Expose audio mixers to mods

osu.Game/Audio/Effects/AudioFilter.cs

@@ -16,7 +16,7 @@ public class AudioFilter : Component, ITransformableFilter
         /// </summary>
         public const int MAX_LOWPASS_CUTOFF = 22049; // nyquist - 1hz
 
-        private readonly AudioMixer mixer;
+        private readonly IAudioMixer mixer;
         private readonly BQFParameters filter;
         private readonly BQFType type;
 
@@ -45,7 +45,7 @@ public int Cutoff
         /// </summary>
         /// <param name="mixer">The mixer this effect should be applied to.</param>
         /// <param name="type">The type of filter (e.g. LowPass, HighPass, etc)</param>
-        public AudioFilter(AudioMixer mixer, BQFType type = BQFType.LowPass)
+        public AudioFilter(IAudioMixer mixer, BQFType type = BQFType.LowPass)
         {
             this.mixer = mixer;
             this.type = type;
@@ -131,7 +131,9 @@ private void ensureDetached()
             if (!isAttached)
                 return;
 
-            Debug.Assert(mixer.Effects.Contains(filter));
+            // Do not assert for the existence of the filter in mixer
+            // There are cases where effects are cleared from the mixer's side,
+            // causing the filter to still think it's attached when it isn't
             mixer.Effects.Remove(filter);
             isAttached = false;
         }

osu.Game/Overlays/MusicController.cs

@@ -7,6 +7,7 @@
 using JetBrains.Annotations;
 using osu.Framework.Allocation;
 using osu.Framework.Audio;
+using osu.Framework.Audio.Mixing;
 using osu.Framework.Audio.Track;
 using osu.Framework.Bindables;
 using osu.Framework.Graphics;
@@ -55,6 +56,15 @@ public class MusicController : CompositeDrawable
         [Resolved]
         private RealmAccess realm { get; set; }
 
+        private readonly DrawableAudioMixer drawableAudioMixer;
+
+        public IAudioMixer AudioMixer => drawableAudioMixer;
+
+        public MusicController()
+        {
+            AddInternal(drawableAudioMixer = new DrawableAudioMixer { Name = $"{nameof(MusicController)} {nameof(DrawableAudioMixer)}" });
+        }
+
         [BackgroundDependencyLoader]
         private void load()
         {
@@ -328,7 +338,7 @@ private void changeTrack()
 
                 if (queuedTrack == CurrentTrack)
                 {
-                    AddInternal(queuedTrack);
+                    drawableAudioMixer.Add(queuedTrack);
                     queuedTrack.VolumeTo(0).Then().VolumeTo(1, 300, Easing.Out);
                 }
                 else
@@ -389,11 +399,14 @@ public void ResetTrackAdjustments()
             CurrentTrack.RemoveAllAdjustments(AdjustableProperty.Frequency);
             CurrentTrack.RemoveAllAdjustments(AdjustableProperty.Tempo);
             CurrentTrack.RemoveAllAdjustments(AdjustableProperty.Volume);
+            drawableAudioMixer.Effects.Clear();
 
             if (allowTrackAdjustments)
             {
                 foreach (var mod in mods.Value.OfType<IApplicableToTrack>())
                     mod.ApplyToTrack(CurrentTrack);
+                foreach (var mod in mods.Value.OfType<IApplicableToTrackMixer>())
+                    mod.ApplyToTrackMixer(drawableAudioMixer);
             }
         }
     }

osu.Game/Rulesets/Mods/IApplicableToTrackMixer.cs

@@ -0,0 +1,15 @@
+// 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.Audio.Mixing;
+
+namespace osu.Game.Rulesets.Mods
+{
+    /// <summary>
+    /// An interface for mods that add effects to the track mixer.
+    /// </summary>
+    public interface IApplicableToTrackMixer : IApplicableMod
+    {
+        void ApplyToTrackMixer(IAudioMixer mixer);
+    }
+}

osu.Game/Screens/Play/Player.cs

@@ -960,6 +960,8 @@ public override void OnEntering(IScreen last)
             musicController.ResetTrackAdjustments();
             foreach (var mod in GameplayState.Mods.OfType<IApplicableToTrack>())
                 mod.ApplyToTrack(musicController.CurrentTrack);
+            foreach (var mod in GameplayState.Mods.OfType<IApplicableToTrackMixer>())
+                mod.ApplyToTrackMixer(musicController.AudioMixer);
 
             updateGameplayState();