Stagefright框架解读（—）音视频Playback流程
最后更新于：2022-04-01 14:45:56
转载请注明出处：[](http://blog.csdn.net/guolin_blog/article/details/42238633)[](http://blog.csdn.net/itachi85/article/details/45041923)http://blog.csdn.net/itachi85/article/details/7216639
从Android 2.0，Google引进了Stagefright，并在android2.3时用Stagefright在Android中是以shared library的形式存在(libstagefright.so)，其中AwesomePlayer可用來播放video/audio。AwesomePlayer提供許多API，可以让上层的应用用程式(Java/JNI)來呼叫，我在这里简单说明一下video playback的流程（采用的是android2.2的源码）。
在Java中，若要播放一个影片，我們通常会这样写：
~~~
MediaPlayer mp = new MediaPlayer();
mp.setDataSource(PATH_TO_FILE);
mp.prepare()；
mp.start();
~~~
在Stagefright中，会看到如下的处理：
1.将影片文件的绝对路径指定给uri:
~~~
status_t AwesomePlayer::setDataSource(
const char *uri, const KeyedVector<String8, String8> *headers) {
Mutex::Autolock autoLock(mLock);
return setDataSource_l(uri, headers);
}
status_t AwesomePlayer::setDataSource_l(
const char *uri, const KeyedVector<String8, String8> *headers) {
reset_l();
mUri = uri;
if (headers) {
mUriHeaders = *headers;
}
// The actual work will be done during preparation in the call to
// ::finishSetDataSource_l to avoid blocking the calling thread in
// setDataSource for any significant time.
return OK;
}
~~~
2.启动mQueue:
~~~
status_t AwesomePlayer::prepare() {
Mutex::Autolock autoLock(mLock);
return prepare_l();
}
status_t AwesomePlayer::prepare_l() {
if (mFlags & PREPARED) {
return OK;
}
if (mFlags & PREPARING) {
return UNKNOWN_ERROR;
}
mIsAsyncPrepare = false;
status_t err = prepareAsync_l();
if (err != OK) {
return err;
}
while (mFlags & PREPARING) {
mPreparedCondition.wait(mLock);
}
return mPrepareResult;
}
status_t AwesomePlayer::prepareAsync() {
Mutex::Autolock autoLock(mLock);
if (mFlags & PREPARING) {
return UNKNOWN_ERROR; // async prepare already pending
}
mIsAsyncPrepare = true;
return prepareAsync_l();
}
status_t AwesomePlayer::prepareAsync_l() {
if (mFlags & PREPARING) {
return UNKNOWN_ERROR; // async prepare already pending
}
if (!mQueueStarted) {
mQueue.start();
mQueueStarted = true;
}
mFlags |= PREPARING;
mAsyncPrepareEvent = new AwesomeEvent(
this, &AwesomePlayer::onPrepareAsyncEvent);
mQueue.postEvent(mAsyncPrepareEvent);
return OK;
}
~~~
3.onprepareAsyncEvent被触发，根据传来文件的header来创建相应的解析器，并初始化音视频解码器：
~~~
void AwesomePlayer::onPrepareAsyncEvent() {
sp<Prefetcher> prefetcher;
{
Mutex::Autolock autoLock(mLock);
if (mFlags & PREPARE_CANCELLED) {
LOGI("prepare was cancelled before doing anything");
abortPrepare(UNKNOWN_ERROR);
return;
}
if (mUri.size() > 0) {
//在这个方法中创建解析器
<strong>status_t err = finishSetDataSource_l();</strong>
if (err != OK) {
abortPrepare(err);
return;
}
}
if (mVideoTrack != NULL && mVideoSource == NULL) {
//初始化视频解码器
<strong>status_t err = initVideoDecoder();</strong>
if (err != OK) {
abortPrepare(err);
return;
}
}
if (mAudioTrack != NULL && mAudioSource == NULL) {
//初始化音频解码器
<strong>status_t err = initAudioDecoder();
</strong>
if (err != OK) {
abortPrepare(err);
return;
}
}
prefetcher = mPrefetcher;
}
if (prefetcher != NULL) {
{
Mutex::Autolock autoLock(mLock);
if (mFlags & PREPARE_CANCELLED) {
LOGI("prepare was cancelled before preparing the prefetcher");
