Fix typo

build/android/app/CMakeLists.txt

@@ -69,6 +69,7 @@ add_library(kaliber SHARED
     ../../../src/engine/audio/audio_bus.cc
     ../../../src/engine/audio/audio_mixer.cc
     ../../../src/engine/audio/audio_sink_oboe.cc
+    ../../../src/engine/audio/mixer_input.cc
     ../../../src/engine/audio/sinc_resampler.cc
     ../../../src/engine/drawable.cc
     ../../../src/engine/engine.cc

build/linux/Makefile

@@ -102,6 +102,7 @@ ENGINE_SRC := \
   $(SRC_ROOT)/engine/audio/audio_bus.cc \
   $(SRC_ROOT)/engine/audio/audio_mixer.cc \
   $(SRC_ROOT)/engine/audio/audio_sink_alsa.cc \
+  $(SRC_ROOT)/engine/audio/mixer_input.cc \
   $(SRC_ROOT)/engine/audio/sinc_resampler.cc \
   $(SRC_ROOT)/engine/drawable.cc \
   $(SRC_ROOT)/engine/engine.cc \

src/demo/demo.cc

@@ -109,10 +109,10 @@ bool Demo::Initialize() {
   }
 
   music_.SetSound("music");
-  music_.SetMaxAplitude(0.5f);
+  music_.SetMaxAmplitude(0.5f);
 
   boss_music_.SetSound("boss_music");
-  boss_music_.SetMaxAplitude(0.5f);
+  boss_music_.SetMaxAmplitude(0.5f);
 
   if (!saved_data_.root().get("audio", Json::Value(true)).asBool())
     Engine::Get().SetEnableAudio(false);

src/demo/enemy.cc

@@ -734,29 +734,29 @@ void Enemy::SpawnUnit(EnemyType enemy_type,
     e.explosion.SetSound("powerup-pick");
 
     e.spawn.SetSound("powerup-spawn");
-    e.spawn.SetMaxAplitude(2.0f);
+    e.spawn.SetMaxAmplitude(2.0f);
     e.spawn.Play(false);
   } else {
     e.explosion.SetSound("explosion");
     e.explosion.SetVariate(true);
     e.explosion.SetSimulateStereo(true);
-    e.explosion.SetMaxAplitude(0.9f);
+    e.explosion.SetMaxAmplitude(0.9f);
   }
 
   e.stealth.SetSound("stealth");
   e.stealth.SetVariate(false);
   e.stealth.SetSimulateStereo(false);
-  e.stealth.SetMaxAplitude(0.7f);
+  e.stealth.SetMaxAmplitude(0.7f);
 
   e.shield_on.SetSound("shield");
   e.shield_on.SetVariate(false);
   e.shield_on.SetSimulateStereo(false);
-  e.shield_on.SetMaxAplitude(0.5f);
+  e.shield_on.SetMaxAmplitude(0.5f);
 
   e.hit.SetSound("hit");
   e.hit.SetVariate(true);
   e.hit.SetSimulateStereo(false);
-  e.hit.SetMaxAplitude(0.5f);
+  e.hit.SetMaxAmplitude(0.5f);
 }
 
 void Enemy::SpawnBoss() {
@@ -834,7 +834,7 @@ void Enemy::SpawnBoss() {
     e.hit.SetSound("hit");
     e.hit.SetVariate(true);
     e.hit.SetSimulateStereo(false);
-    e.hit.SetMaxAplitude(0.5f);
+    e.hit.SetMaxAmplitude(0.5f);
   });
   boss_animator_.Play(Animator::kFrames, true);
   boss_animator_.Play(Animator::kMovement, false);

src/demo/menu.cc

@@ -149,7 +149,7 @@ bool Menu::Initialize() {
   click_.SetSound(click_sound_);
   click_.SetVariate(false);
   click_.SetSimulateStereo(false);
-  click_.SetMaxAplitude(1.5f);
+  click_.SetMaxAmplitude(1.5f);
 
   logo_[0].Create("logo_tex0", {3, 8});
   logo_[0].SetZOrder(41);

src/demo/player.cc

@@ -75,7 +75,7 @@ bool Player::Initialize() {
   nuke_explosion_.SetSound("nuke");
   nuke_explosion_.SetVariate(false);
   nuke_explosion_.SetSimulateStereo(false);
-  nuke_explosion_.SetMaxAplitude(0.8f);
+  nuke_explosion_.SetMaxAmplitude(0.8f);
 
   no_nuke_.SetSound("no_nuke");
 
