Add simple SDL_tray test

Just to get a feel for the API and to see how it works/behaves across operating systems.

SDL3-CS.Tests/TestTray.cs

@@ -0,0 +1,36 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using NUnit.Framework;
+using static SDL.SDL3;
+
+namespace SDL.Tests
+{
+    [Explicit("Uses an interactive tray icon.")]
+    public unsafe class TestTray : TrayTest
+    {
+        private volatile bool running;
+
+        [Test]
+        public void TestBasic()
+        {
+            var checkbox = SDL_InsertTrayEntryAt(RootMenu, -1, "Check box?", SDL_TrayEntryFlags.SDL_TRAYENTRY_CHECKBOX);
+            Assert.That(checkbox != null, SDL_GetError);
+            SetCallback(checkbox, () => Console.WriteLine("Checkbox was toggled."));
+
+            var separator = SDL_InsertTrayEntryAt(RootMenu, -1, (byte*)null, 0);
+            Assert.That(separator != null, SDL_GetError);
+
+            var exit = SDL_InsertTrayEntryAt(RootMenu, -1, "Exit tray", SDL_TrayEntryFlags.SDL_TRAYENTRY_BUTTON);
+            Assert.That(exit != null, SDL_GetError);
+            SetCallback(exit, () => running = false);
+
+            running = true;
+
+            while (running)
+            {
+                SDL_PumpEvents();
+            }
+        }
+    }
+}

SDL3-CS.Tests/TrayTest.cs

@@ -0,0 +1,52 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
+using NUnit.Framework;
+
+namespace SDL.Tests
+{
+    [TestFixture]
+    public abstract unsafe class TrayTest
+    {
+        private SDL_Tray* tray;
+        protected SDL_TrayMenu* RootMenu { get; private set; }
+
+        [SetUp]
+        public void SetUp()
+        {
+            Assert.That(SDL3.SDL_Init(SDL_InitFlags.SDL_INIT_VIDEO), SDL3.SDL_GetError);
+            tray = SDL3.SDL_CreateTray(null, "Test tray");
+            Assert.That(tray != null, SDL3.SDL_GetError);
+            RootMenu = SDL3.SDL_CreateTrayMenu(tray);
+            Assert.That(RootMenu != null, SDL3.SDL_GetError);
+        }
+
+        protected static void SetCallback(SDL_TrayEntry* entry, Action callback)
+        {
+            var objectHandle = new ObjectHandle<Action>(callback, GCHandleType.Normal);
+            SDL3.SDL_SetTrayEntryCallback(entry, &nativeOnSelect, objectHandle.Handle); // this is leaking object handles, fine for tests
+        }
+
+        [UnmanagedCallersOnly(CallConvs = [typeof(CallConvCdecl)])]
+        private static void nativeOnSelect(IntPtr userdata, SDL_TrayEntry* entry)
+        {
+            var objectHandle = new ObjectHandle<Action>(userdata, true);
+
+            if (objectHandle.GetTarget(out var action))
+                action();
+            else
+                Assert.Fail("Accessing disposed object handle.");
+        }
+
+        [TearDown]
+        public void TearDown()
+        {
+            if (tray != null)
+                SDL3.SDL_DestroyTray(tray);
+
+            SDL3.SDL_Quit();
+        }
+    }
+}