音视频播放基础流程
在讲具体的实现之前我们看一下音视频播放的基础流程:
1.png
流程很简单,就是将复用的音视频流解复用出编码后的音频流和编码后的视频流。然后通过音频解码解出PCM数据给音频设备去播放,通过视频解码解出YUV数据给视频设备去播放。
StagefrightPlayer
上一篇文章有讲到MediaPlayerService会通过MediaPlayerFactory创建Player,其中一个创建的就是StagefrightPlayer.但它实际上是一个空壳,只是简单的调用AwesomePlayer的实现而已:
//StagefrightPlayer.h
class StagefrightPlayer : public MediaPlayerInterface {
...
private:
AwesomePlayer *mPlayer;
...
}
//StagefrightPlayer.cpp
status_t StagefrightPlayer::pause() {
ALOGV("pause");
return mPlayer->pause();
}
bool StagefrightPlayer::isPlaying() {
ALOGV("isPlaying");
return mPlayer->isPlaying();
}
status_t StagefrightPlayer::seekTo(int msec) {
ALOGV("seekTo %.2f secs", msec / 1E3);
status_t err = mPlayer->seekTo((int64_t)msec * 1000);
return err;
}
...
所以我们直接看AwesomePlayer的实现。
多线程架构
音视频的处理一般都很耗时,所以AwesomePlayer开了一个子线程去工作,防止阻塞住MediaPlayerService的主线程。
具体的架构如下(这幅图是在这篇博客抄来的,这篇文章写得的确不错,大家感兴趣可以去仔细读一下:
2.png
首先AwesomePlayer内部有个TimedEventQueue对象,所有的操作都会封装成一个个的Event,丢到这个队列里。然后TimedEventQueue创建了一个子线程,不断从队列中拿出Event来执行。
例如prepare操作最后会调到prepareAsync_l,这里面就是创建了个Event,通过postEvent丢到队列里:
status_t AwesomePlayer::prepareAsync_l() {
...
if (!mQueueStarted) {
mQueue.start();
mQueueStarted = true;
}
...
mAsyncPrepareEvent = new AwesomeEvent(
this, &AwesomePlayer::onPrepareAsyncEvent);
mQueue.postEvent(mAsyncPrepareEvent);
return OK;
}
AwesomeEvent继承TimedEventQueue::Event,实现了fire方法,回调了注册的方法:
struct AwesomeEvent : public TimedEventQueue::Event {
AwesomeEvent(
AwesomePlayer *player,
void (AwesomePlayer::*method)())
: mPlayer(player),
mMethod(method) {
}
...
virtual void fire(TimedEventQueue *queue, int64_t /* now_us */) {
(mPlayer->*mMethod)();
}
...
};
TimedEventQueue::start创建了一个子线程,调用TimedEventQueue::threadEntry方法,这里面有个死循环一直在从Event队列中拿出Event,执行fire方法:
void TimedEventQueue::start() {
if (mRunning) {
return;
}
mStopped = false;
pthread_attr_t attr;
pthread_attr_init(&attr);
pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
pthread_create(&mThread, &attr, ThreadWrapper, this);
pthread_attr_destroy(&attr);
mRunning = true;
}
void *TimedEventQueue::ThreadWrapper(void *me) {
androidSetThreadPriority(0, ANDROID_PRIORITY_FOREGROUND);
static_cast<TimedEventQueue *>(me)->threadEntry();
return NULL;
}
void TimedEventQueue::threadEntry() {
...
for (;;) {
...
event = removeEventFromQueue_l(eventID);
if (event != NULL) {
// Fire event with the lock NOT held.
event->fire(this, now_us);
}
}
}
Demux
我们先来看看prepare回调的时候实际是调用了AwesomePlayer::beginPrepareAsync_l()方法,在这里会实际的去设置数据源,然后初始化Demux、视频解码器和音频解码器:
void AwesomePlayer::onPrepareAsyncEvent() {
Mutex::Autolock autoLock(mLock);
beginPrepareAsync_l();
}
void AwesomePlayer::beginPrepareAsync_l() {
...
status_t err = finishSetDataSource_l();
...
status_t err = initVideoDecoder();
...
status_t err = initAudioDecoder();
}
先来看看AwesomePlayer::finishSetDataSource_l实际上是为音视频源找到对应的MediaExtractor,这个MediaExtractor的功能就是实现播放器的基础流程中的Demux,分解出视频流和音频流:
3.png
代码如下:
status_t AwesomePlayer::finishSetDataSource_l() {
...
extractor = MediaExtractor::Create(dataSource, sniffedMIME.empty() ? NULL : sniffedMIME.c_str());
...
status_t err = setDataSource_l(extractor);
...
