Minor adjustments to changes in master + fix failing test (need to tick the timer manually) + add some comments

Avalonia.sln

@@ -269,7 +269,6 @@ EndProject
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Avalonia.Headless.XUnit.UnitTests", "tests\Avalonia.Headless.XUnit.UnitTests\Avalonia.Headless.XUnit.UnitTests.csproj", "{EBA7613E-C36C-4E0C-AB45-71B143F86219}"
 EndProject
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Avalonia.Headless.NUnit.UnitTests", "tests\Avalonia.Headless.NUnit.UnitTests\Avalonia.Headless.NUnit.UnitTests.csproj", "{47025FBC-2130-42EE-98C9-D3989B3B9446}"
-Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Avalonia.Headless.UnitTests", "tests\Avalonia.Headless.UnitTests\Avalonia.Headless.UnitTests.csproj", "{3B2405E8-9E7A-46D1-8E2D-EF9ED124C9F2}"
 EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
@@ -654,10 +653,6 @@ Global
 		{F47F8316-4D4B-4026-8EF3-16B2CFDA8119}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{F47F8316-4D4B-4026-8EF3-16B2CFDA8119}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{F47F8316-4D4B-4026-8EF3-16B2CFDA8119}.Release|Any CPU.Build.0 = Release|Any CPU
-		{3B2405E8-9E7A-46D1-8E2D-EF9ED124C9F2}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
-		{3B2405E8-9E7A-46D1-8E2D-EF9ED124C9F2}.Debug|Any CPU.Build.0 = Debug|Any CPU
-		{3B2405E8-9E7A-46D1-8E2D-EF9ED124C9F2}.Release|Any CPU.ActiveCfg = Release|Any CPU
-		{3B2405E8-9E7A-46D1-8E2D-EF9ED124C9F2}.Release|Any CPU.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE
@@ -741,7 +736,6 @@ Global
 		{EBA7613E-C36C-4E0C-AB45-71B143F86219} = {C5A00AC3-B34C-4564-9BDD-2DA473EF4D8B}
 		{47025FBC-2130-42EE-98C9-D3989B3B9446} = {C5A00AC3-B34C-4564-9BDD-2DA473EF4D8B}
 		{F47F8316-4D4B-4026-8EF3-16B2CFDA8119} = {FF237916-7150-496B-89ED-6CA3292896E7}
-		{3B2405E8-9E7A-46D1-8E2D-EF9ED124C9F2} = {C5A00AC3-B34C-4564-9BDD-2DA473EF4D8B}
 	EndGlobalSection
 	GlobalSection(ExtensibilityGlobals) = postSolution
 		SolutionGuid = {87366D66-1391-4D90-8999-95A620AD786A}

src/Headless/Avalonia.Headless.NUnit/AvaloniaTestMethodCommand.cs

@@ -9,12 +9,20 @@ using NUnit.Framework.Internal.Commands;
 
 namespace Avalonia.Headless.NUnit;
 
-internal class AvaloniaTestMethodCommand : DelegatingTestCommand
+internal class AvaloniaTestMethodCommand : TestCommand
 {
     private readonly HeadlessUnitTestSession _session;
+    private readonly TestCommand _innerCommand;
     private readonly List<Action> _beforeTest;
     private readonly List<Action> _afterTest;
 
+    // There are multiple problems with NUnit integration at the moment when we wrote this integration.
+    // NUnit doesn't have extensibility API for running on custom dispatcher/sync-context.
+    // See https://github.com/nunit/nunit/issues/2917 https://github.com/nunit/nunit/issues/2774
+    // To workaround that we had to replace inner TestMethodCommand with our own implementation while keeping original hierarchy of commands.
+    // Which will respect proper async/await awaiting code that works with our session and can be block-awaited to fit in NUnit.
+    // Also, we need to push BeforeTest/AfterTest callbacks to the very same session call.
+    // I hope there will be a better solution without reflection, but for now that's it.
     private static FieldInfo s_innerCommand = typeof(DelegatingTestCommand)
         .GetField("innerCommand", BindingFlags.Instance | BindingFlags.NonPublic)!;
     private static FieldInfo s_beforeTest = typeof(BeforeAndAfterTestCommand)
@@ -27,9 +35,10 @@ internal class AvaloniaTestMethodCommand : DelegatingTestCommand
         TestCommand innerCommand,
         List<Action> beforeTest,
         List<Action> afterTest)
-        : base(innerCommand)
+        : base(innerCommand.Test)
     {
         _session = session;
+        _innerCommand = innerCommand;
         _beforeTest = beforeTest;
         _afterTest = afterTest;
     }
@@ -78,10 +87,10 @@ internal class AvaloniaTestMethodCommand : DelegatingTestCommand
     {
         _beforeTest.ForEach(a => a());
         
-        var testMethod = innerCommand.Test.Method;
+        var testMethod = _innerCommand.Test.Method;
         var methodInfo = testMethod!.MethodInfo;
 