@@ -306,7 +306,7 @@ void Player::SetupWeapons() {
     laser_shot_[i].SetSound("laser");
     laser_shot_[i].SetVariate(true);
     laser_shot_[i].SetSimulateStereo(false);
-    laser_shot_[i].SetMaxAplitude(0.4f);
+    laser_shot_[i].SetMaxAmplitude(0.4f);
   }
 }
 

src/engine/audio/audio_mixer.cc

@@ -6,6 +6,7 @@
 #include "base/task_runner.h"
 #include "base/thread_pool.h"
 #include "engine/audio/audio_bus.h"
+#include "engine/audio/mixer_input.h"
 
 #if defined(__ANDROID__)
 #include "engine/audio/audio_sink_oboe.h"
@@ -17,10 +18,6 @@ using namespace base;
 
 namespace eng {
 
-AudioMixer::Resource::~Resource() {
-  DLOG(1) << "Destroying audio resource: " << uintptr_t(this);
-}
-
 AudioMixer::AudioMixer()
     : main_thread_task_runner_(TaskRunner::GetThreadLocalTaskRunner()),
 #if defined(__ANDROID__)
@@ -36,98 +33,11 @@ AudioMixer::~AudioMixer() {
   audio_sink_.reset();
 }
 
-std::shared_ptr<void> AudioMixer::CreateResource() {
-  auto resource = std::make_shared<Resource>();
-  DLOG(1) << "Audio resource created: " << uintptr_t(resource.get());
-  return resource;
-}
-
-void AudioMixer::Play(std::shared_ptr<void> resource,
-                      std::shared_ptr<AudioBus> audio_bus,
-                      float amplitude,
-                      bool reset_pos) {
-  if (!audio_enabled_)
-    return;
-
-  auto resource_ptr = std::static_pointer_cast<Resource>(resource);
-
-  if (resource_ptr->active) {
-    if (reset_pos)
-      resource_ptr->flags.fetch_or(kStopped, std::memory_order_relaxed);
-
-    if (resource_ptr->flags.load(std::memory_order_relaxed) & kStopped)
-      resource_ptr->restart_cb = [&, resource, audio_bus, amplitude,
-                                  reset_pos]() -> void {
-        Play(resource, audio_bus, amplitude, reset_pos);
-      };
-
-    return;
-  }
-
-  if (reset_pos) {
-    resource_ptr->src_index = 0;
-    resource_ptr->accumulator = 0;
-    audio_bus->ResetStream();
-  } else if (resource_ptr->src_index >= audio_bus->samples_per_channel()) {
-    return;
-  }
-
-  resource_ptr->active = true;
-  resource_ptr->flags.fetch_and(~kStopped, std::memory_order_relaxed);
-  resource_ptr->audio_bus = audio_bus;
-  if (amplitude >= 0)
-    resource_ptr->amplitude = amplitude;
+void AudioMixer::AddInput(std::shared_ptr<MixerInput> input) {
+  DCHECK(audio_enabled_);
 
