From c3e3b2019dcf169d74c435bde269ec53bf43a8fb Mon Sep 17 00:00:00 2001
From: Dean Herbert <pe@ppy.sh>
Date: Sun, 30 Jan 2022 13:01:41 +0900
Subject: [PATCH] Reduce overhead of `ApplyState` by tracking previous values

Even with pooling applied, there are overheads involved with transforms
when quickly cycling through the carousel.

The main goal here is to reduce the transforms in cases the reuse is
still in the same state. Avoiding firing `FadeIn` and `FadeOut` are the
main areas of saving.
---
 .../Carousel/DrawableCarouselBeatmap.cs       |  3 +
 .../Select/Carousel/DrawableCarouselItem.cs   | 55 ++++++++++++++-----
 2 files changed, 43 insertions(+), 15 deletions(-)

diff --git a/osu.Game/Screens/Select/Carousel/DrawableCarouselBeatmap.cs b/osu.Game/Screens/Select/Carousel/DrawableCarouselBeatmap.cs
index 43ddfc79b1..a3483aa60a 100644
--- a/osu.Game/Screens/Select/Carousel/DrawableCarouselBeatmap.cs
+++ b/osu.Game/Screens/Select/Carousel/DrawableCarouselBeatmap.cs
@@ -71,6 +71,9 @@ namespace osu.Game.Screens.Select.Carousel
         {
             beatmapInfo = panel.BeatmapInfo;
             Item = panel;
+
+            // Difficulty panels should start hidden for a better initial effect.
+            Hide();
         }
 
         [BackgroundDependencyLoader(true)]
diff --git a/osu.Game/Screens/Select/Carousel/DrawableCarouselItem.cs b/osu.Game/Screens/Select/Carousel/DrawableCarouselItem.cs
index 8b38bb9a96..4a23faa650 100644
--- a/osu.Game/Screens/Select/Carousel/DrawableCarouselItem.cs
+++ b/osu.Game/Screens/Select/Carousel/DrawableCarouselItem.cs
@@ -64,8 +64,6 @@ namespace osu.Game.Screens.Select.Carousel
         {
             RelativeSizeAxes = Axes.X;
 
-            Alpha = 0;
-
             InternalChildren = new Drawable[]
             {
                 MovementContainer = new Container
@@ -119,29 +117,56 @@ namespace osu.Game.Screens.Select.Carousel
 
         private void onStateChange(ValueChangedEvent<bool> _) => Scheduler.AddOnce(ApplyState);
 
+        private CarouselItemState? lastAppliedState;
+
         protected virtual void ApplyState()
         {
-            // Use the fact that we know the precise height of the item from the model to avoid the need for AutoSize overhead.
-            // Additionally, AutoSize doesn't work well due to content starting off-screen and being masked away.
-            Height = Item.TotalHeight;
-
             Debug.Assert(Item != null);
 
-            switch (Item.State.Value)
+            if (lastAppliedState != Item.State.Value)
             {
-                case CarouselItemState.NotSelected:
-                    Deselected();
-                    break;
+                lastAppliedState = Item.State.Value;
 
-                case CarouselItemState.Selected:
-                    Selected();
-                    break;
+                // Use the fact that we know the precise height of the item from the model to avoid the need for AutoSize overhead.
+                // Additionally, AutoSize doesn't work well due to content starting off-screen and being masked away.
+                Height = Item.TotalHeight;
+
+                switch (lastAppliedState)
+                {
+                    case CarouselItemState.NotSelected:
+                        Deselected();
+                        break;
+
+                    case CarouselItemState.Selected:
+                        Selected();
+                        break;
+                }
             }
 
             if (!Item.Visible)
-                this.FadeOut(300, Easing.OutQuint);
+                Hide();
             else
-                this.FadeIn(250);
+                Show();
+        }
+
+        private bool isVisible = true;
+
+        public override void Show()
+        {
+            if (isVisible)
+                return;
+
+            isVisible = true;
+            this.FadeIn(250);
+        }
+
+        public override void Hide()
+        {
+            if (!isVisible)
+                return;
+
+            isVisible = false;
+            this.FadeOut(300, Easing.OutQuint);
         }
 
         protected virtual void Selected()