Refactoring and comments for MixerInput

src/engine/audio/audio_mixer.cc

@@ -176,7 +176,12 @@ void AudioMixer::DoStream(std::shared_ptr<MixerInput> input, bool loop) {
 }
 
 void AudioMixer::EndCallback(std::shared_ptr<MixerInput> input) {
-  input->active = false;
+  input->streaming = false;
+
+  if (input->pending_audio_bus) {
+    input->audio_bus = input->pending_audio_bus;
+    input->pending_audio_bus.reset();
+  }
 
   if (input->end_cb)
     input->end_cb();

src/engine/audio/mixer_input.cc

@@ -21,45 +21,41 @@ 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) {
+void MixerInput::SetAudioBus(std::shared_ptr<AudioBus> bus) {
+  if (streaming)
+    pending_audio_bus = bus;
+  else
+    audio_bus = bus;
+}
+
+void MixerInput::Play(AudioMixer* mixer, bool restart) {
   if (!mixer->IsAudioEnabled())
     return;
 
-  if (bus != audio_bus || src_index >= bus->samples_per_channel())
-    restart = true;
-
-  if (active) {
+  // If already streaming check if stream position needs to be reset.
+  if (streaming) {
     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);
-      };
+      restart_cb = [&, mixer, restart]() -> void { Play(mixer, restart); };
 
     return;
   }
 
-  if (restart) {
+  if (restart || audio_bus->EndOfStream()) {
     src_index = 0;
     accumulator = 0;
-    bus->ResetStream();
+    audio_bus->ResetStream();
   }
 
-  active = true;
+  streaming = 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) {
+  if (streaming) {
     restart_cb = nullptr;
     flags.fetch_or(kStopped, std::memory_order_relaxed);
   }
@@ -83,6 +79,10 @@ void MixerInput::SetResampleStep(size_t value) {
   step.store(value + 100, std::memory_order_relaxed);
 }
 
+void MixerInput::SetAmplitude(float value) {
+  amplitude.store(value, std::memory_order_relaxed);
+}
+
 void MixerInput::SetMaxAmplitude(float value) {
   max_amplitude.store(value, std::memory_order_relaxed);
 }

src/engine/audio/mixer_input.h

@@ -20,37 +20,54 @@ struct MixerInput : public std::enable_shared_from_this<MixerInput> {
 
   static std::shared_ptr<MixerInput> Create();
 
-  void Play(AudioMixer* mixer,
-            std::shared_ptr<AudioBus> bus,
-            float amp,
-            bool restart);
+  // Set AudioBus for playback. If this MixerInput is already streaming, keeps
+  // it as pending and sets once playback ends.
+  void SetAudioBus(std::shared_ptr<AudioBus> bus);
+
+  // Starts playback. Remembers last stream position. Resets the stream position
+  // if restart is true.
+  void Play(AudioMixer* mixer, bool restart);
+
+  // Stops playback. Does not reset stream position.
   void Stop();
 
   void SetLoop(bool loop);
+
+  // Simulate stereo effect slightly delays one channel.
   void SetSimulateStereo(bool simulate);
+
+  // Vary basic resampling for sound variations.
   void SetResampleStep(size_t value);
+
+  // Set the current volume, max volume and volume increment steps.
+  void SetAmplitude(float value);
   void SetMaxAmplitude(float value);
   void SetAmplitudeInc(float value);
+
+  // Set a callback to be called once playback ends.
   void SetEndCallback(base::Closure cb);
 
+  // Active AudioBus.
+  std::shared_ptr<AudioBus> audio_bus;
+  // AudioBus to be set as active once playback ends.
+  std::shared_ptr<AudioBus> pending_audio_bus;
+  // Stream position.
+  size_t src_index = 0;
+  size_t accumulator = 0;
+
   // Accessed by main thread only.
-  bool active = false;
+  bool streaming = 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.
+  // Accessed by main thread and audio thread.
   std::atomic<unsigned> flags{0};
   std::atomic<size_t> step{100};
+  std::atomic<float> amplitude{1.0f};
   std::atomic<float> amplitude_inc{0};
   std::atomic<float> max_amplitude{1.0f};
 
-  // Accessed by audio thread and decoder thread.
+  // Accessed by audio thread and decoding worker thread.
   std::atomic<bool> streaming_in_progress{false};
 
  private:

src/engine/sound_player.cc

@@ -18,45 +18,50 @@ SoundPlayer::~SoundPlayer() {
 }
 
 void SoundPlayer::SetSound(const std::string& asset_name) {
-  sound_ = Engine::Get().GetAudioBus(asset_name);
+  input_->SetAudioBus(Engine::Get().GetAudioBus(asset_name));
 }
 
 void SoundPlayer::SetSound(std::shared_ptr<AudioBus> sound) {
-  sound_ = sound;
+  input_->SetAudioBus(sound);
 }
 
 void SoundPlayer::Play(bool loop, float fade_in_duration) {
-  if (!sound_)
+  if (!input_->audio_bus)
     return;
 
   int step = variate_ ? Engine::Get().GetRandomGenerator().Roll(3) - 2 : 0;
   input_->SetResampleStep(step * 12);
   input_->SetLoop(loop);
-  if (fade_in_duration > 0)
-    input_->SetAmplitudeInc(1.0f / (sound_->sample_rate() * fade_in_duration));
-  else
+  if (fade_in_duration > 0) {
+    input_->SetAmplitude(0);
+    input_->SetAmplitudeInc(
+        1.0f / (input_->audio_bus->sample_rate() * fade_in_duration));
+  } else {
+    input_->SetAmplitude(max_amplitude_);
     input_->SetAmplitudeInc(0);
-  input_->Play(Engine::Get().GetAudioMixer(), sound_,
-               fade_in_duration > 0 ? 0 : max_amplitude_, true);
+  }
+  input_->Play(Engine::Get().GetAudioMixer(), true);
 }
 
 void SoundPlayer::Resume(float fade_in_duration) {
-  if (!sound_)
+  if (!input_->audio_bus)
     return;
 
-  if (fade_in_duration > 0)
-    input_->SetAmplitudeInc(1.0f / (sound_->sample_rate() * fade_in_duration));
-  input_->Play(Engine::Get().GetAudioMixer(), sound_,
-               fade_in_duration > 0 ? 0 : -1, false);
+  if (fade_in_duration > 0) {
+    input_->SetAmplitude(0);
+    input_->SetAmplitudeInc(
+        1.0f / (input_->audio_bus->sample_rate() * fade_in_duration));
+  }
+  input_->Play(Engine::Get().GetAudioMixer(), false);
 }
 
 void SoundPlayer::Stop(float fade_out_duration) {
-  if (!sound_)
+  if (!input_->audio_bus)
     return;
 
   if (fade_out_duration > 0)
-    input_->SetAmplitudeInc(-1.0f /
-                            (sound_->sample_rate() * fade_out_duration));
+    input_->SetAmplitudeInc(
+        -1.0f / (input_->audio_bus->sample_rate() * fade_out_duration));
   else
     input_->Stop();
 }

src/engine/sound_player.h

@@ -39,7 +39,6 @@ class SoundPlayer {
 
  private:
   std::shared_ptr<MixerInput> input_;
-  std::shared_ptr<AudioBus> sound_;
 
   float max_amplitude_ = 1.0f;