[Chromecast] Add Observer in CmaBackend::AudioDecoder.

Bug: Internal b/162079069

Merge-With: eureka-internal/424693

Test: Check whether the observer receive the signal on device.

Change-Id: I44c15917a4104cfb94ca619d986587fd96b6857b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2349415Reviewed-by: Yuchen Liu <yucliu@chromium.org>
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Commit-Queue: Ziyang Cheng <ziyangch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798007}

chromecast/media/api/cma_backend.h

@@ -8,6 +8,7 @@
 #include <stdint.h>
 
 #include "base/memory/ref_counted.h"
+#include "base/observer_list_types.h"
 #include "chromecast/media/api/decoder_buffer_base.h"
 #include "chromecast/public/media/decoder_config.h"
 #include "chromecast/public/media/media_pipeline_backend.h"
@@ -48,6 +49,18 @@ class CmaBackend {
     using RenderingDelay = MediaPipelineBackend::AudioDecoder::RenderingDelay;
     using Statistics = MediaPipelineBackend::AudioDecoder::Statistics;
 
+    class Observer : public base::CheckedObserver {
+     public:
+      // Called when the audio is ready to play. In general the audio will start
+      // playing immediately once it's ready to play, so no need to wait for
+      // this signal. But in multizone mode, we delay the playback in order to
+      // allow time for other devices to receive the audio.
+      virtual void OnAudioReadyToPlay() = 0;
+
+     protected:
+      ~Observer() override = default;
+    };
+
     // These methods have the same behavior as the corresponding methods on
     // MediaPipelineBackend::AudioDecoder.
     // See chromecast/public/media/media_pipeline_backend.h for documentation.
@@ -61,6 +74,8 @@ class CmaBackend {
     // change whenever SetConfig() is called or the backend is initialized.
     virtual bool RequiresDecryption() = 0;
 
+    virtual void SetObserver(Observer* observer) = 0;
+
    protected:
     ~AudioDecoder() override = default;
   };

chromecast/media/api/test/mock_cma_backend.h

@@ -41,6 +41,7 @@ class MockCmaBackend : public CmaBackend {
     MOCK_METHOD0(GetRenderingDelay, RenderingDelay());
     MOCK_METHOD1(GetStatistics, void(Statistics*));
     MOCK_METHOD0(RequiresDecryption, bool());
+    MOCK_METHOD1(SetObserver, void(Observer*));
   };
 
   class VideoDecoder : public CmaBackend::VideoDecoder {

chromecast/media/audio/cast_audio_output_stream_unittest.cc

@@ -135,6 +135,7 @@ class FakeAudioDecoder : public CmaBackend::AudioDecoder {
   }
   RenderingDelay GetRenderingDelay() override { return rendering_delay_; }
   bool RequiresDecryption() override { return false; }
+  void SetObserver(CmaBackend::AudioDecoder::Observer* observer) override {}
 
   const AudioConfig& config() const { return config_; }
   float volume() const { return volume_; }

chromecast/media/audio/cma_audio_output.cc

@@ -122,6 +122,14 @@ void CmaAudioOutput::Initialize(
   timestamp_helper_.SetBaseTimestamp(base::TimeDelta());
 }
 
+void CmaAudioOutput::SetObserver(CmaBackend::AudioDecoder::Observer* observer) {
+  DCHECK_CALLED_ON_VALID_THREAD(media_thread_checker_);
+  DCHECK(observer);
+  if (audio_decoder_) {
+    audio_decoder_->SetObserver(observer);
+  }
+}
+
 bool CmaAudioOutput::Start(int64_t start_pts) {
   DCHECK_CALLED_ON_VALID_THREAD(media_thread_checker_);
   if (!cma_backend_ || !cma_backend_->Start(start_pts)) {

chromecast/media/audio/cma_audio_output.h

@@ -41,6 +41,8 @@ class CmaAudioOutput {
   CmaAudioOutput& operator=(const CmaAudioOutput&) = delete;
   ~CmaAudioOutput();
 
+  void SetObserver(CmaBackend::AudioDecoder::Observer* observer);
+
   bool Start(int64_t start_pts);
   void Stop();
   bool Pause();

chromecast/media/cma/backend/audio_decoder_wrapper.cc

@@ -43,6 +43,7 @@ class RevokedAudioDecoderWrapper : public DestructableAudioDecoder {
     *statistics = statistics_;
   }
   bool RequiresDecryption() override { return requires_decryption_; }
+  void SetObserver(CmaBackend::AudioDecoder::Observer* observer) override {}
 
   const RenderingDelay rendering_delay_;
   const Statistics statistics_;

chromecast/media/cma/backend/audio_decoder_wrapper.h

@@ -47,6 +47,7 @@ class ActiveAudioDecoderWrapper : public DestructableAudioDecoder {
   RenderingDelay GetRenderingDelay() override;
   void GetStatistics(Statistics* statistics) override;
   bool RequiresDecryption() override;
+  void SetObserver(CmaBackend::AudioDecoder::Observer* observer) override {}
 
   AudioDecoderSoftwareWrapper decoder_;
   const AudioContentType content_type_;
@@ -85,6 +86,7 @@ class AudioDecoderWrapper : public CmaBackend::AudioDecoder {
   RenderingDelay GetRenderingDelay() override;
   void GetStatistics(Statistics* statistics) override;
   bool RequiresDecryption() override;
+  void SetObserver(CmaBackend::AudioDecoder::Observer* observer) override {}
 
   bool decoder_revoked_;