refactor(KeyCounterDisplay): apply suggestions

I also took the freedom to add type checking, as we can't limit the
usage of `Add()` since it's a Container. The exception thrown also
advises of using the suggested `AddTrigger()` instead.

osu.Game.Tests/Visual/Gameplay/TestSceneAutoplay.cs

@@ -38,7 +38,7 @@ namespace osu.Game.Tests.Visual.Gameplay
             AddUntilStep("key counter counted keys", () => Player.HUDOverlay.KeyCounter.Children.Any(kc => kc.CountPresses.Value > 2));
 
             seekTo(referenceBeatmap.Breaks[0].StartTime);
-            AddAssert("keys not counting", () => !Player.HUDOverlay.KeyCounter.IsCounting);
+            AddAssert("keys not counting", () => !Player.HUDOverlay.KeyCounter.IsCounting.Value);
             AddAssert("overlay displays 100% accuracy", () => Player.BreakOverlay.ChildrenOfType<BreakInfo>().Single().AccuracyDisplay.Current.Value == 1);
 
             AddStep("rewind", () => Player.GameplayClockContainer.Seek(-80000));

osu.Game.Tests/Visual/Gameplay/TestSceneHUDOverlay.cs

@@ -267,7 +267,7 @@ namespace osu.Game.Tests.Visual.Gameplay
                 hudOverlay = new HUDOverlay(null, Array.Empty<Mod>());
 
                 // Add any key just to display the key counter visually.
-                hudOverlay.KeyCounter.Add(hudOverlay.KeyCounter.CreateKeyCounter(new KeyCounterKeyboardTrigger(Key.Space)));
+                hudOverlay.KeyCounter.AddTrigger(new KeyCounterKeyboardTrigger(Key.Space));
 
                 scoreProcessor.Combo.Value = 1;
 

osu.Game.Tests/Visual/Gameplay/TestSceneKeyCounter.cs

@@ -35,7 +35,7 @@ namespace osu.Game.Tests.Visual.Gameplay
             AddStep("Add random", () =>
             {
                 Key key = (Key)((int)Key.A + RNG.Next(26));
-                kc.Add(kc.CreateKeyCounter(new KeyCounterKeyboardTrigger(key)));
+                kc.AddTrigger(new KeyCounterKeyboardTrigger(key));
             });
 
             Key testKey = ((KeyCounterKeyboardTrigger)kc.Children.First().Trigger).Key;
@@ -49,7 +49,7 @@ namespace osu.Game.Tests.Visual.Gameplay
             AddAssert($"Check {testKey} counter after keypress", () => testCounter.CountPresses.Value == 1);
             addPressKeyStep();
             AddAssert($"Check {testKey} counter after keypress", () => testCounter.CountPresses.Value == 2);
-            AddStep("Disable counting", () => testCounter.IsCounting = false);
+            AddStep("Disable counting", () => testCounter.IsCounting.Value = false);
             addPressKeyStep();
             AddAssert($"Check {testKey} count has not changed", () => testCounter.CountPresses.Value == 2);
 

osu.Game.Tests/Visual/Gameplay/TestSceneSkinEditorMultipleSkins.cs

@@ -53,7 +53,7 @@ namespace osu.Game.Tests.Visual.Gameplay
                     };
 
                     // Add any key just to display the key counter visually.
-                    hudOverlay.KeyCounter.Add(hudOverlay.KeyCounter.CreateKeyCounter(new KeyCounterKeyboardTrigger(Key.Space)));
+                    hudOverlay.KeyCounter.AddTrigger(new KeyCounterKeyboardTrigger(Key.Space));
                     scoreProcessor.Combo.Value = 1;
 
                     return new Container

osu.Game.Tests/Visual/Gameplay/TestSceneSkinnableHUDOverlay.cs

@@ -88,7 +88,7 @@ namespace osu.Game.Tests.Visual.Gameplay
                     hudOverlay = new HUDOverlay(null, Array.Empty<Mod>());
 
