[Android] MediaExtractor 和 MediaCodec 如何配合使用 - Audio篇

在[Android] MediaExtractor 和 MediaCodec 如何配合使用 - Video篇裡，
我們示範了如何處理video，
現在我們來看audio部份，

原則上，我們可以使用同一個MediaExtractor來同時處理video/audio，
只要透過selectTrack()來動態切換即可，
不過，不知為何常常會收不到資料，
所以這邊我們就用固定的方式，
video和audio都有自己的MediaExtractor ，

audio處理方式和video大同小異，
除了audio不需要render到SurfaceView，
以及audio需要多一個AudioTrack()來處理decode好的PCM data，

private MediaExtractor extractorAudio;
private MediaCodec decoderAudio;

extractorAudio = new MediaExtractor();
extractorAudio.setDataSource("myTest.mp4");

for (int i = 0; i < extractorAudio.getTrackCount(); i++) {
 MediaFormat format = extractorAudio.getTrackFormat(i);
 String mime = format.getString(MediaFormat.KEY_MIME); 
 if (mime.startsWith("audio/")) {  
  audioTrack = i;  
  extractorAudio.selectTrack(audioTrack);
  formatAudio = format;        
  decoderAudio = MediaCodec.createDecoderByType(mime);
  sampleRate = format.getInteger(MediaFormat.KEY_SAMPLE_RATE);
  decoderAudio.configure(format, null, null, 0);
  break;
 }
}

if (audioTrack >=0) {
 if(decoderAudio == null)
 {
  Log.e(TAG, "Can't find audio info!");
  return;
 }
 else
 {
   // create our AudioTrack instance
   int minBufferSize = AudioTrack.getMinBufferSize(44100, AudioFormat.CHANNEL_CONFIGURATION_STEREO, AudioFormat.ENCODING_PCM_16BIT);
      int bufferSize = 4 * minBufferSize;
  playAudioTrack = new AudioTrack(
    AudioManager.STREAM_MUSIC,
    formatAudio.getInteger(MediaFormat.KEY_SAMPLE_RATE),
    AudioFormat.CHANNEL_OUT_STEREO,
    AudioFormat.ENCODING_PCM_16BIT,
    bufferSize,
    AudioTrack.MODE_STREAM
   );
  playAudioTrack.play();
  decoderAudio.start();
 }
}

extractorAudio一樣是根據MIME的資訊將audio track找出來，
decoderAudio.configure()我們只要傳進track format即可，其它參數都不需要，
另外，我們還需要多一個AudioTrack()，
因為decoderAudio最後解出來的是PCM data，
要實際發出聲音來，需要AudioTrack()幫忙，

下面是decode部份：

ByteBuffer[] inputBuffersAudio=null;
ByteBuffer[] outputBuffersAudio=null;
BufferInfo infoAudio=null;


if (audioTrack >=0)
{
 inputBuffersAudio = decoderAudio.getInputBuffers();
 outputBuffersAudio = decoderAudio.getOutputBuffers();
 infoAudio = new BufferInfo();
}
boolean isEOS = false;
long startMs = System.currentTimeMillis();
long lasAudioStartMs = System.currentTimeMillis();
while (!Thread.interrupted()) { 
 if (audioTrack >=0)
 { 
  if (!isEOS) {
   int inIndex=-1;
   try {
    inIndex = decoderAudio.dequeueInputBuffer(10000);
   } catch (Exception e) {
    e.printStackTrace();    
   }
   if (inIndex >= 0) {    
    ByteBuffer buffer = inputBuffersAudio[inIndex];
    int sampleSize = extractorAudio.readSampleData(buffer, 0);
    if (sampleSize < 0) {
     
     decoderAudio.queueInputBuffer(inIndex, 0, 0, 0, MediaCodec.BUFFER_FLAG_END_OF_STREAM);
     buffer.clear();
     isEOS = true;
    } else {     
     decoderAudio.queueInputBuffer(inIndex, 0, sampleSize, extractorAudio.getSampleTime(), 0);
     buffer.clear();
     extractorAudio.advance();
    }
    
   }
  }

  int outIndex=-1;
  try {
   outIndex = decoderAudio.dequeueOutputBuffer(infoAudio,10000);
  } catch (Exception e) {
   e.printStackTrace();   
  }

  switch (outIndex) {
  case MediaCodec.INFO_OUTPUT_BUFFERS_CHANGED:
   Log.d(TAG, "INFO_OUTPUT_BUFFERS_CHANGED");
   outputBuffersAudio = decoderAudio.getOutputBuffers();
   break;
  case MediaCodec.INFO_OUTPUT_FORMAT_CHANGED:
   Log.d(TAG, "New format " + decoderAudio.getOutputFormat());
   playAudioTrack.setPlaybackRate(formatAudio.getInteger(MediaFormat.KEY_SAMPLE_RATE));
   break;
  case MediaCodec.INFO_TRY_AGAIN_LATER:
   Log.d(TAG, "dequeueOutputBuffer timed out!");
   break;
  default:
   if(outIndex>=0)
   {
    ByteBuffer buffer = outputBuffersAudio[outIndex];
    byte[] chunk = new byte[infoAudio.size];
    buffer.get(chunk);
    buffer.clear();
                if(chunk.length>0){         
                 playAudioTrack.write(chunk,0,chunk.length);
                }                
    decoderAudio.releaseOutputBuffer(outIndex, false);
   }
   break;
  }

  // All decoded frames have been rendered, we can stop playing now
  if ((infoAudio.flags & MediaCodec.BUFFER_FLAG_END_OF_STREAM) != 0) {
   Log.d(TAG, "OutputBuffer BUFFER_FLAG_END_OF_STREAM");
   break;
  }
 }
}

if (audioTrack >=0)
{
 decoderAudio.stop();
 decoderAudio.release();
 playAudioTrack.stop();
}

extractorAudio.release();

和video差不多，就不解釋了，
唯一要注意的是decoderAudio.releaseOutputBuffer(outIndex, false)，
後面的參數記得要設為false，也就是不需要render