Android系统Camera录像过程分析

最近调试系统Camera，遇到如下问题：在录像过程中，拔掉Camera；会出现应用程序卡死现象。

先说说之前的设计架构：

当用户拔掉Camera时，会给应用程序发送广播；当应用程序收到广播后调用Activity类的finish方法（系统会自动调用onPause方法），而我们的onPause方法做了停止录制和关闭Camera的动作。

问题出在：

当调用系统MediaRecorder的stop方法停止录制时，应用程序因为阻塞而卡死。

下面就着重分析下系统Camera和MediaRecorder（libstagefright中MPEG4Writer以及CameraSource的关系）。

首先，通过图示、看看Android系统Camera录像时的调用时序：

1.录像命令时序



2.录像数据回调时序



一、应用部分

1.主Activity启动

packages/apps/Camera/src/com/android/camera/CameraActivity.java

public void onCreate(Bundle state) 
{
  mCurrentModule.init(this, mFrame, true);
}

2.录像Activity初始化

packages/apps/Camera/src/com/android/camera/VideoModule.java

public void init(CameraActivity activity, View root, boolean reuseScreenNail) {
{
  CameraOpenThread cameraOpenThread = new CameraOpenThread();
  /*
  protected class CameraOpenThread extends Thread {
    @Override
    public void run() {
      openCamera();
    }
  }
  */
}

3.开始录制和停止录制

//当用户点击录像后调用
public void onShutterButtonClick() 
{
  startVideoRecording();
}
private void startVideoRecording() 
{
  initializeRecorder();
  /*
  private void initializeRecorder()   
  {
    mMediaRecorder = new MediaRecorder();
    //调用至frameworks/av/media/libmediaplayerservice/StagefrightRecorder.cpp
    /*
    status_t StagefrightRecorder::setCamera(const sp<ICamera> &camera,
                                        const sp<ICameraRecordingProxy> &proxy) {
      mCamera = camera;
    }
    */
    mMediaRecorder.setCamera(mActivity.mCameraDevice.getCamera()); 
    mMediaRecorder.setAudioSource(MediaRecorder.AudioSource.CAMCORDER);
    mMediaRecorder.setVideoSource(MediaRecorder.VideoSource.CAMERA);
    mMediaRecorder.setProfile(mProfile);
    mMediaRecorder.setMaxDuration(mMaxVideoDurationInMs);
    mMediaRecorder.setOutputFile(mVideoFilename);
  }
  */
  mMediaRecorder.start();
}
//当用户点击停止录制后调用
public void onShutterButtonClick() {
  public void onShutterButtonClick() {
}
private void onStopVideoRecording() {
  stopVideoRecording();
}
private boolean stopVideoRecording() {
  mMediaRecorder.setOnErrorListener(null);
  mMediaRecorder.setOnInfoListener(null);
  mMediaRecorder.stop();
  closeCamera(closeEffects);
}

二、框架部分

1.MediaRecorder的API部分

frameworks/base/media/java/android/media/MediaRecorder.java

public native void start() throws IllegalStateException;
public native void stop() throws IllegalStateException;

2.Native部分

frameworks/base/media/jni/android_media_MediaRecorder.cpp

static JNINativeMethod gMethods[] = {
  {"start",  "()V",   (void *)android_media_MediaRecorder_start},
  {"stop",  "()V",   (void *)android_media_MediaRecorder_stop},
}
static void
android_media_MediaRecorder_start(JNIEnv *env, jobject thiz)
{
  ALOGV("start");
  sp<MediaRecorder> mr = getMediaRecorder(env, thiz);
  process_media_recorder_call(env, mr->start(), "java/lang/RuntimeException", "start failed.");
}
static void
android_media_MediaRecorder_stop(JNIEnv *env, jobject thiz)
{
  ALOGV("stop");
  sp<MediaRecorder> mr = getMediaRecorder(env, thiz);
  process_media_recorder_call(env, mr->stop(), "java/lang/RuntimeException", "stop failed.");
}