   std::lock_guard<std::mutex> scoped_lock(lock_);
-  play_list_[0].push_back(resource_ptr);
-}
-
-void AudioMixer::Stop(std::shared_ptr<void> resource) {
-  auto resource_ptr = std::static_pointer_cast<Resource>(resource);
-  if (resource_ptr->active) {
-    resource_ptr->restart_cb = nullptr;
-    resource_ptr->flags.fetch_or(kStopped, std::memory_order_relaxed);
-  }
-}
-
-void AudioMixer::SetLoop(std::shared_ptr<void> resource, bool loop) {
-  auto resource_ptr = std::static_pointer_cast<Resource>(resource);
-  if (loop)
-    resource_ptr->flags.fetch_or(kLoop, std::memory_order_relaxed);
-  else
-    resource_ptr->flags.fetch_and(~kLoop, std::memory_order_relaxed);
-}
-
-void AudioMixer::SetSimulateStereo(std::shared_ptr<void> resource,
-                                   bool simulate) {
-  auto resource_ptr = std::static_pointer_cast<Resource>(resource);
-  if (simulate)
-    resource_ptr->flags.fetch_or(kSimulateStereo, std::memory_order_relaxed);
-  else
-    resource_ptr->flags.fetch_and(~kSimulateStereo, std::memory_order_relaxed);
-}
-
-void AudioMixer::SetResampleStep(std::shared_ptr<void> resource, size_t step) {
-  auto resource_ptr = std::static_pointer_cast<Resource>(resource);
-  resource_ptr->step.store(step + 100, std::memory_order_relaxed);
-}
-
-void AudioMixer::SetMaxAmplitude(std::shared_ptr<void> resource,
-                                 float max_amplitude) {
-  auto resource_ptr = std::static_pointer_cast<Resource>(resource);
-  resource_ptr->max_amplitude.store(max_amplitude, std::memory_order_relaxed);
-}
-
-void AudioMixer::SetAmplitudeInc(std::shared_ptr<void> resource,
-                                 float amplitude_inc) {
-  auto resource_ptr = std::static_pointer_cast<Resource>(resource);
-  resource_ptr->amplitude_inc.store(amplitude_inc, std::memory_order_relaxed);
-}
-
-void AudioMixer::SetEndCallback(std::shared_ptr<void> resource,
-                                base::Closure cb) {
-  auto resource_ptr = std::static_pointer_cast<Resource>(resource);
-  resource_ptr->end_cb = std::move(cb);
+  inputs_[0].push_back(input);
 }
 
 void AudioMixer::Suspend() {
@@ -146,17 +56,17 @@ void AudioMixer::RenderAudio(float* output_buffer, size_t num_frames) {
   {
     std::unique_lock<std::mutex> scoped_lock(lock_, std::try_to_lock);
     if (scoped_lock)
-      play_list_[1].splice(play_list_[1].end(), play_list_[0]);
+      inputs_[1].splice(inputs_[1].end(), inputs_[0]);
   }
 
   memset(output_buffer, 0, sizeof(float) * num_frames * kChannelCount);
 
-  for (auto it = play_list_[1].begin(); it != play_list_[1].end();) {
+  for (auto it = inputs_[1].begin(); it != inputs_[1].end();) {
     auto audio_bus = (*it)->audio_bus.get();
     unsigned flags = (*it)->flags.load(std::memory_order_relaxed);
     bool marked_for_removal = false;
 
-    if (flags & kStopped) {
+    if (flags & MixerInput::kStopped) {
       marked_for_removal = true;
     } else {
       const float* src[2] = {audio_bus->GetChannelData(0),
@@ -173,8 +83,9 @@ void AudioMixer::RenderAudio(float* output_buffer, size_t num_frames) {
           (*it)->amplitude_inc.load(std::memory_order_relaxed);
       float max_amplitude =
           (*it)->max_amplitude.load(std::memory_order_relaxed);
-      size_t channel_offset =
-          (flags & kSimulateStereo) ? audio_bus->sample_rate() / 10 : 0;
+      size_t channel_offset = (flags & MixerInput::kSimulateStereo)
+                                  ? audio_bus->sample_rate() / 10
+                                  : 0;
 
       DCHECK(num_samples > 0);
 
@@ -187,7 +98,7 @@ void AudioMixer::RenderAudio(float* output_buffer, size_t num_frames) {
           size_t ind = channel_offset + src_index;
           if (ind < num_samples)
             output_buffer[i++] += src[1][ind] * amplitude;
-          else if (flags & kLoop)
+          else if (flags & MixerInput::kLoop)
             output_buffer[i++] += src[1][ind % num_samples] * amplitude;
           else
             i++;
@@ -208,7 +119,7 @@ void AudioMixer::RenderAudio(float* output_buffer, size_t num_frames) {
         } else {
           if (audio_bus->EndOfStream()) {
             src_index %= num_samples;
-            marked_for_removal = !(flags & kLoop);
+            marked_for_removal = !(flags & MixerInput::kLoop);
             break;
           }
 
@@ -226,7 +137,8 @@ void AudioMixer::RenderAudio(float* output_buffer, size_t num_frames) {
 
