Android(安卓)MediaRecorder系统结构

前面有分析过Camera的实现，现在来看看MediaRecorder的实现，这里我不会太去关注它的分层结构，我更关注它的逻辑！

APP层 /path/to/aosp/frameworks/base/media/java/android/media/MediaRecorder.java
JNI层 /path/to/aosp/frameworks/base/media/jni/android_media_MediaRecorder.cpp
调用NATIVE层的MediaRecorder(这里是BnMediaRecorderClient)
header /path/to/aosp/frameworks/av/include/media/mediarecorder.h
implementation /path/to/aosp/frameworks/av/media/libmedia/mediarecorder.cpp

1 2 3 4 5 6 7 8 9 10 11 12 13 14 MediaRecorder::MediaRecorder() : mSurfaceMediaSource(NULL) { ALOGV( "constructor" ); const sp& service(getMediaPlayerService()); if (service != NULL) { mMediaRecorder = service->createMediaRecorder(getpid()); } if (mMediaRecorder != NULL) { mCurrentState = MEDIA_RECORDER_IDLE; } doCleanUp(); }

getMediaPlayerService()这个方法位于/path/to/aosp/frameworks/av/include/media/IMediaDeathNotifier.h

获取到MediaPlayerService(这个是BpMediaPlayerService)之后
调用IMediaPlayerService当中的

1 2 3 4 5 6 7 8 9 sp MediaPlayerService::createMediaRecorder(pid_t pid) { sp recorder = new MediaRecorderClient( this , pid); wp w = recorder; Mutex::Autolock lock(mLock); mMediaRecorderClients.add(w); ALOGV( "Create new media recorder client from pid %d" , pid); return recorder; }

创建MediaRecorderClient(这里是BnMediaRecorder)

但是通过binder拿到的是BpMediaRecorder
因为有如下的interface_cast过程

1 2 3 4 5 6 7 8 virtual sp createMediaRecorder(pid_t pid) { Parcel data, reply; data.writeInterfaceToken(IMediaPlayerService::getInterfaceDescriptor()); data.writeInt32(pid); remote()->transact(CREATE_MEDIA_RECORDER, data, &reply); return interface_cast(reply.readStrongBinder()); }

而MediaRecorderClient当中又会创建StagefrightRecorder(MediaRecorderBase)，它位于
/path/to/aosp/frameworks/av/media/libmediaplayerservice/StagefrightRecorder.cpp

目前我们可以认为在APP/JNI/NATIVE这边是在一个进程当中，在MediaPlayerService当中的MediaRecorderClient/StagefrightRecorder是在另外一个进程当中，他们之间通过binder通信，而且Bp和Bn我们也都有拿到，后面我们将不再仔细区分Bp和Bn。

客户端这边
BnMediaRecorderClient
BpMediaRecorder
BpMediaPlayerService

服务端这边
BpMediaRecorderClient(如果需要通知客户端的话，它可以获得这个Bp)
BnMediaRecorder
BnMediaPlayerService

这有张图(点过去看原始大图)

我们以开始录影为例子，比如start()

在这里就兵分两路，一个CameraSource，一个MPEG4Writer(sp mWriter)
这两个class都位于/path/to/aosp/frameworks/av/media/libstagefright/当中

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 status_t StagefrightRecorder::startMPEG4Recording() { int32_t totalBitRate; status_t err = setupMPEG4Recording( mOutputFd, mVideoWidth, mVideoHeight, mVideoBitRate, &totalBitRate, &mWriter); if (err != OK) { return err; } int64_t startTimeUs = systemTime() / 1000 ; sp meta = new MetaData; setupMPEG4MetaData(startTimeUs, totalBitRate, &meta); err = mWriter->start(meta.get()); if (err != OK) { return err; } return OK; }