                     // Add any key just to display the key counter visually.
-                    hudOverlay.KeyCounter.Add(hudOverlay.KeyCounter.CreateKeyCounter(new KeyCounterKeyboardTrigger(Key.Space)));
+                    hudOverlay.KeyCounter.AddTrigger(new KeyCounterKeyboardTrigger(Key.Space));
 
                     action?.Invoke(hudOverlay);
 

osu.Game/Rulesets/UI/RulesetInputManager.cs

@@ -162,11 +162,11 @@ namespace osu.Game.Rulesets.UI
             KeyBindingContainer.Add(receptor);
 
             keyCounter.SetReceptor(receptor);
-            keyCounter.AddRange(KeyBindingContainer.DefaultKeyBindings
+            keyCounter.AddTriggerRange(KeyBindingContainer.DefaultKeyBindings
                                                           .Select(b => b.GetAction<T>())
                                                           .Distinct()
                                                           .OrderBy(action => action)
-                                                   .Select(action => keyCounter.CreateKeyCounter(new KeyCounterActionTrigger<T>(action))));
+                                                          .Select(action => new KeyCounterActionTrigger<T>(action)));
         }
 
         private partial class ActionReceptor : KeyCounterDisplay.Receptor, IKeyBindingHandler<T>

osu.Game/Screens/Play/DefaultKeyCounterDisplay.cs

@@ -13,7 +13,9 @@ namespace osu.Game.Screens.Play
         private const int duration = 100;
         private const double key_fade_time = 80;
 
-        protected override Container<KeyCounter> Content => KeyFlow;
+        private readonly FillFlowContainer<KeyCounter> keyFlow = new FillFlowContainer<KeyCounter>();
+
+        protected override Container<KeyCounter> Content => keyFlow;
 
         public new IReadOnlyList<DefaultKeyCounter> Children
         {
@@ -22,12 +24,13 @@ namespace osu.Game.Screens.Play
         }
 
         public DefaultKeyCounterDisplay()
+            : base(typeof(DefaultKeyCounter))
         {
-            KeyFlow.Direction = FillDirection.Horizontal;
+            keyFlow.Direction = FillDirection.Horizontal;
-            KeyFlow.AutoSizeAxes = Axes.Both;
+            keyFlow.AutoSizeAxes = Axes.Both;
-            KeyFlow.Alpha = 0;
+            keyFlow.Alpha = 0;
 
-            InternalChild = KeyFlow;
+            InternalChild = keyFlow;
         }
 
         protected override void Update()
@@ -36,12 +39,22 @@ namespace osu.Game.Screens.Play
 
             // Don't use autosize as it will shrink to zero when KeyFlow is hidden.
             // In turn this can cause the display to be masked off screen and never become visible again.
-            Size = KeyFlow.Size;
+            Size = keyFlow.Size;
+        }
+
+        public override void AddTrigger(KeyCounter.InputTrigger trigger)
+        {
+            DefaultKeyCounter key = new DefaultKeyCounter(trigger);
+            Add(key);
+            key.FadeTime = key_fade_time;
+            key.KeyDownTextColor = KeyDownTextColor;
+            key.KeyUpTextColor = KeyUpTextColor;
         }
 
         public override void Add(KeyCounter key)
         {
             base.Add(key);
+
             DefaultKeyCounter defaultKey = (DefaultKeyCounter)key;
 
             defaultKey.FadeTime = key_fade_time;
@@ -51,7 +64,7 @@ namespace osu.Game.Screens.Play
 
         protected override void UpdateVisibility() =>
             // Isolate changing visibility of the key counters from fading this component.
-            KeyFlow.FadeTo(AlwaysVisible.Value || ConfigVisibility.Value ? 1 : 0, duration);
+            keyFlow.FadeTo(AlwaysVisible.Value || ConfigVisibility.Value ? 1 : 0, duration);
 
         private Color4 keyDownTextColor = Color4.DarkGray;
 

osu.Game/Screens/Play/KeyCounter.cs

@@ -21,7 +21,7 @@ namespace osu.Game.Screens.Play
             MinValue = 0
         };
 
-        public bool IsCounting { get; set; } = true;
+        public Bindable<bool> IsCounting { get; } = new BindableBool(true);
 
         public IBindable<int> CountPresses => countPresses;
 
@@ -46,7 +46,7 @@ namespace osu.Game.Screens.Play
 
         private void increment()
         {
-            if (!IsCounting)
+            if (!IsCounting.Value)
                 return;
 
             countPresses.Value++;
@@ -54,7 +54,7 @@ namespace osu.Game.Screens.Play
 
         private void decrement()
         {
-            if (!IsCounting)
+            if (!IsCounting.Value)
                 return;
 
             countPresses.Value--;

osu.Game/Screens/Play/KeyCounterDisplay.cs

@@ -2,9 +2,11 @@
 // See the LICENCE file in the repository root for full licence text.
 