             ThreadPool::Get().PostTask(
                 HERE,
-                std::bind(&AudioMixer::DoStream, this, *it, flags & kLoop),
+                std::bind(&AudioMixer::DoStream, this, *it,
+                          flags & MixerInput::kLoop),
                 true);
           } else {
             DLOG(0) << "Mixer buffer underrun!";
@@ -240,37 +152,37 @@ void AudioMixer::RenderAudio(float* output_buffer, size_t num_frames) {
     }
 
     if (marked_for_removal) {
-      end_list_.push_back(*it);
-      it = play_list_[1].erase(it);
+      removed_inputs_.push_back(*it);
+      it = inputs_[1].erase(it);
     } else {
       ++it;
     }
   }
 
-  for (auto it = end_list_.begin(); it != end_list_.end();) {
+  for (auto it = removed_inputs_.begin(); it != removed_inputs_.end();) {
     if (!(*it)->streaming_in_progress.load(std::memory_order_relaxed)) {
       main_thread_task_runner_->PostTask(
           HERE, std::bind(&AudioMixer::EndCallback, this, *it));
-      it = end_list_.erase(it);
+      it = removed_inputs_.erase(it);
     } else {
       ++it;
     }
   }
 }
 
-void AudioMixer::DoStream(std::shared_ptr<Resource> resource, bool loop) {
-  resource->audio_bus->Stream(loop);
-  resource->streaming_in_progress.store(false, std::memory_order_release);
+void AudioMixer::DoStream(std::shared_ptr<MixerInput> input, bool loop) {
+  input->audio_bus->Stream(loop);
+  input->streaming_in_progress.store(false, std::memory_order_release);
 }
 
-void AudioMixer::EndCallback(std::shared_ptr<Resource> resource) {
-  resource->active = false;
+void AudioMixer::EndCallback(std::shared_ptr<MixerInput> input) {
+  input->active = false;
 
-  if (resource->end_cb)
-    resource->end_cb();
-  if (resource->restart_cb) {
-    resource->restart_cb();
-    resource->restart_cb = nullptr;
+  if (input->end_cb)
+    input->end_cb();
+  if (input->restart_cb) {
+    input->restart_cb();
+    input->restart_cb = nullptr;
   }
 }
 

src/engine/audio/audio_mixer.h

@@ -1,7 +1,6 @@
 #ifndef ENGINE_AUDIO_AUDIO_MIXER_H
 #define ENGINE_AUDIO_AUDIO_MIXER_H
 
-#include <atomic>
 #include <list>
 #include <memory>
 #include <mutex>
@@ -16,31 +15,24 @@ class TaskRunner;
 namespace eng {
 
 class AudioBus;
+struct MixerInput;
 
-// Mix and render audio with low overhead. A platform specific AudioSink
-// implementation is expected to periodically call RenderAudio() in a background
-// thread.
+// Mix and render audio with low overhead. The mixer has zero or more inputs
+// which can be added at any time. The mixer will pull from each input source
+// when it needs more data. Input source will be removed once end-of-stream is
+// reached. Any unfilled frames will be filled with silence. The mixer always
+// outputs audio when active, even if input sources underflow. A platform
+// specific AudioSink implementation is expected to periodically call
+// RenderAudio() in a background thread.
 class AudioMixer : public AudioSink::Delegate {
  public:
   AudioMixer();
   ~AudioMixer();
 
-  std::shared_ptr<void> CreateResource();
-
-  void Play(std::shared_ptr<void> resource,
-            std::shared_ptr<AudioBus> audio_bus,
-            float amplitude,
-            bool reset_pos);
-  void Stop(std::shared_ptr<void> resource);
-
-  void SetLoop(std::shared_ptr<void> resource, bool loop);
-  void SetSimulateStereo(std::shared_ptr<void> resource, bool simulate);
-  void SetResampleStep(std::shared_ptr<void> resource, size_t step);
-  void SetMaxAmplitude(std::shared_ptr<void> resource, float max_amplitude);
-  void SetAmplitudeInc(std::shared_ptr<void> resource, float amplitude_inc);
-  void SetEndCallback(std::shared_ptr<void> resource, base::Closure cb);
+  void AddInput(std::shared_ptr<MixerInput> input);
 
   void SetEnableAudio(bool enable) { audio_enabled_ = enable; }
+  bool IsAudioEnabled() const { return audio_enabled_; }
 
   void Suspend();
   void Resume();
@@ -48,38 +40,12 @@ class AudioMixer : public AudioSink::Delegate {
   size_t GetHardwareSampleRate();
 
  private:
-  enum SampleFlags { kLoop = 1, kStopped = 2, kSimulateStereo = 4 };
   static constexpr int kChannelCount = 2;
 