prefetcher.clear();
abortPrepare(UNKNOWN_ERROR);
return;
}
}
LOGI("calling prefetcher->prepare()");
status_t result =
prefetcher->prepare(&AwesomePlayer::ContinuePreparation, this);
prefetcher.clear();
if (result == OK) {
LOGI("prefetcher is done preparing");
} else {
Mutex::Autolock autoLock(mLock);
CHECK_EQ(result, -EINTR);
LOGI("prefetcher->prepare() was cancelled early.");
abortPrepare(UNKNOWN_ERROR);
return;
}
}
Mutex::Autolock autoLock(mLock);
if (mIsAsyncPrepare) {
if (mVideoWidth < 0 || mVideoHeight < 0) {
notifyListener_l(MEDIA_SET_VIDEO_SIZE, 0, 0);
} else {
notifyListener_l(MEDIA_SET_VIDEO_SIZE, mVideoWidth, mVideoHeight);
}
notifyListener_l(MEDIA_PREPARED);
}
mPrepareResult = OK;
mFlags &= ~(PREPARING|PREPARE_CANCELLED);
mFlags |= PREPARED;
mAsyncPrepareEvent = NULL;
mPreparedCondition.broadcast();
postBufferingEvent_l();
}
~~~
~~~
status_t AwesomePlayer::finishSetDataSource_l() {
sp<DataSource> dataSource;
if (!strncasecmp("http://", mUri.string(), 7)) {
mConnectingDataSource = new HTTPDataSource(mUri, &mUriHeaders);
mLock.unlock();
status_t err = mConnectingDataSource->connect();
mLock.lock();
if (err != OK) {
mConnectingDataSource.clear();
LOGI("mConnectingDataSource->connect() returned %d", err);
return err;
}
dataSource = new CachingDataSource(
mConnectingDataSource, 64 * 1024, 10);
mConnectingDataSource.clear();
} else {
dataSource = DataSource::CreateFromURI(mUri.string(), &mUriHeaders);
}
if (dataSource == NULL) {
return UNKNOWN_ERROR;
}
<strong>sp<MediaExtractor> extractor = MediaExtractor::Create(dataSource);
</strong>
if (extractor == NULL) {
return UNKNOWN_ERROR;
}
dataSource->getDrmInfo(&mDecryptHandle, &mDrmManagerClient);
if (mDecryptHandle != NULL
&& RightsStatus::RIGHTS_VALID != mDecryptHandle->status) {
notifyListener_l(MEDIA_ERROR, MEDIA_ERROR_UNKNOWN, ERROR_NO_LICENSE);
}
if (dataSource->flags() & DataSource::kWantsPrefetching) {
mPrefetcher = new Prefetcher;
}
<strong> return setDataSource_l(extractor)</strong>;
}
~~~
4.使用extractor对文件进行A/V分离：
~~~
status_t AwesomePlayer::setDataSource_l(const sp<MediaExtractor> &extractor) {
bool haveAudio = false;
bool haveVideo = false;
for (size_t i = 0; i < extractor->countTracks(); ++i) {
sp<MetaData> meta = extractor->getTrackMetaData(i);
const char *mime;
CHECK(meta->findCString(kKeyMIMEType, &mime));
if (!haveVideo && !strncasecmp(mime, "video/", 6)) {
setVideoSource(extractor->getTrack(i));
haveVideo = true;
} else if (!haveAudio && !strncasecmp(mime, "audio/", 6)) {
setAudioSource(extractor->getTrack(i));
haveAudio = true;
}
if (haveAudio && haveVideo) {
break;
}
}
if (!haveAudio && !haveVideo) {
return UNKNOWN_ERROR;
}
mExtractorFlags = extractor->flags();
return OK;
}
~~~
5.将解析后的音视频数据分别交给VideoTrack和AudioTrack:
~~~
void AwesomePlayer::setVideoSource(sp<MediaSource> source) {
CHECK(source != NULL);
if (mPrefetcher != NULL) {
source = mPrefetcher->addSource(source);
}
mVideoTrack = source;
}
void AwesomePlayer::setAudioSource(sp<MediaSource> source) {
CHECK(source != NULL);
if (mPrefetcher != NULL) {
source = mPrefetcher->addSource(source);
