Initial implementation of LowHealthLayer

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

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+// 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.Allocation;
+using osu.Framework.Bindables;
+using osu.Framework.Graphics;
+using osu.Framework.Graphics.Shapes;
+using osu.Game.Configuration;
+using osu.Game.Graphics;
+
+namespace osu.Game.Screens.Play.HUD
+{
+    public class LowHealthLayer : HealthDisplay
+    {
+        private const float max_alpha = 0.4f;
+
+        private const double fade_time = 300;
+
+        private readonly Box box;
+
+        private Bindable<bool> configFadeRedWhenLowHealth;
+
+        public LowHealthLayer()
+        {
+            Child = box = new Box
+            {
+                RelativeSizeAxes = Axes.Both,
+                Alpha = 0
+            };
+        }
+
+        [BackgroundDependencyLoader]
+        private void load(OsuConfigManager config, OsuColour color)
+        {
+            configFadeRedWhenLowHealth = config.GetBindable<bool>(OsuSetting.FadePlayfieldWhenLowHealth);
+            box.Colour = color.Red;
+
+            configFadeRedWhenLowHealth.BindValueChanged(value =>
+            {
+                if (value.NewValue)
+                    this.FadeIn(fade_time, Easing.OutQuint);
+                else
+                    this.FadeOut(fade_time, Easing.OutQuint);
+            }, true);
+        }
+    }
+}