-  // An audio resource that gets mixed and rendered to the audio sink.
-  struct Resource {
-    // Accessed by main thread only.
-    bool active = false;
-    base::Closure end_cb;
-    base::Closure restart_cb;
-
-    // Initialized by main thread, used by audio thread.
-    std::shared_ptr<AudioBus> audio_bus;
-    size_t src_index = 0;
-    size_t accumulator = 0;
-    float amplitude = 1.0f;
-
-    // Write accessed by main thread, read-only accessed by audio thread.
-    std::atomic<unsigned> flags{0};
-    std::atomic<size_t> step{100};
-    std::atomic<float> amplitude_inc{0};
-    std::atomic<float> max_amplitude{1.0f};
-
-    // Accessed by audio thread and decoder thread.
-    std::atomic<bool> streaming_in_progress{false};
-
-    ~Resource();
-  };
-
-  std::list<std::shared_ptr<Resource>> play_list_[2];
+  std::list<std::shared_ptr<MixerInput>> inputs_[2];
   std::mutex lock_;
 
-  std::list<std::shared_ptr<Resource>> end_list_;
+  std::list<std::shared_ptr<MixerInput>> removed_inputs_;
 
   std::shared_ptr<base::TaskRunner> main_thread_task_runner_;
 
@@ -91,9 +57,9 @@ class AudioMixer : public AudioSink::Delegate {
   int GetChannelCount() final { return kChannelCount; }
   void RenderAudio(float* output_buffer, size_t num_frames) final;
 
-  void DoStream(std::shared_ptr<Resource> sample, bool loop);
+  void DoStream(std::shared_ptr<MixerInput> sample, bool loop);
 
-  void EndCallback(std::shared_ptr<Resource> sample);
+  void EndCallback(std::shared_ptr<MixerInput> sample);
 
   AudioMixer(const AudioMixer&) = delete;
   AudioMixer& operator=(const AudioMixer&) = delete;

src/engine/audio/mixer_input.cc

@@ -0,0 +1,98 @@
+#include "engine/audio/mixer_input.h"
+
+#include "base/log.h"
+#include "engine/audio/audio_bus.h"
+#include "engine/audio/audio_mixer.h"
+
+using namespace base;
+
+namespace eng {
+
+MixerInput::MixerInput() {
+  DLOG(1) << "MixerInput created: " << uintptr_t(this);
+}
+
+MixerInput::~MixerInput() {
+  DLOG(1) << "Destroying MixerInput: " << uintptr_t(this);
+}
+
+// static
+std::shared_ptr<MixerInput> MixerInput::Create() {
+  return std::shared_ptr<MixerInput>(new MixerInput());
+}
+
+void MixerInput::Play(AudioMixer* mixer,
+                      std::shared_ptr<AudioBus> bus,
+                      float amp,
+                      bool restart) {
+  if (!mixer->IsAudioEnabled())
+    return;
+
+  if (bus != audio_bus || src_index >= bus->samples_per_channel())
+    restart = true;
+
+  if (active) {
+    if (restart)
+      flags.fetch_or(kStopped, std::memory_order_relaxed);
+
+    if (flags.load(std::memory_order_relaxed) & kStopped)
+      restart_cb = [&, mixer, bus, amp, restart]() -> void {
+        Play(mixer, bus, amp, restart);
+      };
+
+    return;
+  }
+
+  if (restart) {
+    src_index = 0;
+    accumulator = 0;
+    bus->ResetStream();
+  }
+
+  active = true;
+  flags.fetch_and(~kStopped, std::memory_order_relaxed);
+  audio_bus = bus;
+  if (amp >= 0)
+    amplitude = amp;
+
+  mixer->AddInput(shared_from_this());
+}
+
+void MixerInput::Stop() {
+  if (active) {
+    restart_cb = nullptr;
+    flags.fetch_or(kStopped, std::memory_order_relaxed);
+  }
+}
+
+void MixerInput::SetLoop(bool loop) {
+  if (loop)
+    flags.fetch_or(kLoop, std::memory_order_relaxed);
+  else
+    flags.fetch_and(~kLoop, std::memory_order_relaxed);
+}
+
+void MixerInput::SetSimulateStereo(bool simulate) {
+  if (simulate)
+    flags.fetch_or(kSimulateStereo, std::memory_order_relaxed);
+  else
+    flags.fetch_and(~kSimulateStereo, std::memory_order_relaxed);
+}
+
+void MixerInput::SetResampleStep(size_t value) {
+  step.store(value + 100, std::memory_order_relaxed);
+}
+
+void MixerInput::SetMaxAmplitude(float value) {
+  max_amplitude.store(value, std::memory_order_relaxed);
+}
+
+void MixerInput::SetAmplitudeInc(float value) {
+  amplitude_inc.store(value, std::memory_order_relaxed);
+}
+
+void MixerInput::SetEndCallback(base::Closure cb) {
+  end_cb = std::move(cb);
+}
+
+}  // namespace eng

