From 6ef235c4c5290f3254ed247b2f20054dde1aceaf Mon Sep 17 00:00:00 2001
From: smoogipoo <smoogipoo@smgi.me>
Date: Wed, 17 Feb 2021 21:42:22 +0900
Subject: [PATCH] Fix beatmap panel flickering multiple times

---
 .../Match/BeatmapSelectionControl.cs          | 17 ++------
 .../Screens/OnlinePlay/OnlinePlayComposite.cs | 41 +++++++++++++++++--
 2 files changed, 41 insertions(+), 17 deletions(-)

diff --git a/osu.Game/Screens/OnlinePlay/Multiplayer/Match/BeatmapSelectionControl.cs b/osu.Game/Screens/OnlinePlay/Multiplayer/Match/BeatmapSelectionControl.cs
index 3cf0767cf8..3af0d5b715 100644
--- a/osu.Game/Screens/OnlinePlay/Multiplayer/Match/BeatmapSelectionControl.cs
+++ b/osu.Game/Screens/OnlinePlay/Multiplayer/Match/BeatmapSelectionControl.cs
@@ -1,15 +1,12 @@
 // 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.Diagnostics;
-using System.Linq;
 using osu.Framework.Allocation;
 using osu.Framework.Graphics;
 using osu.Framework.Graphics.Containers;
 using osu.Framework.Graphics.UserInterface;
 using osu.Framework.Screens;
 using osu.Game.Online.API;
-using osu.Game.Online.Rooms;
 using osu.Game.Screens.OnlinePlay.Match.Components;
 
 namespace osu.Game.Screens.OnlinePlay.Multiplayer.Match
@@ -61,10 +58,7 @@ namespace osu.Game.Screens.OnlinePlay.Multiplayer.Match
         {
             base.LoadComplete();
 
-            Debug.Assert(SelectedItem != null);
-            SelectedItem.BindValueChanged(_ => updateBeatmap());
-            Playlist.BindCollectionChanged((_, __) => updateBeatmap(), true);
-
+            SelectedItem.BindValueChanged(_ => updateBeatmap(), true);
             Host.BindValueChanged(host =>
             {
                 if (RoomID.Value == null || host.NewValue?.Equals(api.LocalUser.Value) == true)
@@ -76,15 +70,10 @@ namespace osu.Game.Screens.OnlinePlay.Multiplayer.Match
 
         private void updateBeatmap()
         {
-            Debug.Assert(SelectedItem != null);
-
-            // When the selected item is null, the match hasn't yet been created. Use the playlist directly, which is mutated by song selection.
-            PlaylistItem item = SelectedItem.Value ?? Playlist.FirstOrDefault();
-
-            if (item == null)
+            if (SelectedItem.Value == null)
                 beatmapPanelContainer.Clear();
             else
-                beatmapPanelContainer.Child = new DrawableRoomPlaylistItem(item, false, false);
+                beatmapPanelContainer.Child = new DrawableRoomPlaylistItem(SelectedItem.Value, false, false);
         }
     }
 }
diff --git a/osu.Game/Screens/OnlinePlay/OnlinePlayComposite.cs b/osu.Game/Screens/OnlinePlay/OnlinePlayComposite.cs
index a7058d0ede..f203ef927c 100644
--- a/osu.Game/Screens/OnlinePlay/OnlinePlayComposite.cs
+++ b/osu.Game/Screens/OnlinePlay/OnlinePlayComposite.cs
@@ -2,6 +2,8 @@
 // See the LICENCE file in the repository root for full licence text.
 
 using System;
+using System.Collections.Specialized;
+using System.Linq;
 using JetBrains.Annotations;
 using osu.Framework.Allocation;
 using osu.Framework.Bindables;
@@ -57,11 +59,44 @@ namespace osu.Game.Screens.OnlinePlay
         protected Bindable<TimeSpan?> Duration { get; private set; }
 
         /// <summary>
-        /// The currently selected item in the <see cref="RoomSubScreen"/>.
-        /// May be null if this <see cref="OnlinePlayComposite"/> is not inside a <see cref="RoomSubScreen"/>.
+        /// The currently selected item in the <see cref="RoomSubScreen"/>, or the first item from <see cref="Playlist"/>
+        /// if this <see cref="OnlinePlayComposite"/> is not within a <see cref="RoomSubScreen"/>.
         /// </summary>
+        protected IBindable<PlaylistItem> SelectedItem => selectedItem;
+
+        private readonly Bindable<PlaylistItem> selectedItem = new Bindable<PlaylistItem>();
+
         [CanBeNull]
         [Resolved(CanBeNull = true)]
-        protected IBindable<PlaylistItem> SelectedItem { get; private set; }
+        private IBindable<PlaylistItem> subScreenSelectedItem { get; set; }
+
+        protected override void LoadComplete()
+        {
+            base.LoadComplete();
+
+            if (subScreenSelectedItem != null)
+                subScreenSelectedItem.BindValueChanged(onSelectedItemChanged, true);
+            else
+                Playlist.BindCollectionChanged(onPlaylistChanged, true);
+        }
+
+        /// <summary>
+        /// Invoked when the selected item from within a <see cref="RoomSubScreen"/> changes.
+        /// Does not occur when this <see cref="OnlinePlayComposite"/> is outside a <see cref="RoomSubScreen"/>.
+        /// </summary>
+        private void onSelectedItemChanged(ValueChangedEvent<PlaylistItem> item)
+        {
+            // If the room hasn't been created yet, fall-back to the first item from the playlist.
+            selectedItem.Value = RoomID.Value == null ? Playlist.FirstOrDefault() : item.NewValue;
+        }
+
+        /// <summary>
+        /// Invoked when the playlist changes.
+        /// Does not occur when this <see cref="OnlinePlayComposite"/> is inside a <see cref="RoomSubScreen"/>.
+        /// </summary>
+        private void onPlaylistChanged(object sender, NotifyCollectionChangedEventArgs e)
+        {
+            selectedItem.Value = Playlist.FirstOrDefault();
+        }
     }
 }