Add RulesetStore for use where realm is not present (ie. other projects)

osu.Game/Rulesets/AssemblyRulesetStore.cs

@@ -0,0 +1,35 @@
+// 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.Generic;
+using System.Linq;
+using osu.Framework.Extensions.ObjectExtensions;
+using osu.Framework.Platform;
+
+#nullable enable
+
+namespace osu.Game.Rulesets
+{
+    public class AssemblyRulesetStore : RulesetStore
+    {
+        public override IEnumerable<RulesetInfo> AvailableRulesets => availableRulesets;
+
+        private readonly List<RulesetInfo> availableRulesets = new List<RulesetInfo>();
+
+        public AssemblyRulesetStore(Storage? storage = null)
+            : base(storage)
+
+        {
+            List<Ruleset> instances = LoadedAssemblies.Values
+                                                      .Select(r => Activator.CreateInstance(r) as Ruleset)
+                                                      .Where(r => r != null)
+                                                      .Select(r => r.AsNonNull())
+                                                      .ToList();
+
+            // add all legacy rulesets first to ensure they have exclusive choice of primary key.
+            foreach (var r in instances.Where(r => r is ILegacyRuleset))
+                availableRulesets.Add(new RulesetInfo(r.RulesetInfo.ShortName, r.RulesetInfo.Name, r.RulesetInfo.InstantiationInfo, r.RulesetInfo.OnlineID));
+        }
+    }
+}