Add InvokeOnUIAttribute for NUnit

- so you can more easily run tests on the UI thread.

src/Eto.UnitTest.NUnit/Eto.UnitTest.NUnit.csproj

@@ -12,7 +12,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="NUnit" Version="3.11.0" />
+    <PackageReference Include="NUnit" Version="3.12.0" />
   </ItemGroup>
 
   <ItemGroup>

src/Eto.UnitTest.NUnit/InvokeOnUIAttribute.cs

@@ -0,0 +1,58 @@
+using System;
+using System.Runtime.ExceptionServices;
+using Eto.Forms;
+using NUnit.Framework.Interfaces;
+using NUnit.Framework.Internal;
+using NUnit.Framework.Internal.Commands;
+
+namespace Eto.UnitTest.NUnit
+{
+
+    /// <summary>
+    /// Runs the test on the UI thread
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Method | AttributeTargets.Class, Inherited = true, AllowMultiple = false)]
+    public sealed class InvokeOnUIAttribute : Attribute, IWrapSetUpTearDown
+    {
+        /// <summary>
+        /// Wraps the specified command
+        /// </summary>
+        /// <param name="command">command to wrap</param>
+        /// <returns>A new command that wraps the specified command in a UI invoke</returns>
+        public TestCommand Wrap(TestCommand command) => new RunOnUICommand(command);
+
+        class RunOnUICommand : DelegatingTestCommand
+        {
+            public RunOnUICommand(TestCommand innerCommand)
+                : base(innerCommand)
+            {
+            }
+
+            public override TestResult Execute(TestExecutionContext context)
+            {
+                Exception exception = null;
+
+                var result = Application.Instance.Invoke(() =>
+                {
+                    try
+                    {
+                        context.EstablishExecutionEnvironment();
+                        return innerCommand.Execute(context);
+                    }
+                    catch (Exception ex)
+                    {
+                        exception = ex;
+                        return null;
+                    }
+                });
+
+                if (exception != null)
+                {
+                    ExceptionDispatchInfo.Capture(exception).Throw();
+                }
+
+                return result;
+            }
+        }
+    }
+}

src/Eto.UnitTest.NUnit/MyTests.cs

@@ -1,6 +1,8 @@
 using System;
 using NUnit.Framework;
 using System.Threading;
+using Eto.Forms;
+using System.Threading.Tasks;
 
 #if INCLUDE_TESTS
 
@@ -121,6 +123,39 @@ public void Test4()
             Thread.Sleep(1000);
         }
     }
+
+    [TestFixture]
+    public class UITests
+    {
+        [Test]
+        [InvokeOnUI]
+        public void RunThisOnUIThread()
+        {
+            Thread currentThread = Thread.CurrentThread;
+            Thread uiThread = null;
+            Application.Instance.Invoke(() =>
+            {
+                uiThread = Thread.CurrentThread;
+            });
+
+            Assert.IsNotNull(uiThread, "#1. UI thread was not found");
+            Assert.AreEqual(currentThread.ManagedThreadId, uiThread.ManagedThreadId, "#2. UI Thread should be the same as the test thread");
+        }
+
+        [Test]
+        public void RunThisOnTestThread()
+        {
+            Thread currentThread = Thread.CurrentThread;
+            Thread uiThread = null;
+            Application.Instance.Invoke(() =>
+            {
+                uiThread = Thread.CurrentThread;
+            });
+
+            Assert.IsNotNull(uiThread, "#1. UI thread was not found");
+            Assert.AreNotEqual(currentThread.ManagedThreadId, uiThread.ManagedThreadId, "#2. UI Thread should not be the same as the test thread");
+        }
+    }
 }
 
 #endif