Factor out pooling logic from Playfield

osu.Game/Rulesets/Objects/Pooling/HitObjectEntryManager.cs

@@ -0,0 +1,69 @@
+// 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.
+
+#nullable enable
+
+using System;
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+
+namespace osu.Game.Rulesets.Objects.Pooling
+{
+    /// <summary>
+    /// Manages a mapping between <see cref="HitObject"/> and <see cref="HitObjectLifetimeEntry"/>
+    /// </summary>
+    internal class HitObjectEntryManager
+    {
+        /// <summary>
+        /// All entries, including entries of the nested hit objects.
+        /// </summary>
+        public IEnumerable<HitObjectLifetimeEntry> AllEntries => entryMap.Values;
+
+        public event Action<HitObjectLifetimeEntry, HitObject?>? OnEntryAdded;
+        public event Action<HitObjectLifetimeEntry, HitObject?>? OnEntryRemoved;
+
+        private readonly Func<HitObject, HitObjectLifetimeEntry> createLifetimeEntry;
+
+        private readonly Dictionary<HitObject, HitObjectLifetimeEntry> entryMap = new Dictionary<HitObject, HitObjectLifetimeEntry>();
+        private readonly Dictionary<HitObject, HitObject> parentMap = new Dictionary<HitObject, HitObject>();
+
+        public HitObjectEntryManager(Func<HitObject, HitObjectLifetimeEntry> createLifetimeEntry)
+        {
+            this.createLifetimeEntry = createLifetimeEntry;
+        }
+
+        public HitObjectLifetimeEntry Add(HitObject hitObject, HitObject? parentHitObject)
+        {
+            if (parentHitObject != null && !entryMap.TryGetValue(parentHitObject, out var parentEntry))
+                throw new InvalidOperationException($@"The parent {nameof(HitObject)} must be added to this {nameof(HitObjectEntryManager)} before nested {nameof(HitObject)} is added.");
+
+            if (entryMap.ContainsKey(hitObject))
+                throw new InvalidOperationException($@"The {nameof(HitObject)} is already added to this {nameof(HitObjectEntryManager)}.");
+
+            if (parentHitObject != null)
+                parentMap[hitObject] = parentHitObject;
+
+            var entry = createLifetimeEntry(hitObject);
+            entryMap[hitObject] = entry;
+
+            OnEntryAdded?.Invoke(entry, parentHitObject);
+            return entry;
+        }
+
+        public bool Remove(HitObject hitObject)
+        {
+            if (!entryMap.TryGetValue(hitObject, out var entry))
+                return false;
+
+            parentMap.Remove(hitObject, out var parentHitObject);
+
+            OnEntryRemoved?.Invoke(entry, parentHitObject);
+            return true;
+        }
+
+        public bool TryGet(HitObject hitObject, [MaybeNullWhen(false)] out HitObjectLifetimeEntry entry)
+        {
+            return entryMap.TryGetValue(hitObject, out entry);
+        }
+    }
+}