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 status_t StagefrightRecorder::setupMPEG4Recording( int outputFd, int32_t videoWidth, int32_t videoHeight, int32_t videoBitRate, int32_t *totalBitRate, sp *mediaWriter) { mediaWriter->clear(); *totalBitRate = 0 ; status_t err = OK; sp writer = new MPEG4Writer(outputFd); if (mVideoSource < VIDEO_SOURCE_LIST_END) { sp mediaSource; err = setupMediaSource(&mediaSource); // very important if (err != OK) { return err; } sp encoder; err = setupVideoEncoder(mediaSource, videoBitRate, &encoder); // very important if (err != OK) { return err; } writer->addSource(encoder); *totalBitRate += videoBitRate; } // Audio source is added at the end if it exists. // This help make sure that the "recoding" sound is suppressed for // camcorder applications in the recorded files. if (!mCaptureTimeLapse && (mAudioSource != AUDIO_SOURCE_CNT)) { err = setupAudioEncoder(writer); // very important if (err != OK) return err; *totalBitRate += mAudioBitRate; } ... writer->setListener(mListener); *mediaWriter = writer; return OK; }

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 // Set up the appropriate MediaSource depending on the chosen option status_t StagefrightRecorder::setupMediaSource( sp *mediaSource) { if (mVideoSource == VIDEO_SOURCE_DEFAULT || mVideoSource == VIDEO_SOURCE_CAMERA) { sp cameraSource; status_t err = setupCameraSource(&cameraSource); if (err != OK) { return err; } *mediaSource = cameraSource; } else if (mVideoSource == VIDEO_SOURCE_GRALLOC_BUFFER) { // If using GRAlloc buffers, setup surfacemediasource. // Later a handle to that will be passed // to the client side when queried status_t err = setupSurfaceMediaSource(); if (err != OK) { return err; } *mediaSource = mSurfaceMediaSource; } else { return INVALID_OPERATION; } return OK; }

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 status_t StagefrightRecorder::setupCameraSource( sp *cameraSource) { status_t err = OK; if ((err = checkVideoEncoderCapabilities()) != OK) { return err; } Size videoSize; videoSize.width = mVideoWidth; videoSize.height = mVideoHeight; if (mCaptureTimeLapse) { if (mTimeBetweenTimeLapseFrameCaptureUs < 0 ) { ALOGE( "Invalid mTimeBetweenTimeLapseFrameCaptureUs value: %lld" , mTimeBetweenTimeLapseFrameCaptureUs); return BAD_VALUE; } mCameraSourceTimeLapse = CameraSourceTimeLapse::CreateFromCamera( mCamera, mCameraProxy, mCameraId, videoSize, mFrameRate, mPreviewSurface, mTimeBetweenTimeLapseFrameCaptureUs); *cameraSource = mCameraSourceTimeLapse; } else { *cameraSource = CameraSource::CreateFromCamera( mCamera, mCameraProxy, mCameraId, videoSize, mFrameRate, mPreviewSurface, true /*storeMetaDataInVideoBuffers*/ ); } mCamera.clear(); mCameraProxy.clear(); if (*cameraSource == NULL) { return UNKNOWN_ERROR; } if ((*cameraSource)->initCheck() != OK) { (*cameraSource).clear(); *cameraSource = NULL; return NO_INIT; } // When frame rate is not set, the actual frame rate will be set to // the current frame rate being used. if (mFrameRate == - 1 ) { int32_t frameRate = 0 ; CHECK ((*cameraSource)->getFormat()->findInt32( kKeyFrameRate, &frameRate)); ALOGI( "Frame rate is not explicitly set. Use the current frame " "rate (%d fps)" , frameRate); mFrameRate = frameRate; } CHECK(mFrameRate != - 1 ); mIsMetaDataStoredInVideoBuffers = (*cameraSource)->isMetaDataStoredInVideoBuffers(); return OK; }

