[时间：2016-11] [状态：Open]
[关键词：android，nuplayer，开源播放器，播放框架，渲染器，render]
0 导读
之前我们分析了NuPlayer的实现代码，本文将重点聚焦于其中的一部分——渲染器（Renderer）。
从功能安排来说，Renderer的主要功能有：
- 音视频原始数据缓存操作
- 音频播放（到声卡）
- 视频显示（到显卡）
- 音视频同步
- 其他辅助播放器控制的操作
- 其他获取渲染状态/属性的接口
接下来主要从Renderer的对外接口和实现说明下其中的处理逻辑。
本文是我的NuPlayer播放框架的第四篇。
1 NuPlayer::Renderer对外接口及主要成员
// code frome ~/frameworks/av/media/libmediaplayerservice/nuplayer/NuPlayerRenderer.h
struct NuPlayer::Renderer : public AHandler {
Renderer(const sp<MediaPlayerBase::AudioSink> &sink,
const sp<AMessage> ¬ify, uint32_t flags = 0);
static size_t AudioSinkCallback(MediaPlayerBase::AudioSink *audioSink,
void *data, size_t size, void *me,
MediaPlayerBase::AudioSink::cb_event_t event);
// 缓冲音视频原始数据
void queueBuffer(bool audio,
const sp<ABuffer> &buffer, const sp<AMessage> ¬ifyConsumed);
void queueEOS(bool audio, status_t finalResult);
status_t setPlaybackSettings(const AudioPlaybackRate &rate /* sanitized */);
status_t getPlaybackSettings(AudioPlaybackRate *rate /* nonnull */);
status_t setSyncSettings(const AVSyncSettings &sync, float videoFpsHint);
status_t getSyncSettings(AVSyncSettings *sync /* nonnull */, float *videoFps /* nonnull */);
void flush(bool audio, bool notifyComplete);
void signalTimeDiscontinuity();
void signalAudioSinkChanged();
void signalDisableOffloadAudio();
void signalEnableOffloadAudio();
void pause();
void resume();
void setVideoFrameRate(float fps);
status_t getCurrentPosition(int64_t *mediaUs);
int64_t getVideoLateByUs();
status_t openAudioSink( const sp<AMessage> &format, bool offloadOnly,
bool hasVideo, uint32_t flags, bool *isOffloaded);
void closeAudioSink();
private:
struct QueueEntry {
sp<ABuffer> mBuffer;
sp<AMessage> mNotifyConsumed;
size_t mOffset;
status_t mFinalResult;
int32_t mBufferOrdinal;
};
static const int64_t kMinPositionUpdateDelayUs;
sp<MediaPlayerBase::AudioSink> mAudioSink;
bool mUseVirtualAudioSink;
sp<AMessage> mNotify;
Mutex mLock;
uint32_t mFlags;
List<QueueEntry> mAudioQueue; // 音频缓冲
List<QueueEntry> mVideoQueue; // 视频缓冲
uint32_t mNumFramesWritten;
sp<VideoFrameScheduler> mVideoScheduler;
sp<MediaClock> mMediaClock;
float mPlaybackRate; // audio track rate
}
首先看到的是Renderer本身是AHandler的子类。还记得之前的AHandler和ALooper配合使用的机制嘛？其中ALooper位于NuPlayer中，变量名为mRendererLooper。
2 NuPlayer中调用的Renderer接口
先回顾下NuPlayer源码解析中的调用接口。
- 构造/析构函数
- 设置播放控制参数——setPlaybackSettings/getPlaybackSettings/setVideoFrameRate/setSyncSettings/getSyncSettings
- AudioSink相关——openAudioSink/closeAudioSink
- 控制接口——pause/flush/resume/queueEOS
- 音频状态更新——signalEnableOffloadAudio/signalDisableOffloadAudio
- 音视频原始数据输入——queueBuffer
NuPlayer中并未显示调用，而是将Renderer设置给ADecoder使用
if (mVideoDecoder != NULL) {
mVideoDecoder->setRenderer(mRenderer);
}
if (mAudioDecoder != NULL) {
mAudioDecoder->setRenderer(mRenderer);
}
3 Renderer具体接口分析
构造函数喝析构函数
构造函数最主要的是创建一个MediaClock，用于同步和计时。主要代码如下：
mMediaClock = new MediaClock;
mPlaybackRate = mPlaybackSettings.mSpeed;
mMediaClock->setPlaybackRate(mPlaybackRate);
由于AHandler是智能指针，可以不考虑析构函数。不过可以看下代码中实现：
NuPlayer::Renderer::~Renderer() {
if (offloadingAudio()) {
mAudioSink->stop(); // 主要是针对AudioSink的处理
mAudioSink->flush();
mAudioSink->close();
}
}
设置播放控制参数类接口
音频回放参数设置-setPlaybackSettings/getPlaybackSettings
主要接口定义及参数如下：
status_t setPlaybackSettings(const AudioPlaybackRate &rate /* sanitized */);
status_t getPlaybackSettings(AudioPlaybackRate *rate /* nonnull */);
struct AudioPlaybackRate {
float mSpeed; // 播放倍速
float mPitch; // 声调参数
enum AudioTimestretchStretchMode mStretchMode; // 拉伸模式
