Add test coverage of various selection examples

Where possible I've tried to match the test and method names of
`TestSceneBeatmapCarousel` for easy coverage comparison.

Author: peppy

osu.Game.Tests/Visual/SongSelect/TestSceneBeatmapCarouselV2Selection.cs

@@ -0,0 +1,216 @@
+// 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.Linq;
+using NUnit.Framework;
+using osu.Framework.Testing;
+using osu.Game.Beatmaps;
+using osu.Game.Screens.SelectV2;
+using osuTK.Input;
+
+namespace osu.Game.Tests.Visual.SongSelect
+{
+    [TestFixture]
+    public partial class TestSceneBeatmapCarouselV2Selection : BeatmapCarouselV2TestScene
+    {
+        /// <summary>
+        /// Keyboard selection via up and down arrows doesn't actually change the selection until
+        /// the select key is pressed.
+        /// </summary>
+        [Test]
+        public void TestKeyboardSelectionKeyRepeat()
+        {
+            AddBeatmaps(10);
+            WaitForDrawablePanels();
+            checkNoSelection();
+
+            select();
+            checkNoSelection();
+
+            AddStep("press down arrow", () => InputManager.PressKey(Key.Down));
+            checkSelectionIterating(false);
+
+            AddStep("press up arrow", () => InputManager.PressKey(Key.Up));
+            checkSelectionIterating(false);
+
+            AddStep("release down arrow", () => InputManager.ReleaseKey(Key.Down));
+            checkSelectionIterating(false);
+
+            AddStep("release up arrow", () => InputManager.ReleaseKey(Key.Up));
+            checkSelectionIterating(false);
+
+            select();
+            checkHasSelection();
+        }
+
+        /// <summary>
+        /// Keyboard selection via left and right arrows moves between groups, updating the selection
+        /// immediately.
+        /// </summary>
+        [Test]
+        public void TestGroupSelectionKeyRepeat()
+        {
+            AddBeatmaps(10);
+            WaitForDrawablePanels();
+            checkNoSelection();
+
+            AddStep("press right arrow", () => InputManager.PressKey(Key.Right));
+            checkSelectionIterating(true);
+
+            AddStep("press left arrow", () => InputManager.PressKey(Key.Left));
+            checkSelectionIterating(true);
+
+            AddStep("release right arrow", () => InputManager.ReleaseKey(Key.Right));
+            checkSelectionIterating(true);
+
+            AddStep("release left arrow", () => InputManager.ReleaseKey(Key.Left));
+            checkSelectionIterating(false);
+        }
+
+        [Test]
+        public void TestCarouselRemembersSelection()
+        {
+            AddBeatmaps(10);
+            WaitForDrawablePanels();
+
+            selectNextGroup();
+
+            object? selection = null;
+
+            AddStep("store drawable selection", () => selection = getSelectedPanel()?.Item?.Model);
+
+            checkHasSelection();
+            AddAssert("drawable selection non-null", () => selection, () => Is.Not.Null);
+            AddAssert("drawable selection matches carousel selection", () => selection, () => Is.EqualTo(Carousel.CurrentSelection));
+
+            RemoveAllBeatmaps();
+            AddUntilStep("no drawable selection", getSelectedPanel, () => Is.Null);
+
+            AddBeatmaps(10);
+            WaitForDrawablePanels();
+
+            checkHasSelection();
+            AddAssert("no drawable selection", getSelectedPanel, () => Is.Null);
+
+            AddStep("add previous selection", () => BeatmapSets.Add(((BeatmapInfo)selection!).BeatmapSet!));
+
+            AddUntilStep("drawable selection restored", () => getSelectedPanel()?.Item?.Model, () => Is.EqualTo(selection));
+            AddAssert("drawable selection matches carousel selection", () => selection, () => Is.EqualTo(Carousel.CurrentSelection));
+
+            BeatmapCarouselPanel? getSelectedPanel() => Carousel.ChildrenOfType<BeatmapCarouselPanel>().SingleOrDefault(p => p.Selected.Value);
+        }
+
+        [Test]
+        public void TestTraversalBeyondStart()
+        {
+            const int total_set_count = 200;
+
+            AddBeatmaps(total_set_count);
+            WaitForDrawablePanels();
+
+            selectNextGroup();
+            waitForSelection(0, 0);
+            selectPrevGroup();
+            waitForSelection(total_set_count - 1, 0);
+        }
+
+        [Test]
+        public void TestTraversalBeyondEnd()
+        {
+            const int total_set_count = 200;
+
+            AddBeatmaps(total_set_count);
+            WaitForDrawablePanels();
+
+            selectPrevGroup();
+            waitForSelection(total_set_count - 1, 0);
+            selectNextGroup();
+            waitForSelection(0, 0);
+        }
+
+        [Test]
+        public void TestKeyboardSelection()
+        {
+            AddBeatmaps(10, 3);
+            WaitForDrawablePanels();
+
+            selectNextPanel();
+            selectNextPanel();
+            selectNextPanel();
+            selectNextPanel();
+            checkNoSelection();
+
+            select();
+            waitForSelection(3, 0);
+
+            selectNextPanel();
+            waitForSelection(3, 0);
+
+            select();
+            waitForSelection(3, 1);
+
+            selectNextPanel();
+            waitForSelection(3, 1);
+
+            select();
+            waitForSelection(3, 2);
+
+            selectNextPanel();
+            waitForSelection(3, 2);
+
+            select();
+            waitForSelection(4, 0);
+        }
+
+        [Test]
+        public void TestEmptyTraversal()
+        {
+            selectNextPanel();
+            checkNoSelection();
+
+            selectNextGroup();
+            checkNoSelection();
+
+            selectPrevPanel();
+            checkNoSelection();
+
+            selectPrevGroup();
+            checkNoSelection();
+        }
+
+        private void waitForSelection(int set, int? diff = null)
+        {
+            AddUntilStep($"selected is set{set}{(diff.HasValue ? $" diff{diff.Value}" : "")}", () =>
+            {
+                if (diff != null)
+                    return ReferenceEquals(Carousel.CurrentSelection, BeatmapSets[set].Beatmaps[diff.Value]);
+
+                return BeatmapSets[set].Beatmaps.Contains(Carousel.CurrentSelection);
+            });
+        }
+
+        private void selectNextPanel() => AddStep("select next panel", () => InputManager.Key(Key.Down));
+        private void selectPrevPanel() => AddStep("select prev panel", () => InputManager.Key(Key.Up));
+        private void selectNextGroup() => AddStep("select next group", () => InputManager.Key(Key.Right));
+        private void selectPrevGroup() => AddStep("select prev group", () => InputManager.Key(Key.Left));
+
+        private void select() => AddStep("select", () => InputManager.Key(Key.Enter));
+
+        private void checkNoSelection() => AddAssert("has no selection", () => Carousel.CurrentSelection, () => Is.Null);
+        private void checkHasSelection() => AddAssert("has selection", () => Carousel.CurrentSelection, () => Is.Not.Null);
+
+        private void checkSelectionIterating(bool isIterating)
+        {
+            object? selection = null;
+
+            for (int i = 0; i < 3; i++)
+            {
+                AddStep("store selection", () => selection = Carousel.CurrentSelection);
+                if (isIterating)
+                    AddUntilStep("selection changed", () => Carousel.CurrentSelection != selection);
+                else
+                    AddUntilStep("selection not changed", () => Carousel.CurrentSelection == selection);
+            }
+        }
+    }
+}