSequencer Current { get; }
+    public static ReactiveUI.Primitives.Concurrency.DispatcherSequencer Main { get; }
     public System.DateTimeOffset Now { get; }
     public System.Windows.Threading.DispatcherPriority Priority { get; }
     public long Timestamp { get; }

src/ReactiveUI.Primitives.Wpf/PublicAPI/net11.0-windows/PublicAPI.txt

@@ -8,6 +8,8 @@ public sealed class DispatcherSequencer : ReactiveUI.Primitives.Concurrency.ISeq
     public DispatcherSequencer(System.Windows.Threading.Dispatcher dispatcher) { }
     public DispatcherSequencer(System.Windows.Threading.Dispatcher dispatcher, System.Windows.Threading.DispatcherPriority priority) { }
     public System.Windows.Threading.Dispatcher Dispatcher { get; }
+    public static ReactiveUI.Primitives.Concurrency.DispatcherSequencer Current { get; }
+    public static ReactiveUI.Primitives.Concurrency.DispatcherSequencer Main { get; }
     public System.DateTimeOffset Now { get; }
     public System.Windows.Threading.DispatcherPriority Priority { get; }
     public long Timestamp { get; }

src/ReactiveUI.Primitives.Wpf/PublicAPI/net462/PublicAPI.txt

@@ -7,6 +7,8 @@ public sealed class DispatcherSequencer : ReactiveUI.Primitives.Concurrency.ISeq
     public DispatcherSequencer(System.Windows.Threading.Dispatcher dispatcher) { }
     public DispatcherSequencer(System.Windows.Threading.Dispatcher dispatcher, System.Windows.Threading.DispatcherPriority priority) { }
     public System.Windows.Threading.Dispatcher Dispatcher { get; }
+    public static ReactiveUI.Primitives.Concurrency.DispatcherSequencer Current { get; }
+    public static ReactiveUI.Primitives.Concurrency.DispatcherSequencer Main { get; }
     public System.DateTimeOffset Now { get; }
     public System.Windows.Threading.DispatcherPriority Priority { get; }
     public long Timestamp { get; }