}
mAudioTrack = source;
}
~~~
6.根据mVideoTrck中的编码类型来选择 video decoder 同理根据mAudioTrack中的编码类型来选择 audio decoder:
~~~
status_t AwesomePlayer::initVideoDecoder() {
mVideoSource = OMXCodec::Create(
mClient.interface(), mVideoTrack->getFormat(),
false, // createEncoder
mVideoTrack);
if (mVideoSource != NULL) {
int64_t durationUs;
if (mVideoTrack->getFormat()->findInt64(kKeyDuration, &durationUs)) {
Mutex::Autolock autoLock(mMiscStateLock);
if (mDurationUs < 0 || durationUs > mDurationUs) {
mDurationUs = durationUs;
}
}
CHECK(mVideoTrack->getFormat()->findInt32(kKeyWidth, &mVideoWidth));
CHECK(mVideoTrack->getFormat()->findInt32(kKeyHeight, &mVideoHeight));
status_t err = mVideoSource->start();
if (err != OK) {
mVideoSource.clear();
return err;
}
}
return mVideoSource != NULL ? OK : UNKNOWN_ERROR;
}
~~~
7.将mVideoEvent放入mQueue中，开始解码播放，并交由mvideoRenderer来画出 audio的数据则交由audioplayer来管理，它最终将解码的数据交给audioTrack并由audioTrack和audioFlinger进行交互，最终将数据交给audio hal层，这个我们以后会做讲解：
~~~
status_t AwesomePlayer::play() {
Mutex::Autolock autoLock(mLock);
return play_l();
}
status_t AwesomePlayer::play_l() {
if (mFlags & PLAYING) {
return OK;
}
if (!(mFlags & PREPARED)) {
status_t err = prepare_l();
if (err != OK) {
return err;
}
}
mFlags |= PLAYING;
mFlags |= FIRST_FRAME;
bool deferredAudioSeek = false;
if (mAudioSource != NULL) {
if (mAudioPlayer == NULL) {
if (mAudioSink != NULL) {
//音频数据由audioplayer进行管理
mAudioPlayer = new AudioPlayer(mAudioSink);
mAudioPlayer->setSource(mAudioSource);
// We've already started the MediaSource in order to enable
// the prefetcher to read its data.
//调用audioPlayer的start方法则是调用audioSource对数据进行解码
//并将解码似得数据最终交给audioTrack,并调用audioTrack的start方法与audioFlinger进行交互
status_t err = mAudioPlayer->start(
true /* sourceAlreadyStarted */);
if (err != OK) {
delete mAudioPlayer;
mAudioPlayer = NULL;
mFlags &= ~(PLAYING | FIRST_FRAME);
return err;
}
delete mTimeSource;
mTimeSource = mAudioPlayer;
deferredAudioSeek = true;
mWatchForAudioSeekComplete = false;
mWatchForAudioEOS = true;
}
} else {
mAudioPlayer->resume();
}
postCheckAudioStatusEvent_l();
}
if (mTimeSource == NULL && mAudioPlayer == NULL) {
mTimeSource = new SystemTimeSource;
}
if (mVideoSource != NULL) {
// Kick off video playback
//将mVideoEvent放入queue中
postVideoEvent_l();
}
if (deferredAudioSeek) {
// If there was a seek request while we were paused
// and we're just starting up again, honor the request now.
seekAudioIfNecessary_l();
}
if (mFlags & AT_EOS) {
// Legacy behaviour, if a stream finishes playing and then
// is started again, we play from the start...
seekTo_l(0);
}
if (mDecryptHandle != NULL) {
int64_t position;
getPosition(&position);
mDrmManagerClient->setPlaybackStatus(mDecryptHandle,
Playback::START, position / 1000);
}
return OK;
}
void AwesomePlayer::postVideoEvent_l(int64_t delayUs) {
if (mVideoEventPending) {
return;
}
mVideoEventPending = true;
mQueue.postEventWithDelay(mVideoEvent, delayUs < 0 ? 10000 : delayUs);
}
~~~
~~~
void AwesomePlayer::onVideoEvent()
{
mVideoSource->read(&mVideoBuffer, &options);
mVideoRenderer->render(mVideoBuffer);
postVideoEvent_l();
}
~~~