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 status_t StagefrightRecorder::setupVideoEncoder( sp cameraSource, int32_t videoBitRate, sp *source) { source->clear(); sp enc_meta = new MetaData; enc_meta->setInt32(kKeyBitRate, videoBitRate); enc_meta->setInt32(kKeyFrameRate, mFrameRate); switch (mVideoEncoder) { case VIDEO_ENCODER_H263: enc_meta->setCString(kKeyMIMEType, MEDIA_MIMETYPE_VIDEO_H263); break ; case VIDEO_ENCODER_MPEG_4_SP: enc_meta->setCString(kKeyMIMEType, MEDIA_MIMETYPE_VIDEO_MPEG4); break ; case VIDEO_ENCODER_H264: enc_meta->setCString(kKeyMIMEType, MEDIA_MIMETYPE_VIDEO_AVC); break ; default : CHECK(! "Should not be here, unsupported video encoding." ); break ; } sp meta = cameraSource->getFormat(); int32_t width, height, stride, sliceHeight, colorFormat; CHECK(meta->findInt32(kKeyWidth, &width)); CHECK(meta->findInt32(kKeyHeight, &height)); CHECK(meta->findInt32(kKeyStride, &stride)); CHECK(meta->findInt32(kKeySliceHeight, &sliceHeight)); CHECK(meta->findInt32(kKeyColorFormat, &colorFormat)); enc_meta->setInt32(kKeyWidth, width); enc_meta->setInt32(kKeyHeight, height); enc_meta->setInt32(kKeyIFramesInterval, mIFramesIntervalSec); enc_meta->setInt32(kKeyStride, stride); enc_meta->setInt32(kKeySliceHeight, sliceHeight); enc_meta->setInt32(kKeyColorFormat, colorFormat); if (mVideoTimeScale > 0 ) { enc_meta->setInt32(kKeyTimeScale, mVideoTimeScale); } if (mVideoEncoderProfile != - 1 ) { enc_meta->setInt32(kKeyVideoProfile, mVideoEncoderProfile); } if (mVideoEncoderLevel != - 1 ) { enc_meta->setInt32(kKeyVideoLevel, mVideoEncoderLevel); } OMXClient client; CHECK_EQ(client.connect(), (status_t)OK); uint32_t encoder_flags = 0 ; if (mIsMetaDataStoredInVideoBuffers) { encoder_flags |= OMXCodec::kStoreMetaDataInVideoBuffers; } // Do not wait for all the input buffers to become available. // This give timelapse video recording faster response in // receiving output from video encoder component. if (mCaptureTimeLapse) { encoder_flags |= OMXCodec::kOnlySubmitOneInputBufferAtOneTime; } sp encoder = OMXCodec::Create( client. interface (), enc_meta, true /* createEncoder */ , cameraSource, NULL, encoder_flags); if (encoder == NULL) { ALOGW( "Failed to create the encoder" ); // When the encoder fails to be created, we need // release the camera source due to the camera's lock // and unlock mechanism. cameraSource->stop(); return UNKNOWN_ERROR; } *source = encoder; return OK; }

这里和OMXCodec关联起来
有一个叫media_codecs.xml的配置文件来表明设备支持哪些codec

我们录制MPEG 4的时候还会有声音，所以后面还有个setupAudioEncoder，具体的方法就不展开了，总之就是把声音也作为一个Track加入到MPEG4Writer当中去。
这里插个题外话，Google说把setupAudioEncoder放到后面是为了避免开始录影的那一个提示声音也被录制进去，但是实际发现它这样做还是会有bug，在一些设备上还是会把那声录制进去，这个遇到的都是靠APP自己来播放声音来绕过这个问题的。

另外MPEG4Writer当中有个
start(MetaData*)
启动两个方法
a) startWriterThread

启动一个thread去写

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 void MPEG4Writer::threadFunc() { ALOGV( "threadFunc" ); prctl(PR_SET_NAME, (unsigned long ) "MPEG4Writer" , 0 , 0 , 0 ); Mutex::Autolock autoLock(mLock); while (!mDone) { Chunk chunk; bool chunkFound = false ; while (!mDone && !(chunkFound = findChunkToWrite(&chunk))) { mChunkReadyCondition.wait(mLock); } // Actual write without holding the lock in order to // reduce the blocking time for media track threads. if (chunkFound) { mLock.unlock(); writeChunkToFile(&chunk); mLock.lock(); } } writeAllChunks(); }

b) startTracks

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 status_t MPEG4Writer::startTracks(MetaData *params) { for (List::iterator it = mTracks.begin(); it != mTracks.end(); ++it) { status_t err = (*it)->start(params); if (err != OK) { for (List::iterator it2 = mTracks.begin(); it2 != it; ++it2) { (*it2)->stop(); } return err; } } return OK; }

然后调用每个Track的start方法

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 status_t MPEG4Writer::Track::start(MetaData *params) { ... initTrackingProgressStatus(params); ... status_t err = mSource->start(meta.get()); // 这里会去执行CameraSource(start)，这两个是相互关联的 ... pthread_create(&mThread, &attr, ThreadWrapper, this ); return OK; } void *MPEG4Writer::Track::ThreadWrapper( void *me) { Track *track = static_cast(me); status_t err = track->threadEntry(); return ( void *) err; }