}
status_t AwesomePlayer::setDataSource_l(const sp<MediaExtractor> &extractor) {
...
for (size_t i = 0; i < extractor->countTracks(); ++i) {
sp<MetaData> meta = extractor->getTrackMetaData(i);
const char *_mime;
CHECK(meta->findCString(kKeyMIMEType, &_mime));
String8 mime = String8(_mime);
...
if (!haveVideo && !strncasecmp(mime.string(), "video/", 6)) {
setVideoSource(extractor->getTrack(i));
...
} else if (!haveAudio && !strncasecmp(mime.string(), "audio/", 6)) {
setAudioSource(extractor->getTrack(i));
...
}
...
}
...
}
MediaExtractor::Create的实现也是蛮粗暴的,判断媒体类型,然后创建不同的MediaExtractor,如MPEG4Extractor、MP3Extractor等:
sp<MediaExtractor> MediaExtractor::Create(const sp<DataSource> &source, const char *mime) {
..
MediaExtractor *ret = NULL;
if (!strcasecmp(mime, MEDIA_MIMETYPE_CONTAINER_MPEG4)
|| !strcasecmp(mime, "audio/mp4")) {
ret = new MPEG4Extractor(source);
} else if (!strcasecmp(mime, MEDIA_MIMETYPE_AUDIO_MPEG)) {
ret = new MP3Extractor(source, meta);
} else if (!strcasecmp(mime, MEDIA_MIMETYPE_AUDIO_AMR_NB)
|| !strcasecmp(mime, MEDIA_MIMETYPE_AUDIO_AMR_WB)) {
ret = new AMRExtractor(source);
} else if (!strcasecmp(mime, MEDIA_MIMETYPE_AUDIO_FLAC)) {
ret = new FLACExtractor(source);
} else if (!strcasecmp(mime, MEDIA_MIMETYPE_CONTAINER_WAV)) {
ret = new WAVExtractor(source);
} else if (!strcasecmp(mime, MEDIA_MIMETYPE_CONTAINER_OGG)) {
ret = new OggExtractor(source);
} else if (!strcasecmp(mime, MEDIA_MIMETYPE_CONTAINER_MATROSKA)) {
ret = new MatroskaExtractor(source);
} else if (!strcasecmp(mime, MEDIA_MIMETYPE_CONTAINER_MPEG2TS)) {
ret = new MPEG2TSExtractor(source);
} else if (!strcasecmp(mime, MEDIA_MIMETYPE_CONTAINER_WVM)) {
// Return now. WVExtractor should not have the DrmFlag set in the block below.
return new WVMExtractor(source);
} else if (!strcasecmp(mime, MEDIA_MIMETYPE_AUDIO_AAC_ADTS)) {
ret = new AACExtractor(source, meta);
} else if (!strcasecmp(mime, MEDIA_MIMETYPE_CONTAINER_MPEG2PS)) {
ret = new MPEG2PSExtractor(source);
}
...
}
4.png
解码器
然后AwesomePlayer::initVideoDecoder、AwesomePlayer::initAudioDecoder里面就是调用OMXCodec去做解码,OMXCodec其实是OpenMax的一层封装。OpenMax就是具体的解码器实现了:
status_t AwesomePlayer::initVideoDecoder(uint32_t flags) {
...
mVideoSource = OMXCodec::Create(
mClient.interface(), mVideoTrack->getFormat(),
false, // createEncoder
mVideoTrack,
NULL, flags, USE_SURFACE_ALLOC ? mNativeWindow : NULL);
...
}
status_t AwesomePlayer::initAudioDecoder() {
...
mOmxSource = OMXCodec::Create(
mClient.interface(), mAudioTrack->getFormat(),
false, // createEncoder
mAudioTrack);
...
}
播放流程
应用在java层调用MediaPlayer.start,最终会通过IPC去到MediaPlayerService里调用到StagefrightPlayer::start方法,我们直接从这里开始往下挖:
//从这里开始是StagefrightPlayer.cpp里的代码
status_t StagefrightPlayer::start() {
return mPlayer->play();
}
//从这里开始是AwesomePlayer.cpp里的代码
status_t AwesomePlayer::play() {
...
return play_l();
}
status_t AwesomePlayer::play_l() {
...
createAudioPlayer_l();
...
postVideoEvent_l();
...
return OK;
}
void AwesomePlayer::postVideoEvent_l(int64_t delayUs) {
...
mQueue.postEventWithDelay(mVideoEvent, delayUs < 0 ? 10000 : delayUs);
}
在AwesomePlayer::play_l方法里面调用AwesomePlayer::createAudioPlayer_l创建了一个AudioPlayer,然后调用AwesomePlayer::postVideoEvent_l往mQueue里丢了一个事件。
还记得这个mVideoEvent吗?它对应的是AwesomePlayer::onVideoEvent方法,也就是说把这个Event丢到mQueue里面之后AwesomePlayer::onVideoEvent就会在子线程中被调用
mVideoEvent = new AwesomeEvent(this, &AwesomePlayer::onVideoEvent);
让我们继续看看AwesomePlayer::onVideoEvent方法里面干了什么:
void AwesomePlayer::onVideoEvent() {
...
