Add beat snap grid to osu!taiko editor

Addresses https://github.com/ppy/osu/discussions/25150.

osu.Game.Rulesets.Taiko/Edit/TaikoBeatSnapGrid.cs

@@ -0,0 +1,19 @@
+// 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.Collections.Generic;
+using osu.Framework.Graphics.Containers;
+using osu.Game.Rulesets.Edit;
+using osu.Game.Rulesets.Taiko.UI;
+using osu.Game.Screens.Edit.Compose.Components;
+
+namespace osu.Game.Rulesets.Taiko.Edit
+{
+    public partial class TaikoBeatSnapGrid : BeatSnapGrid
+    {
+        protected override IEnumerable<Container> GetTargetContainers(HitObjectComposer composer) => new[]
+        {
+            ((TaikoPlayfield)composer.Playfield).UnderlayElements
+        };
+    }
+}

osu.Game.Rulesets.Taiko/Edit/TaikoHitObjectComposer.cs

@@ -2,10 +2,14 @@
 // See the LICENCE file in the repository root for full licence text.
 
 using System.Collections.Generic;
+using System.Linq;
+using osu.Framework.Allocation;
+using osu.Framework.Input;
 using osu.Game.Beatmaps;
 using osu.Game.Rulesets.Edit;
 using osu.Game.Rulesets.Edit.Tools;
 using osu.Game.Rulesets.Mods;
+using osu.Game.Rulesets.Objects;
 using osu.Game.Rulesets.Taiko.Objects;
 using osu.Game.Rulesets.UI;
 using osu.Game.Screens.Edit.Compose.Components;
@@ -16,6 +20,9 @@ namespace osu.Game.Rulesets.Taiko.Edit
     {
         protected override bool ApplyHorizontalCentering => false;
 
+        private TaikoBeatSnapGrid beatSnapGrid = null!;
+        private InputManager inputManager = null!;
+
         public TaikoHitObjectComposer(TaikoRuleset ruleset)
             : base(ruleset)
         {
@@ -33,5 +40,41 @@ namespace osu.Game.Rulesets.Taiko.Edit
 
         protected override ComposeBlueprintContainer CreateBlueprintContainer()
             => new TaikoBlueprintContainer(this);
+
+        [BackgroundDependencyLoader]
+        private void load()
+        {
+            AddInternal(beatSnapGrid = new TaikoBeatSnapGrid());
+        }
+
+        protected override void LoadComplete()
+        {
+            base.LoadComplete();
+
+            inputManager = GetContainingInputManager();
+        }
+
+        protected override void UpdateAfterChildren()
+        {
+            base.UpdateAfterChildren();
+
+            if (BlueprintContainer.CurrentTool is SelectTool)
+            {
+                if (EditorBeatmap.SelectedHitObjects.Any())
+                {
+                    beatSnapGrid.SelectionTimeRange = (EditorBeatmap.SelectedHitObjects.Min(h => h.StartTime), EditorBeatmap.SelectedHitObjects.Max(h => h.GetEndTime()));
+                }
+                else
+                    beatSnapGrid.SelectionTimeRange = null;
+            }
+            else
+            {
+                var result = FindSnappedPositionAndTime(inputManager.CurrentState.Mouse.Position);
+                if (result.Time is double time)
+                    beatSnapGrid.SelectionTimeRange = (time, time);
+                else
+                    beatSnapGrid.SelectionTimeRange = null;
+            }
+        }
     }
 }

osu.Game.Rulesets.Taiko/UI/TaikoPlayfield.cs

@@ -40,6 +40,8 @@ namespace osu.Game.Rulesets.Taiko.UI
         /// </summary>
         public Bindable<bool> ClassicHitTargetPosition = new BindableBool();
 
+        public Container UnderlayElements { get; private set; } = null!;
+
         private Container<HitExplosion> hitExplosionContainer;
         private Container<KiaiHitExplosion> kiaiExplosionContainer;
         private JudgementContainer<DrawableTaikoJudgement> judgementContainer;
@@ -130,7 +132,14 @@ namespace osu.Game.Rulesets.Taiko.UI
                         {
                             Name = "Bar line content",
                             RelativeSizeAxes = Axes.Both,
-                            Child = barLinePlayfield = new BarLinePlayfield(),
+                            Children = new Drawable[]
+                            {
+                                UnderlayElements = new Container
+                                {
+                                    RelativeSizeAxes = Axes.Both,
+                                },
+                                barLinePlayfield = new BarLinePlayfield(),
+                            }
                         },
                         hitObjectContent = new Container
                         {

osu.Game/Screens/Edit/Compose/Components/BeatSnapGrid.cs

@@ -173,9 +173,6 @@ namespace osu.Game.Screens.Edit.Compose.Components
             public DrawableGridLine()
                 : base(new HitObject())
             {
-                RelativeSizeAxes = Axes.X;
-                Height = 2;
-
                 AddInternal(new Box { RelativeSizeAxes = Axes.Both });
             }
 
@@ -191,6 +188,19 @@ namespace osu.Game.Screens.Edit.Compose.Components
                 Origin = Anchor = direction.NewValue == ScrollingDirection.Up
                     ? Anchor.TopLeft
                     : Anchor.BottomLeft;
+
+                bool isHorizontal = direction.NewValue == ScrollingDirection.Left || direction.NewValue == ScrollingDirection.Right;
+
+                if (isHorizontal)
+                {
+                    RelativeSizeAxes = Axes.Y;
+                    Width = 2;
+                }
+                else
+                {
+                    RelativeSizeAxes = Axes.X;
+                    Height = 2;
+                }
             }
 
             protected override void UpdateInitialTransforms()