通过status_t MPEG4Writer::Track::threadEntry()
是新启动另外一个thread，它里面会通过一个循环来不断读取CameraSource(read)里面的数据，CameraSource里面的数据当然是从driver返回过来的(可以参见CameraSourceListener，CameraSource用一个叫做mFrameReceived的List专门存放从driver过来的数据，如果收到数据会调用mFrameAvailableCondition.signal，若还没有开始录影，这个时候收到的数据是被丢弃的，当然MediaWriter先启动的是CameraSource的start方法，再启动写Track)，然后写到文件当中。
注意：准确来说这里MPEG4Writer读取的是OMXCodec里的数据，因为数据先到CameraSource，codec负责编码之后，MPEG4Writer才负责写到文件当中！关于数据在CameraSource/OMXCodec/MPEG4Writer之间是怎么传递的，可以参见http://guoh.org/lifelog/2013/06/interaction-between-stagefright-and-codec/当中讲Buffer的传输过程。

回头再来看，Stagefright做了什么事情？我更觉得它只是一个粘合剂(glue)的用处，它工作在MediaPlayerService这一层，把MediaSource，MediaWriter，Codec以及上层的MediaRecorder绑定在一起，这应该就是它最大的作用，Google用它来替换Opencore也是符合其一贯的工程派作风(相比复杂的学术派而言，虽然Google很多东西也很复杂，但是它一般都是以尽量简单的方式来解决问题)。
让大家觉得有点不习惯的是，它把MediaRecorder放在MediaPlayerService当中，这两个看起来是对立的事情，或者某一天它们会改名字，或者是两者分开，不知道～～

当然这只是个简单的大体介绍，Codec相关的后面争取专门来分析一下！

有些细节的东西在这里没有列出，需要的话会把一些注意点列出来：

1. 时光流逝录影
CameraSource对应的就是CameraSourceTimeLapse

具体做法就是在
dataCallbackTimestamp
当中有skipCurrentFrame

当然它是用些变量来记录和计算
mTimeBetweenTimeLapseVideoFramesUs(1E6/videoFrameRate) // 两个frame之间的间隔时间
记录上一个frame的(mLastTimeLapseFrameRealTimestampUs) // 上一个frame发生的时间
然后通过frame rate计算出两个frame之间的相距离时间，中间的都透过releaseOneRecordingFrame来drop掉
也就是说driver返回的东西都不变，只是在SW这层我们自己来处理掉

关于Time-lapse相关的可以参阅
https://en.wikipedia.org/wiki/Time-lapse_photography

2. 录影当中需要用到Camera的话是通过ICameraRecordingProxy，即Camera当中的RecordingProxy(这是一个BnCameraRecordingProxy)
当透过binder，将ICameraRecordingProxy传到服务端进程之后，它就变成了Bp，如下：

1 2 3 4 5 6 7 8 9 case SET_CAMERA: { ALOGV( "SET_CAMERA" ); CHECK_INTERFACE(IMediaRecorder, data, reply); sp camera = interface_cast(data.readStrongBinder()); sp proxy = interface_cast(data.readStrongBinder()); reply->writeInt32(setCamera(camera, proxy)); return NO_ERROR; } break ;

在CameraSource当中会这样去使用

1 2 3 4 5 6 7 8 // We get the proxy from Camera, not ICamera. We need to get the proxy // to the remote Camera owned by the application. Here mCamera is a // local Camera object created by us. We cannot use the proxy from // mCamera here. mCamera = Camera::create(camera); if (mCamera == 0 ) return -EBUSY; mCameraRecordingProxy = proxy; mCameraFlags |= FLAGS_HOT_CAMERA;

疑问点：

CameraSource当中这个
List > mFramesBeingEncoded;
有什么用？
每编码完一个frame，CameraSource就会将其保存起来，Buffer被release的时候，会反过来release掉这些frame(s)，这种做法是为了效率么？为什么不编码完一个frame就将其release掉？
另外不得不再感叹下Google经常的delete this;行为，精妙，但是看起来反常！

原文地址：　http://guoh.org/lifelog/2013/06/android-mediarecorder-architecture/

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