-        var result = methodInfo.Invoke(context.TestObject, innerCommand.Test.Arguments);
+        var result = methodInfo.Invoke(context.TestObject, _innerCommand.Test.Arguments);
         // Only Task, non generic ValueTask are supported in async context. No ValueTask<> nor F# tasks.
         if (result is Task task)
         {

src/Headless/Avalonia.Headless/AvaloniaHeadlessPlatform.cs

@@ -21,20 +21,21 @@ namespace Avalonia.Headless
             private Action? _forceTick; 
             protected override IDisposable StartCore(Action<TimeSpan> tick)
             {
-                bool cancelled = false;
                 var st = Stopwatch.StartNew();
                 _forceTick = () => tick(st.Elapsed);
-                DispatcherTimer.Run(() =>
+
+                var timer = new DispatcherTimer(DispatcherPriority.Render)
                 {
-                    if (cancelled)
-                        return false;
-                    tick(st.Elapsed);
-                    return !cancelled;
-                }, TimeSpan.FromSeconds(1.0 / _framesPerSecond), DispatcherPriority.Render);
+                    Interval = TimeSpan.FromSeconds(1.0 / _framesPerSecond),
+                    Tag = "HeadlessRenderTimer"
+                };
+                timer.Tick += (s, e) => tick(st.Elapsed);
+                timer.Start();
+
                 return Disposable.Create(() =>
                 {
                     _forceTick = null;
-                    cancelled = true;
+                    timer.Stop();
                 });
             }
 

src/Headless/Avalonia.Headless/HeadlessUnitTestSession.cs

@@ -7,6 +7,7 @@ using System.Runtime.ExceptionServices;
 using System.Threading;
 using System.Threading.Tasks;
 using Avalonia.Controls.Platform;
+using Avalonia.Metadata;
 using Avalonia.Reactive;
 using Avalonia.Rendering;
 using Avalonia.Threading;
@@ -15,10 +16,10 @@ namespace Avalonia.Headless;
 
 /// <summary>
 /// Headless unit test session that needs to be used by the actual testing framework.
-/// All UI tests are supposed to be executed from the <see cref="Dispatcher"/> or <see cref="SynchronizationContext"/>
-/// to keep execution flow on the UI thread.
+/// All UI tests are supposed to be executed from one of the <see cref="Dispatch"/> methods to keep execution flow on the UI thread.
 /// Disposing unit test session stops internal dispatcher loop. 
 /// </summary>
+[Unstable("This API is experimental and might be unstable. Use on your risk. API might or might not be changed in a minor update.")]
 public sealed class HeadlessUnitTestSession : IDisposable
 {
     private static readonly ConcurrentDictionary<Assembly, HeadlessUnitTestSession> s_session = new();
@@ -82,14 +83,13 @@ public sealed class HeadlessUnitTestSession : IDisposable
             using var application = EnsureApplication();
 
             var cts = new CancellationTokenSource();
-            using var globalCts = token.Register(s => ((CancellationTokenSource)s!).Cancel(), cts, true);
-            using var localCts = cancellationToken.Register(s => ((CancellationTokenSource)s!).Cancel(), cts, true);
+            using var globalCts = token.Register(s => ((CancellationTokenSource)s!).Cancel(), cts);
+            using var localCts = cancellationToken.Register(s => ((CancellationTokenSource)s!).Cancel(), cts);
 
             try
             {
                 var task = action();
-                task.ContinueWith((_, s) => ((CancellationTokenSource)s!).Cancel(), cts,
-                    TaskScheduler.FromCurrentSynchronizationContext());
+                task.ContinueWith((_, s) => ((CancellationTokenSource)s!).Cancel(), cts);
 
                 if (cts.IsCancellationRequested)
                 {
@@ -97,7 +97,7 @@ public sealed class HeadlessUnitTestSession : IDisposable
                 }
 
                 var frame = new DispatcherFrame();
-                using var innerCts = cts.Token.Register(() => frame.Continue = false, true);
+                using var innerCts = cts.Token.Register(() => frame.Continue = false);
                 Dispatcher.UIThread.PushFrame(frame);
 
                 var result = task.GetAwaiter().GetResult();

src/Headless/Avalonia.Headless/HeadlessWindowExtensions.cs

@@ -89,6 +89,8 @@ public static class HeadlessWindowExtensions
 
     private static void RunJobsOnImpl(this TopLevel topLevel, Action<IHeadlessWindow> action)
     {
+        Dispatcher.UIThread.RunJobs();
+        AvaloniaHeadlessPlatform.ForceRenderTimerTick();
         Dispatcher.UIThread.RunJobs();
         action(GetImpl(topLevel));
         Dispatcher.UIThread.RunJobs();

tests/Avalonia.Headless.NUnit.UnitTests/Avalonia.Headless.NUnit.UnitTests.csproj

@@ -19,6 +19,8 @@
 
   <ItemGroup>
     <Compile Include="..\Avalonia.Headless.UnitTests\**\*.cs" />
+    <Compile Remove="..\Avalonia.Headless.UnitTests\bin\**\*.cs" />
+    <Compile Remove="..\Avalonia.Headless.UnitTests\obj\**\*.cs" />
   </ItemGroup>
   
   <ItemGroup>

tests/Avalonia.Headless.UnitTests/InputTests.cs

@@ -13,6 +13,7 @@ public class InputTests
 #endif
 {
     private Window _window;
+    private Application _setupApp;
 
 #if NUNIT
     [SetUp]
@@ -21,6 +22,7 @@ public class InputTests
     public InputTests()
 #endif
     {
+        _setupApp = Application.Current;
         Dispatcher.UIThread.VerifyAccess();
         _window = new Window
         {
@@ -32,6 +34,8 @@ public class InputTests
     [AvaloniaFact]
     public void Should_Click_Button_On_Window()
     {
+        Assert.True(_setupApp == Application.Current);
+        
         var buttonClicked = false;
         var button = new Button
         {
@@ -57,6 +61,8 @@ public class InputTests
     public void Dispose()
 #endif
     {
+        Assert.True(_setupApp == Application.Current);
+
         Dispatcher.UIThread.VerifyAccess();
         _window.Close();
     }