Abstract statefulness of new menu item type

osu.Game/Graphics/UserInterface/StatefulMenuItem.cs

@@ -0,0 +1,56 @@
+// 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 osu.Framework.Bindables;
+using osu.Framework.Graphics.Sprites;
+
+namespace osu.Game.Graphics.UserInterface
+{
+    public abstract class StatefulMenuItem : OsuMenuItem
+    {
+        public readonly Bindable<object> State = new Bindable<object>();
+
+        protected StatefulMenuItem(string text, MenuItemType type = MenuItemType.Standard)
+            : this(text, type, null)
+        {
+        }
+
+        protected StatefulMenuItem(string text, MenuItemType type, Func<object, object> changeStateFunc)
+            : base(text, type)
+        {
+            Action.Value = () => State.Value = changeStateFunc?.Invoke(State.Value) ?? State.Value;
+        }
+
+        public abstract IconUsage? GetIconForState(object state);
+    }
+
+    public abstract class StatefulMenuItem<T> : StatefulMenuItem
+        where T : struct
+    {
+        public new readonly Bindable<T> State = new Bindable<T>();
+
+        protected StatefulMenuItem(string text, MenuItemType type = MenuItemType.Standard)
+            : this(text, type, null)
+        {
+        }
+
+        protected StatefulMenuItem(string text, MenuItemType type, Func<T, T> changeStateFunc)
+            : base(text, type, o => changeStateFunc?.Invoke((T)o) ?? o)
+        {
+            base.State.BindValueChanged(state =>
+            {
+                if (state.NewValue == null)
+                    base.State.Value = default(T);
+
+                State.Value = (T)base.State.Value;
+            }, true);
+
+            State.BindValueChanged(state => base.State.Value = state.NewValue);
+        }
+
+        public sealed override IconUsage? GetIconForState(object state) => GetIconForState((T)state);
+
+        public abstract IconUsage? GetIconForState(T state);
+    }
+}