src/engine/audio/mixer_input.h

@@ -0,0 +1,62 @@
+#ifndef ENGINE_AUDIO_MIXER_INPUT_H
+#define ENGINE_AUDIO_MIXER_INPUT_H
+
+#include <atomic>
+#include <memory>
+
+#include "base/closure.h"
+
+namespace eng {
+
+class AudioBus;
+class AudioMixer;
+
+// An audio input stream that gets mixed and rendered to the audio sink. Handles
+// playback and volume control.
+struct MixerInput : public std::enable_shared_from_this<MixerInput> {
+  enum Flags { kLoop = 1, kStopped = 2, kSimulateStereo = 4 };
+
+  ~MixerInput();
+
+  static std::shared_ptr<MixerInput> Create();
+
+  void Play(AudioMixer* mixer,
+            std::shared_ptr<AudioBus> bus,
+            float amp,
+            bool restart);
+  void Stop();
+
+  void SetLoop(bool loop);
+  void SetSimulateStereo(bool simulate);
+  void SetResampleStep(size_t value);
+  void SetMaxAmplitude(float value);
+  void SetAmplitudeInc(float value);
+  void SetEndCallback(base::Closure cb);
+
+  // Accessed by main thread only.
+  bool active = false;
+  base::Closure end_cb;
+  base::Closure restart_cb;
+
+  // Initialized by main thread, used by audio thread.
+  std::shared_ptr<AudioBus> audio_bus;
+  size_t src_index = 0;
+  size_t accumulator = 0;
+  float amplitude = 1.0f;
+
+  // Write accessed by main thread, read-only accessed by audio thread.
+  std::atomic<unsigned> flags{0};
+  std::atomic<size_t> step{100};
+  std::atomic<float> amplitude_inc{0};
+  std::atomic<float> max_amplitude{1.0f};
+
+  // Accessed by audio thread and decoder thread.
+  std::atomic<bool> streaming_in_progress{false};
+
+ private:
+  MixerInput();
+};
+
+}  // namespace eng
+
+#endif  // ENGINE_AUDIO_MIXER_INPUT_H

src/engine/sound_player.cc

@@ -4,17 +4,17 @@
 #include "base/log.h"
 #include "engine/audio/audio_bus.h"
 #include "engine/audio/audio_mixer.h"
+#include "engine/audio/mixer_input.h"
 #include "engine/engine.h"
 
 using namespace base;
 
 namespace eng {
 
-SoundPlayer::SoundPlayer()
-    : resource_(Engine::Get().GetAudioMixer()->CreateResource()) {}
+SoundPlayer::SoundPlayer() : input_{MixerInput::Create()} {}
 