 using System;
+using System.Collections.Generic;
 using System.Linq;
 using osu.Framework.Allocation;
 using osu.Framework.Bindables;
+using osu.Framework.Extensions.IEnumerableExtensions;
 using osu.Framework.Graphics;
 using osu.Framework.Graphics.Containers;
 using osu.Framework.Input.Events;
@@ -15,22 +17,58 @@ namespace osu.Game.Screens.Play
 {
     public abstract partial class KeyCounterDisplay : Container<KeyCounter>
     {
-        protected readonly Bindable<bool> ConfigVisibility = new Bindable<bool>();
-
-        protected readonly FillFlowContainer<KeyCounter> KeyFlow = new FillFlowContainer<KeyCounter>();
-
-        protected override Container<KeyCounter> Content => KeyFlow;
-
         /// <summary>
         /// Whether the key counter should be visible regardless of the configuration value.
         /// This is true by default, but can be changed.
         /// </summary>
-        public readonly Bindable<bool> AlwaysVisible = new Bindable<bool>(true);
+        public Bindable<bool> AlwaysVisible { get; } = new Bindable<bool>(true);
+
+        public Bindable<bool> IsCounting { get; } = new BindableBool(true);
+
+        public IReadOnlyList<KeyCounter.InputTrigger> Triggers
+        {
+            get => Children.Select(c => c.Trigger).ToArray();
+            set
+            {
+                Clear();
+                value.ForEach(AddTrigger);
+            }
+        }
+
+        protected readonly Bindable<bool> ConfigVisibility = new Bindable<bool>();
+
+        protected abstract void UpdateVisibility();
+
+        private Receptor? receptor;
+
+        private readonly Type[] acceptedTypes;
+
+        protected KeyCounterDisplay(params Type[] acceptedTypes)
+        {
+            this.acceptedTypes = acceptedTypes;
+        }
+
+        public void SetReceptor(Receptor receptor)
+        {
+            if (this.receptor != null)
+                throw new InvalidOperationException("Cannot set a new receptor when one is already active");
+
+            this.receptor = receptor;
+        }
+
+        public abstract void AddTrigger(KeyCounter.InputTrigger trigger);
+
+        public void AddTriggerRange(IEnumerable<KeyCounter.InputTrigger> triggers) => triggers.ForEach(AddTrigger);
+
+        private bool checkType(KeyCounter key) => acceptedTypes.Length == 0 || acceptedTypes.Any(t => t.IsInstanceOfType(key));
 
         public override void Add(KeyCounter key)
         {
+            if (!checkType(key))
+                throw new InvalidOperationException($"{key.GetType()} is not a supported counter type. (hint: you may want to use {nameof(AddTrigger)} instead.)");
+
             base.Add(key);
-            key.IsCounting = IsCounting;
+            key.IsCounting.BindTo(IsCounting);
         }
 
         [BackgroundDependencyLoader]
@@ -47,38 +85,10 @@ namespace osu.Game.Screens.Play
             ConfigVisibility.BindValueChanged(_ => UpdateVisibility(), true);
         }
 
-        private bool isCounting = true;
-
-        public bool IsCounting
-        {
-            get => isCounting;
-            set
-            {
-                if (value == isCounting) return;
-
-                isCounting = value;
-                foreach (var child in Children)
-                    child.IsCounting = value;
-            }
-        }
-
-        protected abstract void UpdateVisibility();
-
         public override bool HandleNonPositionalInput => receptor == null;
+
         public override bool HandlePositionalInput => receptor == null;
 
-        private Receptor? receptor;
-
-        public void SetReceptor(Receptor receptor)
-        {
-            if (this.receptor != null)
-                throw new InvalidOperationException("Cannot set a new receptor when one is already active");
-
-            this.receptor = receptor;
-        }
-
-        public virtual KeyCounter CreateKeyCounter(KeyCounter.InputTrigger trigger) => new DefaultKeyCounter(trigger);
-
         public partial class Receptor : Drawable
         {
             protected readonly KeyCounterDisplay Target;

osu.Game/Screens/Play/Player.cs

@@ -440,8 +440,11 @@ namespace osu.Game.Screens.Play
                         },
                         KeyCounter =
                         {
+                            IsCounting =
+                            {
+                                Value = false
+                            },
                             AlwaysVisible = { BindTarget = DrawableRuleset.HasReplayLoaded },
-                            IsCounting = false
                         },
                         Anchor = Anchor.Centre,
                         Origin = Anchor.Centre
@@ -481,7 +484,7 @@ namespace osu.Game.Screens.Play
         {
             updateGameplayState();
             updatePauseOnFocusLostState();
-            HUDOverlay.KeyCounter.IsCounting = !isBreakTime.NewValue;
+            HUDOverlay.KeyCounter.IsCounting.Value = !isBreakTime.NewValue;
         }
 
         private void updateGameplayState()