status_t err = mVideoSource->read(&mVideoBuffer, &options);
...
if ((mNativeWindow != NULL)
&& (mVideoRendererIsPreview || mVideoRenderer == NULL)) {
mVideoRendererIsPreview = false;
initRenderer_l();
}
...
if (mAudioPlayer != NULL && !(mFlags & (AUDIO_RUNNING | SEEK_PREVIEW))) {
startAudioPlayer_l();
}
...
if (mVideoRenderer != NULL) {
...
mVideoRenderer->render(mVideoBuffer);
...
}
...
postVideoEvent_l();
}
这个方法最重要的就是创建一个VideoRender,从mVideoSource读取解码好的视频帧去渲染,渲染完之后再调AwesomePlayer::postVideoEvent_l再往队列丢入一个VideoEvent。于是画面就不断的刷新了。
可以看到,这个方法内部也启动了音频播放器去播放音频。而且其实它还做了一些音视频同步的工作,但是考虑到逻辑比较啰嗦,我这里就省略了。
VideoRender
最后让我们来看看VideoRendere是怎么来的
void AwesomePlayer::initRenderer_l() {
...
if (USE_SURFACE_ALLOC
&& !strncmp(component, "OMX.", 4)
&& strncmp(component, "OMX.google.", 11)
&& strcmp(component, "OMX.Nvidia.mpeg2v.decode")) {
mVideoRenderer =
new AwesomeNativeWindowRenderer(mNativeWindow, rotationDegrees);
} else {
mVideoRenderer = new AwesomeLocalRenderer(mNativeWindow, meta);
}
}
可以看到,是根据解码器类型用mNativeWindow创建了不同的AwesomeNativeWindowRenderer或者AwesomeLocalRenderer。这个mNativeWindow就是画面最终需要渲染到的地方
我们看看mNativeWindow是怎么来的:
// AwesomePlayer.cpp
status_t AwesomePlayer::setNativeWindow_l(const sp<ANativeWindow> &native) {
mNativeWindow = native;
...
}
status_t AwesomePlayer::setSurfaceTexture(const sp<IGraphicBufferProducer> &bufferProducer) {
...
err = setNativeWindow_l(new Surface(bufferProducer));
...
}
//StagefrightPlayer.cpp
status_t StagefrightPlayer::setVideoSurfaceTexture(
const sp<IGraphicBufferProducer> &bufferProducer) {
ALOGV("setVideoSurfaceTexture");
return mPlayer->setSurfaceTexture(bufferProducer);
}
//MediaPlayerService.cpp
status_t MediaPlayerService::Client::setVideoSurfaceTexture(
...
sp<MediaPlayerBase> p = getPlayer();
...
status_t err = p->setVideoSurfaceTexture(bufferProducer);
...
}
//MediaPlayer.cpp
status_t MediaPlayer::setVideoSurfaceTexture(
const sp<IGraphicBufferProducer>& bufferProducer)
{
...
return mPlayer->setVideoSurfaceTexture(bufferProducer);
}
//android_media_MediaPlayer.cpp
static void setVideoSurface(JNIEnv *env, jobject thiz, jobject jsurface, jboolean mediaPlayerMustBeAlive)
{
sp<MediaPlayer> mp = getMediaPlayer(env, thiz);
...
sp<Surface> surface(android_view_Surface_getSurface(env, jsurface));
...
new_st = surface->getIGraphicBufferProducer();
...
mp->setVideoSurfaceTexture(new_st);
}
static void android_media_MediaPlayer_setVideoSurface(JNIEnv *env, jobject thiz, jobject jsurface)
{
setVideoSurface(env, thiz, jsurface, true /* mediaPlayerMustBeAlive */);
}
//android.media.MediaPlayer.java
public class MediaPlayer extends PlayerBase
implements SubtitleController.Listener
, VolumeAutomation
, AudioRouting
{
...
private native void _setVideoSurface(Surface surface);
...
public void setDisplay(SurfaceHolder sh) {
mSurfaceHolder = sh;
Surface surface;
if (sh != null) {
surface = sh.getSurface();
} else {
surface = null;
}
_setVideoSurface(surface);
updateSurfaceScreenOn();
}
...
}
可以看到,VideoRendere最终是根据MediaPlayer.setDisplay这个方法设置的SurfaceHolder创建的到的。这就解释了画面是怎么渲染到指定的SurfaceView上的。
完整架构图
整个渲染的架构如下:
5.png
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