 SoundPlayer::~SoundPlayer() {
-  Engine::Get().GetAudioMixer()->Stop(resource_);
+  input_->Stop();
 }
 
 void SoundPlayer::SetSound(const std::string& asset_name) {
@@ -30,15 +30,14 @@ void SoundPlayer::Play(bool loop, float fade_in_duration) {
     return;
 
   int step = variate_ ? Engine::Get().GetRandomGenerator().Roll(3) - 2 : 0;
-  Engine::Get().GetAudioMixer()->SetResampleStep(resource_, step * 12);
-  Engine::Get().GetAudioMixer()->SetLoop(resource_, loop);
+  input_->SetResampleStep(step * 12);
+  input_->SetLoop(loop);
   if (fade_in_duration > 0)
-    Engine::Get().GetAudioMixer()->SetAmplitudeInc(
-        resource_, 1.0f / (sound_->sample_rate() * fade_in_duration));
+    input_->SetAmplitudeInc(1.0f / (sound_->sample_rate() * fade_in_duration));
   else
-    Engine::Get().GetAudioMixer()->SetAmplitudeInc(resource_, 0);
-  Engine::Get().GetAudioMixer()->Play(
-      resource_, sound_, fade_in_duration > 0 ? 0 : max_amplitude_, true);
+    input_->SetAmplitudeInc(0);
+  input_->Play(Engine::Get().GetAudioMixer(), sound_,
+               fade_in_duration > 0 ? 0 : max_amplitude_, true);
 }
 
 void SoundPlayer::Resume(float fade_in_duration) {
@@ -46,9 +45,8 @@ void SoundPlayer::Resume(float fade_in_duration) {
     return;
 
   if (fade_in_duration > 0)
-    Engine::Get().GetAudioMixer()->SetAmplitudeInc(
-        resource_, 1.0f / (sound_->sample_rate() * fade_in_duration));
-  Engine::Get().GetAudioMixer()->Play(resource_, sound_,
+    input_->SetAmplitudeInc(1.0f / (sound_->sample_rate() * fade_in_duration));
+  input_->Play(Engine::Get().GetAudioMixer(), sound_,
                fade_in_duration > 0 ? 0 : -1, false);
 }
 
@@ -57,10 +55,10 @@ void SoundPlayer::Stop(float fade_out_duration) {
     return;
 
   if (fade_out_duration > 0)
-    Engine::Get().GetAudioMixer()->SetAmplitudeInc(
-        resource_, -1.0f / (sound_->sample_rate() * fade_out_duration));
+    input_->SetAmplitudeInc(-1.0f /
+                            (sound_->sample_rate() * fade_out_duration));
   else
-    Engine::Get().GetAudioMixer()->Stop(resource_);
+    input_->Stop();
 }
 
 void SoundPlayer::SetVariate(bool variate) {
@@ -68,16 +66,16 @@ void SoundPlayer::SetVariate(bool variate) {
 }
 
 void SoundPlayer::SetSimulateStereo(bool simulate) {
-  Engine::Get().GetAudioMixer()->SetSimulateStereo(resource_, simulate);
+  input_->SetSimulateStereo(simulate);
 }
 
-void SoundPlayer::SetMaxAplitude(float max_amplitude) {
+void SoundPlayer::SetMaxAmplitude(float max_amplitude) {
   max_amplitude_ = max_amplitude;
-  Engine::Get().GetAudioMixer()->SetMaxAmplitude(resource_, max_amplitude);
+  input_->SetMaxAmplitude(max_amplitude);
 }
 
 void SoundPlayer::SetEndCallback(base::Closure cb) {
-  Engine::Get().GetAudioMixer()->SetEndCallback(resource_, cb);
+  input_->SetEndCallback(cb);
 }
 
 }  // namespace eng

src/engine/sound_player.h

@@ -9,6 +9,7 @@
 namespace eng {
 
 class AudioBus;
+struct MixerInput;
 
 class SoundPlayer {
  public:
@@ -31,13 +32,13 @@ class SoundPlayer {
   // Enable or disable stereo simulation effect. Disabled by default.
   void SetSimulateStereo(bool simulate);
 
-  void SetMaxAplitude(float max_amplitude);
+  void SetMaxAmplitude(float max_amplitude);
 
   // Set callback to be called once playback stops.
   void SetEndCallback(base::Closure cb);
 
  private:
-  std::shared_ptr<void> resource_;
+  std::shared_ptr<MixerInput> input_;
   std::shared_ptr<AudioBus> sound_;
 
   float max_amplitude_ = 1.0f;