Move children to CarouselGroup

osu.Game/Screens/Select/BeatmapCarousel.cs

@@ -53,7 +53,7 @@ namespace osu.Game.Screens.Select
 
         public override bool HandleInput => AllowSelection;
 
-        private IEnumerable<CarouselBeatmapSet> beatmapSets => root.Children?.OfType<CarouselBeatmapSet>() ?? new CarouselBeatmapSet[] { };
+        private IEnumerable<CarouselBeatmapSet> beatmapSets => root.Children.OfType<CarouselBeatmapSet>();
 
         public IEnumerable<BeatmapSetInfo> BeatmapSets
         {
@@ -277,7 +277,7 @@ namespace osu.Game.Screens.Select
 
         private void applyActiveCriteria(bool debounce, bool scroll)
         {
-            if (root.Children?.Any() != true) return;
+            if (root.Children.Any() != true) return;
 
             void perform()
             {

osu.Game/Screens/Select/Carousel/CarouselGroup.cs

@@ -14,15 +14,63 @@ namespace osu.Game.Screens.Select.Carousel
 
         protected override DrawableCarouselItem CreateDrawableRepresentation() => null;
 
-        public override void AddChild(CarouselItem i)
+        public IReadOnlyList<CarouselItem> Children => InternalChildren;
+
+        protected List<CarouselItem> InternalChildren = new List<CarouselItem>();
+
+        public override List<DrawableCarouselItem> Drawables
+        {
+            get
+            {
+                var drawables = base.Drawables;
+                foreach (var c in InternalChildren)
+                    drawables.AddRange(c.Drawables);
+                return drawables;
+            }
+        }
+
+        public virtual void RemoveChild(CarouselItem i)
+        {
+            InternalChildren.Remove(i);
+
+            // it's important we do the deselection after removing, so any further actions based on
+            // State.ValueChanged make decisions post-removal.
+            i.State.Value = CarouselItemState.Collapsed;
+        }
+
+        public virtual void AddChild(CarouselItem i)
         {
             i.State.ValueChanged += v => ChildItemStateChanged(i, v);
-            base.AddChild(i);
+            InternalChildren.Add(i);
         }
 
         public CarouselGroup(List<CarouselItem> items = null)
         {
             if (items != null) InternalChildren = items;
+
+            State.ValueChanged += v =>
+            {
+                switch (v)
+                {
+                    case CarouselItemState.Collapsed:
+                    case CarouselItemState.NotSelected:
+                        InternalChildren.ForEach(c => c.State.Value = CarouselItemState.Collapsed);
+                        break;
+                    case CarouselItemState.Selected:
+                        InternalChildren.ForEach(c =>
+                        {
+                            if (c.State == CarouselItemState.Collapsed) c.State.Value = CarouselItemState.NotSelected;
+                        });
+                        break;
+                }
+            };
+        }
+
+        public override void Filter(FilterCriteria criteria)
+        {
+            base.Filter(criteria);
+            InternalChildren.Sort((x, y) => x.CompareTo(criteria, y));
+            InternalChildren.ForEach(c => c.Filter(criteria));
         }
 
         protected virtual void ChildItemStateChanged(CarouselItem item, CarouselItemState value)

osu.Game/Screens/Select/Carousel/CarouselItem.cs

@@ -13,66 +13,28 @@ namespace osu.Game.Screens.Select.Carousel
 
         public readonly Bindable<CarouselItemState> State = new Bindable<CarouselItemState>(CarouselItemState.NotSelected);
 
-        public IReadOnlyList<CarouselItem> Children => InternalChildren;
-
-        protected List<CarouselItem> InternalChildren { get; set; }
-
         /// <summary>
         /// This item is not in a hidden state.
         /// </summary>
         public bool Visible => State.Value != CarouselItemState.Collapsed && !Filtered;
 
-        public IEnumerable<DrawableCarouselItem> Drawables
+        public virtual List<DrawableCarouselItem> Drawables
         {
             get
             {
-                List<DrawableCarouselItem> items = new List<DrawableCarouselItem>();
+                var items = new List<DrawableCarouselItem>();
 
                 var self = drawableRepresentation.Value;
                 if (self?.IsPresent == true) items.Add(self);
 
-                if (InternalChildren != null)
-                    foreach (var c in InternalChildren)
-                        items.AddRange(c.Drawables);
-
                 return items;
             }
         }
 
-        public virtual void AddChild(CarouselItem i) => (InternalChildren ?? (InternalChildren = new List<CarouselItem>())).Add(i);
-
-        public virtual void RemoveChild(CarouselItem i)
-        {
-            InternalChildren?.Remove(i);
-
-            // it's important we do the deselection after removing, so any further actions based on
-            // State.ValueChanged make decisions post-removal.
-            i.State.Value = CarouselItemState.Collapsed;
-        }
-
         protected CarouselItem()
         {
             drawableRepresentation = new Lazy<DrawableCarouselItem>(CreateDrawableRepresentation);
 
-            State.ValueChanged += v =>
-            {
-                if (InternalChildren == null) return;
-
-                switch (v)
-                {
-                    case CarouselItemState.Collapsed:
-                    case CarouselItemState.NotSelected:
-                        InternalChildren.ForEach(c => c.State.Value = CarouselItemState.Collapsed);
-                        break;
-                    case CarouselItemState.Selected:
-                        InternalChildren.ForEach(c =>
-                        {
-                            if (c.State == CarouselItemState.Collapsed) c.State.Value = CarouselItemState.NotSelected;
-                        });
-                        break;
-                }
-            };
-
             Filtered.ValueChanged += v =>
             {
                 if (v && State == CarouselItemState.Selected)
@@ -86,8 +48,6 @@ namespace osu.Game.Screens.Select.Carousel
 
         public virtual void Filter(FilterCriteria criteria)
         {
-            InternalChildren?.Sort((x, y) => x.CompareTo(criteria, y));
-            InternalChildren?.ForEach(c => c.Filter(criteria));
         }
 
         public virtual int CompareTo(FilterCriteria criteria, CarouselItem other) => GetHashCode().CompareTo(other.GetHashCode());