Proof of concept realm subscriptions via Register

osu.Game/Database/EmptyRealmSet.cs

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+// 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;
+using System.Collections;
+using System.Collections.Generic;
+using System.Collections.Specialized;
+using System.ComponentModel;
+using Realms;
+using Realms.Schema;
+
+#nullable enable
+
+namespace osu.Game.Database
+{
+    public class EmptyRealmSet<T> : IRealmCollection<T>
+    {
+        private static List<T> emptySet => new List<T>();
+
+        public IEnumerator<T> GetEnumerator()
+        {
+            return emptySet.GetEnumerator();
+        }
+
+        IEnumerator IEnumerable.GetEnumerator()
+        {
+            return ((IEnumerable)emptySet).GetEnumerator();
+        }
+
+        public int Count => emptySet.Count;
+
+        public T this[int index] => emptySet[index];
+
+        public event NotifyCollectionChangedEventHandler? CollectionChanged
+        {
+            add => throw new NotImplementedException();
+            remove => throw new NotImplementedException();
+        }
+
+        public event PropertyChangedEventHandler? PropertyChanged
+        {
+            add => throw new NotImplementedException();
+            remove => throw new NotImplementedException();
+        }
+
+        public int IndexOf(object item)
+        {
+            return emptySet.IndexOf((T)item);
+        }
+
+        public bool Contains(object item)
+        {
+            return emptySet.Contains((T)item);
+        }
+
+        public IRealmCollection<T> Freeze()
+        {
+            throw new NotImplementedException();
+        }
+
+        public IDisposable SubscribeForNotifications(NotificationCallbackDelegate<T> callback)
+        {
+            throw new NotImplementedException();
+        }
+
+        public bool IsValid => throw new NotImplementedException();
+
+        public Realm Realm => throw new NotImplementedException();
+
+        public ObjectSchema ObjectSchema => throw new NotImplementedException();
+
+        public bool IsFrozen => throw new NotImplementedException();
+    }
+}