Setup skinnable combo counter component with default implementation

osu.Game/Graphics/UserInterface/SimpleComboCounter.cs

@@ -5,13 +5,14 @@ using System;
 using osu.Framework.Allocation;
 using osu.Framework.Graphics;
 using osu.Game.Graphics.Sprites;
+using osu.Game.Screens.Play.HUD;
 
 namespace osu.Game.Graphics.UserInterface
 {
     /// <summary>
     /// Used as an accuracy counter. Represented visually as a percentage.
     /// </summary>
-    public class SimpleComboCounter : RollingCounter<int>
+    public class SimpleComboCounter : RollingCounter<int>, IComboCounter
     {
         protected override double RollingDuration => 750;
 

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

@@ -0,0 +1,58 @@
+// 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.Bindables;
+using osu.Framework.Graphics;
+using osu.Game.Graphics.UserInterface;
+using osu.Game.Skinning;
+using osuTK.Graphics;
+
+namespace osu.Game.Screens.Play.HUD
+{
+    public class SkinnableComboCounter : SkinnableDrawable, IComboCounter
+    {
+        public SkinnableComboCounter()
+            : base(new HUDSkinComponent(HUDSkinComponents.ComboCounter), createDefault)
+        {
+        }
+
+        private IComboCounter skinnedCounter;
+
+        protected override void SkinChanged(ISkinSource skin, bool allowFallback)
+        {
+            // todo: unnecessary?
+            if (skinnedCounter != null)
+            {
+                Current.UnbindFrom(skinnedCounter.Current);
+            }
+
+            base.SkinChanged(skin, allowFallback);
+
+            // temporary layout code, will eventually be replaced by the skin layout system.
+            if (Drawable is SimpleComboCounter)
+            {
+                Drawable.BypassAutoSizeAxes = Axes.X;
+                Drawable.Anchor = Anchor.TopRight;
+                Drawable.Origin = Anchor.TopLeft;
+                Drawable.Margin = new MarginPadding { Top = 5, Left = 20 };
+            }
+            else
+            {
+                Drawable.BypassAutoSizeAxes = Axes.X;
+                Drawable.Anchor = Anchor.BottomLeft;
+                Drawable.Origin = Anchor.BottomLeft;
+                Drawable.Margin = new MarginPadding { Top = 5, Left = 20 };
+            }
+
+            skinnedCounter = (IComboCounter)Drawable;
+
+            Current.BindTo(skinnedCounter.Current);
+        }
+
+        private static Drawable createDefault(ISkinComponent skinComponent) => new SimpleComboCounter();
+
+        public Bindable<int> Current { get; } = new Bindable<int>();
+
+        public void UpdateCombo(int combo, Color4? hitObjectColour = null) => Current.Value = combo;
+    }
+}

osu.Game/Screens/Play/HUDOverlay.cs

@@ -28,7 +28,7 @@ namespace osu.Game.Screens.Play
         private const Easing fade_easing = Easing.Out;
 
         public readonly KeyCounterDisplay KeyCounter;
-        public readonly RollingCounter<int> ComboCounter;
+        public readonly SkinnableComboCounter ComboCounter;
         public readonly ScoreCounter ScoreCounter;
         public readonly RollingCounter<double> AccuracyCounter;
         public readonly HealthDisplay HealthDisplay;
@@ -275,13 +275,7 @@ namespace osu.Game.Screens.Play
             Origin = Anchor.TopCentre,
         };
 
-        protected virtual RollingCounter<int> CreateComboCounter() => new SimpleComboCounter
+        protected virtual SkinnableComboCounter CreateComboCounter() => new SkinnableComboCounter();
-        {
-            BypassAutoSizeAxes = Axes.X,
-            Anchor = Anchor.TopRight,
-            Origin = Anchor.TopLeft,
-            Margin = new MarginPadding { Top = 5, Left = 20 },
-        };
 
         protected virtual HealthDisplay CreateHealthDisplay() => new StandardHealthDisplay
         {