Android 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
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MediaRecorder::MediaRecorder() : mSurfaceMediaSource(NULL)
{
ALOGV("constructor");
const sp<IMediaPlayerService>& service(getMediaPlayerService());
if (service != NULL) {
mMediaRecorder = service->createMediaRecorder(getpid());
}
if (mMediaRecorder != NULL) {
mCurrentState = MEDIA_RECORDER_IDLE;
}
doCleanUp();
}
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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
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sp<IMediaRecorder> MediaPlayerService::createMediaRecorder(pid_t pid)
{
sp<MediaRecorderClient> recorder = new MediaRecorderClient(this, pid);
wp<MediaRecorderClient> w = recorder;
Mutex::Autolock lock(mLock);
mMediaRecorderClients.add(w);
ALOGV("Create new media recorder client from pid %d", pid);
return recorder;
}
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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
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virtual sp<IMediaRecorder> createMediaRecorder(pid_t pid)
{
Parcel data, reply;
data.writeInterfaceToken(IMediaPlayerService::getInterfaceDescriptor());
data.writeInt32(pid);
remote()->transact(CREATE_MEDIA_RECORDER, data, &reply);
return interface_cast<IMediaRecorder>(reply.readStrongBinder());
}
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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
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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<MetaData> meta = new MetaData;
setupMPEG4MetaData(startTimeUs, totalBitRate, &meta);
err = mWriter->start(meta.get());
if (err != OK) {
return err;
}
return OK;
}
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status_t StagefrightRecorder::setupMPEG4Recording(
int outputFd,
int32_t videoWidth, int32_t videoHeight,
int32_t videoBitRate,
int32_t *totalBitRate,
sp<MediaWriter> *mediaWriter) {
mediaWriter->clear();
*totalBitRate = 0;
status_t err = OK;
sp<MediaWriter> writer = new MPEG4Writer(outputFd);
if (mVideoSource < VIDEO_SOURCE_LIST_END) {
sp<MediaSource> mediaSource;
err = setupMediaSource(&mediaSource); // very important
if (err != OK) {
return err;
}
sp<MediaSource> 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;
}
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// Set up the appropriate MediaSource depending on the chosen option
status_t StagefrightRecorder::setupMediaSource(
sp<MediaSource> *mediaSource) {
if (mVideoSource == VIDEO_SOURCE_DEFAULT
|| mVideoSource == VIDEO_SOURCE_CAMERA) {
sp<CameraSource> 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;
}
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status_t StagefrightRecorder::setupCameraSource(
sp<CameraSource> *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;
}
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status_t StagefrightRecorder::setupVideoEncoder(
sp<MediaSource> cameraSource,
int32_t videoBitRate,
sp<MediaSource> *source) {
source->clear();
sp<MetaData> 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<MetaData> 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<MediaSource> 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;
}
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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
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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();
}
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b) startTracks
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status_t MPEG4Writer::startTracks(MetaData *params) {
for (List<Track *>::iterator it = mTracks.begin();
it != mTracks.end(); ++it) {
status_t err = (*it)->start(params);
if (err != OK) {
for (List<Track *>::iterator it2 = mTracks.begin();
it2 != it; ++it2) {
(*it2)->stop();
}
return err;
}
}
return OK;
}
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然后调用每个Track的start方法
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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<Track *>(me);
status_t err = track->threadEntry();
return (void *) err;
}
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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：
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case SET_CAMERA: {
ALOGV("SET_CAMERA");
CHECK_INTERFACE(IMediaRecorder, data, reply);
sp<ICamera> camera = interface_cast<ICamera>(data.readStrongBinder());
sp<ICameraRecordingProxy> proxy =
interface_cast<ICameraRecordingProxy>(data.readStrongBinder());
reply->writeInt32(setCamera(camera, proxy));
return NO_ERROR;
} break;
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在CameraSource当中会这样去使用
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// 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;
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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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