3.C++部分

frameworks/av/media/libmedia/MediaRecorder.cpp

status_t MediaRecorder::start()
{
  status_t ret = mMediaRecorder->start();
  /*
  MediaRecorder::MediaRecorder() : mSurfaceMediaSource(NULL)
  {
    const sp<IMediaPlayerService>& service(getMediaPlayerService());
    mMediaRecorder = service->createMediaRecorder(getpid());
    //frameworks/av/media/libmediaplayerservice/MediaPlayerService.cpp
    //sp<IMediaRecorder> MediaPlayerService::createMediaRecorder(pid_t pid)
    //{
    //  sp<MediaRecorderClient> recorder = new MediaRecorderClient(this, pid);
    //  return recorder;
    //}
  }
  */
}

4.服务端（Android的Binder）

frameworks/av/media/libmediaplayerservice/MediaRecorderClient.cpp

MediaRecorderClient::MediaRecorderClient(const sp<MediaPlayerService>& service, pid_t pid)
{
  ALOGV("Client constructor");
  mPid = pid;
  mRecorder = new StagefrightRecorder;
  mMediaPlayerService = service;
}
status_t MediaRecorderClient::start()
{
  return mRecorder->start();
}

Android4.2多媒体使用Stagefright架构

frameworks/av/media/libmediaplayerservice/StagefrightRecorder.cpp

status_t StagefrightRecorder::start() {
  switch (mOutputFormat) {
    case OUTPUT_FORMAT_MPEG_4:
      status = startMPEG4Recording();
      break;
    case OUTPUT_FORMAT_AMR_WB:
      status = startAMRRecording();
      break;
    case OUTPUT_FORMAT_AAC_ADTS:
      status = startAACRecording();    
      break;
    case OUTPUT_FORMAT_RTP_***P:  
      status = startRTPRecording();
      break;
    case OUTPUT_FORMAT_MPEG2TS:
      status = startMPEG2TSRecording();
      break;
  }
}
status_t StagefrightRecorder::startMPEG4Recording() {
  status_t err = setupMPEG4Recording(
            mOutputFd, mVideoWidth, mVideoHeight,
            mVideoBitRate, &totalBitRate, &mWriter);
  /*
  status_t StagefrightRecorder::setupMPEG4Recording(
        int outputFd,
        int32_t videoWidth, int32_t videoHeight,
        int32_t videoBitRate,
        int32_t *totalBitRate,
        sp<MediaWriter> *mediaWriter) {
    sp<MediaWriter> writer = new MPEG4Writer(outputFd);
    if (mVideoSource < VIDEO_SOURCE_LIST_END) {
      sp<MediaSource> mediaSource;
      //创建CameraSource:
      //这步使用mCamera，mMediaRecorder.setCamera(mActivity.mCameraDevice.getCamera())应用设置过来的
      err = setupMediaSource(&mediaSource); 
      sp<MediaSource> encoder;
      //重要！！！！！设置输入类型，如：YUV420SP等
      err = setupVideoEncoder(mediaSource, videoBitRate, &encoder); 
      //sp<MetaData> meta = cameraSource->getFormat();
      //CameraSource.cpp
      ////mColorFormat = getColorFormat(params.get(CameraParameters::KEY_VIDEO_FRAME_FORMAT));
      //CameraHal.cpp:
      ////p.set(CameraParameters::KEY_VIDEO_FRAME_FORMAT, (const char *) CameraParameters::PIXEL_FORMAT_YUV420SP);
      //setVideoInputFormat(mMIME, meta);
      //setVideoPortFormatType
      writer->addSource(encoder);
    }
    if (!mCaptureTimeLapse && (mAudioSource != AUDIO_SOURCE_CNT)) {
      //创建AudioSource；这步使用new AudioRecord
      err = setupAudioEncoder(writer); 
      if (err != OK) return err;
      *totalBitRate += mAudioBitRate;
    }
  }
  */  
  err = mWriter->start(meta.get());
}

