Use ISkinnableComponent wherever possible (and expose as BindableList)

osu.Game/Screens/Play/HUD/SkinnableElementTargetContainer.cs

@@ -1,8 +1,10 @@
 // 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 System.Collections.Generic;
 using System.Linq;
+using osu.Framework.Bindables;
 using osu.Framework.Graphics;
 using osu.Game.Extensions;
 using osu.Game.Skinning;
@@ -15,30 +17,46 @@ namespace osu.Game.Screens.Play.HUD
 
         public SkinnableTarget Target { get; }
 
+        public IBindableList<ISkinnableComponent> Components => components;
+
+        private readonly BindableList<ISkinnableComponent> components = new BindableList<ISkinnableComponent>();
+
         public SkinnableElementTargetContainer(SkinnableTarget target)
         {
             Target = target;
         }
 
-        public IReadOnlyList<Drawable> Children => content?.Children;
-
         public void Reload()
         {
+            ClearInternal();
+            components.Clear();
+
             content = CurrentSkin.GetDrawableComponent(new SkinnableTargetComponent(Target)) as SkinnableTargetWrapper;
 
-            ClearInternal();
-
             if (content != null)
-                LoadComponentAsync(content, AddInternal);
+            {
+                LoadComponentAsync(content, wrapper =>
+                {
+                    AddInternal(wrapper);
+                    components.AddRange(wrapper.Children.OfType<ISkinnableComponent>());
+                });
+            }
         }
 
-        public void Add(Drawable drawable)
+        public void Add(ISkinnableComponent component)
         {
+            if (content == null)
+                throw new NotSupportedException("Attempting to add a new component to a target container which is not supported by the current skin.");
+
+            if (!(component is Drawable drawable))
+                throw new ArgumentException("Provided argument must be of type {nameof(ISkinnableComponent)}.", nameof(drawable));
+
             content.Add(drawable);
+            components.Add(component);
         }
 
         public IEnumerable<SkinnableInfo> CreateSerialisedChildren() =>
-            content.Select(d => d.CreateSerialisedInformation());
+            components.Select(d => ((Drawable)d).CreateSerialisedInformation());
 
         protected override void SkinChanged(ISkinSource skin, bool allowFallback)
         {

osu.Game/Screens/Play/HUDOverlay.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using osu.Framework.Allocation;
 using osu.Framework.Bindables;
 using osu.Framework.Extensions.EnumExtensions;
@@ -207,10 +208,8 @@ namespace osu.Game.Screens.Play
 
             Vector2 lowestScreenSpace = Vector2.Zero;
 
-            // TODO: may be null during skin switching. not sure if there's a better way of exposing these children.
+            // LINQ cast can be removed when IDrawable interface includes Anchor / RelativeSizeAxes.
-            if (mainComponents.Children != null)
+            foreach (var element in mainComponents.Components.Cast<Drawable>())
-            {
-                foreach (var element in mainComponents.Children)
             {
                 // for now align top-right components with the bottom-edge of the lowest top-anchored hud element.
                 if (!element.Anchor.HasFlagFast(Anchor.TopRight) && !element.RelativeSizeAxes.HasFlagFast(Axes.X))
@@ -228,7 +227,6 @@ namespace osu.Game.Screens.Play
             topRightElements.Y = TopScoringElementsHeight = ToLocalSpace(lowestScreenSpace).Y;
             bottomRightElements.Y = -Progress.Height;
         }
-        }
 
         private void updateVisibility()
         {

osu.Game/Skinning/ISkinnableTarget.cs

@@ -1,8 +1,6 @@
 // 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.Graphics;
-
 namespace osu.Game.Skinning
 {
     /// <summary>
@@ -20,6 +18,6 @@ namespace osu.Game.Skinning
         /// <summary>
         /// Add the provided item to this target.
         /// </summary>
-        public void Add(Drawable drawable);
+        public void Add(ISkinnableComponent drawable);
     }
 }

osu.Game/Skinning/SkinnableTargetWrapper.cs

@@ -8,7 +8,7 @@ using osu.Framework.Graphics.Containers;
 namespace osu.Game.Skinning
 {
     /// <summary>
-    /// A container which is serialised and can encapsulate multiple skinnable elements into a single return type.
+    /// A container which is serialised and can encapsulate multiple skinnable elements into a single return type (for consumption via <see cref="ISkin.GetDrawableComponent"/>.
     /// Will also optionally apply default cross-element layout dependencies when initialised from a non-deserialised source.
     /// </summary>
     public class SkinnableTargetWrapper : Container, ISkinSerialisable