Split OnlinePlayComposite to remove if-statement

osu.Game/Screens/OnlinePlay/RoomSubScreenComposite.cs

@@ -0,0 +1,38 @@
+// 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 osu.Framework.Allocation;
+using osu.Framework.Bindables;
+using osu.Game.Online.Rooms;
+using osu.Game.Screens.OnlinePlay.Match;
+
+namespace osu.Game.Screens.OnlinePlay
+{
+    /// <summary>
+    /// An <see cref="OnlinePlayComposite"/> with additional logic tracking the currently-selected <see cref="PlaylistItem"/> inside a <see cref="RoomSubScreen"/>.
+    /// </summary>
+    public class RoomSubScreenComposite : OnlinePlayComposite
+    {
+        [Resolved]
+        private IBindable<PlaylistItem> subScreenSelectedItem { get; set; }
+
+        protected override void LoadComplete()
+        {
+            base.LoadComplete();
+
+            subScreenSelectedItem.BindValueChanged(_ => UpdateSelectedItem(), true);
+        }
+
+        protected override void UpdateSelectedItem()
+        {
+            if (RoomID.Value == null)
+            {
+                // If the room hasn't been created yet, fall-back to the base logic.
+                base.UpdateSelectedItem();
+                return;
+            }
+
+            SelectedItem.Value = subScreenSelectedItem.Value;
+        }
+    }
+}