MPEG4编码器部分

frameworks/av/media/libstagefright/MPEG4Writer.cpp

status_t MPEG4Writer::Track::start(MetaData *params) {
  pthread_create(&mThread, &attr, ThreadWrapper, this);
  /*
  void *MPEG4Writer::Track::ThreadWrapper(void *me) {
    Track *track = static_cast<Track *>(me);
    status_t err = track->threadEntry();
    return (void *) err;
  }
  status_t MPEG4Writer::Track::threadEntry() {
    while (!mDone && (err = mSource->read(&buffer)) == OK) {
      //如上read即是frameworks/av/media/libstagefright/CameraSource.cpp的read
    }
  }
  */  
}
status_t MPEG4Writer::Track::stop() {
  mDone = true;
  void *dummy;
  pthread_join(mThread, &dummy); //等待刚才的主线程退出
}

当有数据来时CameraSource的dataCallbackTimestamp函数会被调用，如此、完成视频录制。

三、分析问题

我们的问题就出在直接拔掉Camera时；应用程序调用mMediaRecorder.stop()超时卡死；经分析是上述框架部分“Camera拔出时录制的主线程不能退出、导致接口阻塞”。后调试发现：mDone变量并不能在两个线程间传参数；后打入之前一个patcher（see bug 4724339），修改了主线程中的mSource->read、并在相应的while循环中做判断，问题解决。

frameworks/av/media/libstagefright/CameraSource.cpp

status_t CameraSource::read(
        MediaBuffer **buffer, const ReadOptions *options) {
  ALOGW("Timed out waiting for incoming camera video frames: %lld us",
                    mLastFrameTimestampUs);
  //add by tankai
  // For funtion OMXCodec::read timeout return in writer, then Writer (e.g. VECaptureWriter) thread can exit  when
  // media recorder stop
  // reason: when media recorder start with no frame send to OMXCodec(with camera source),
  // media recorder can not stop always (because writer can't exit)
  // this change impact cameras: vecapture fake camera and Webcam
  return ERROR_END_OF_STREAM;
  //end tankai
}

四、补充，分析MPEG4中Audio流程；接分析二中的实现

1.Audio录音

frameworks/av/media/libmediaplayerservice/StagefrightRecorder.cpp

status_t StagefrightRecorder::setupAudioEncoder(const sp<MediaWriter>& writer) {() {
  sp<MediaSource> audioEncoder = createAudioSource();
  writer->addSource(audioEncoder);
}
sp<MediaSource> StagefrightRecorder::createAudioSource() {
  sp<AudioSource> audioSource =
        new AudioSource(
                mAudioSource,
                mSampleRate,
                mAudioChannels);
}

frameworks/av/media/libstagefright/AudioSource.cpp

AudioSource::AudioSource(
        audio_source_t inputSource, uint32_t sampleRate, uint32_t channelCount)
    : mRecord(NULL),
      mStarted(false),
      mSampleRate(sampleRate),
      mPrevSampleTimeUs(0),
      mNumFramesReceived(0),
      mNumClientOwnedBuffers(0) {
  mRecord = new AudioRecord(
                    inputSource, sampleRate, AUDIO_FORMAT_PCM_16_BIT,
                    audio_channel_in_mask_from_count(channelCount),
                    bufCount * frameCount,
                    AudioRecordCallbackFunction,
                    this,
                    frameCount);
}

至此，MediaRecorder与AudioFlinger建立联系。

2.Audio放音

MediaPlayer播放音频服务端（后边有时间在具体分析应用程序/客户端流程）：

frameworks/av/media/libmediaplayerservice/MediaPlayerService.cpp

status_t MediaPlayerService::AudioOutput::open(
        uint32_t sampleRate, int channelCount, audio_channel_mask_t channelMask,
        audio_format_t format, int bufferCount,
        AudioCallback cb, void *cookie,
        audio_output_flags_t flags)
{
  AudioTrack *t;
  //最终放声音使用AudioTrack
  t = new AudioTrack(
                mStreamType,
                sampleRate,
                format,
                channelMask,
                frameCount,
                flags,
                CallbackWrapper,
                newcbd,
                0,  // notification frames
                mSessionId);
}