enum AudioTimestretchFallbackMode mFallbackMode; // 备用模式
};
从实际接口含义来看主要控制音频播放速率。最终设置函数将参数传递给mMediaClock->setPlaybackRate函数。
视频播放帧率参数-setVideoFrameRate
函数原型如下：void setVideoFrameRate(float fps);。只有一个参数视频播放帧率fps，最终实现函数将该参数设置给mVideoScheduler。实现如下：
void NuPlayer::Renderer::onSetVideoFrameRate(float fps) {
if (mVideoScheduler == NULL) {
mVideoScheduler = new VideoFrameScheduler();
}
mVideoScheduler->init(fps);
}
音视频同步参数-setSyncSettings/getSyncSettings
接口声明及主要参数如下：
status_t setSyncSettings(const AVSyncSettings &sync, float videoFpsHint);
status_t getSyncSettings(AVSyncSettings *sync /* nonnull */, float *videoFps /* nonnull */);
// from ~/frameworks/av/include/media/AVSyncSettings.h
struct AVSyncSettings {
AVSyncSource mSource; // 同步基准
AVSyncAudioAdjustMode mAudioAdjustMode; // 音频调整方式
float mTolerance; // 最大容忍的调速时间
AVSyncSettings()
: mSource(AVSYNC_SOURCE_DEFAULT),
mAudioAdjustMode(AVSYNC_AUDIO_ADJUST_MODE_DEFAULT),
mTolerance(.044f) { }
};
看代码实现就会发现，Renderer中并没有实现setSyncSettings，只是判断了必须使用必须使用默认的同步方式，判断逻辑如下：
status_t NuPlayer::Renderer::onConfigSync(const AVSyncSettings &sync, float videoFpsHint __unused) {
if (sync.mSource != AVSYNC_SOURCE_DEFAULT) {
return BAD_VALUE;
}
// TODO: support sync sources
return INVALID_OPERATION;
}
至于这里涉及的MediaClock、AudioSink、VideoFrameSchedule后续有专门介绍。
AudioSink相关-openAudioSink/closeAudioSink
主要用于创建和关闭AudioSink，声明如下：
status_t openAudioSink(
const sp<AMessage> &format,
bool offloadOnly,
bool hasVideo,
uint32_t flags,
bool *isOffloaded);
void closeAudioSink();
后续会解释两个接口。
控制接口-pause/flush/resume/queueEOS
pause/resume接口
暂停和恢复接口，实现类似，pause接口最终实现是在onPause中：
void NuPlayer::Renderer::onPause() {
if (mPaused) {
return;
}
{
Mutex::Autolock autoLock(mLock);
// we do not increment audio drain generation so that we fill audio buffer during pause.
++mVideoDrainGeneration;
prepareForMediaRenderingStart_l();
mPaused = true;
mMediaClock->setPlaybackRate(0.0); // 设置成0.0，后面解释为什么
}
mDrainAudioQueuePending = false;
mDrainVideoQueuePending = false;
// Note: audio data may not have been decoded, and the AudioSink may not be opened.
mAudioSink->pause();
startAudioOffloadPauseTimeout();
}
其最终通过mMediaClock->setPlaybackRate和mAudioSink->pause接口实现暂停功能。
resume接口最终实现是在onResume中，代码如下：
void NuPlayer::Renderer::onResume() {
if (!mPaused) {
return;
}
// Note: audio data may not have been decoded, and the AudioSink may not be opened.
cancelAudioOffloadPauseTimeout();
if (mAudioSink->ready()) {
status_t err = mAudioSink->start();
if (err != OK) {
ALOGE("cannot start AudioSink err %d", err);
notifyAudioTearDown(kDueToError);
}
}
{
Mutex::Autolock autoLock(mLock);
mPaused = false;
// rendering started message may have been delayed if we were paused.
if (mRenderingDataDelivered) {
notifyIfMediaRenderingStarted_l();
}
// configure audiosink as we did not do it when pausing
if (mAudioSink != NULL && mAudioSink->ready()) {
mAudioSink->setPlaybackRate(mPlaybackSettings);
}
mMediaClock->setPlaybackRate(mPlaybackRate);
if (!mAudioQueue.empty()) {
postDrainAudioQueue_l();
}
}
if (!mVideoQueue.empty()) {
postDrainVideoQueue();
}
}
基本上是通过mAudioSink->start()和mMediaClock->setPlaybackRate实现，这过程中也有音视频队列清空的操作。
flush接口
主要分为针对音频的flush和针对视频的flush，具体实现时，音频主要是使用AudioSink的pause/flush/start接口，视频主要是使用清空缓冲队列和mVideoScheduler->restart实现。详细实现建议参考NuPlayer::Renderer::onFlush的代码。
queueEOS
添加流结束标志，最终实现是在onQueueEOS接口中，代码如下：
void NuPlayer::Renderer::onQueueEOS(const sp<AMessage> &msg) {
int32_t audio;
CHECK(msg->findInt32("audio", &audio));
if (dropBufferIfStale(audio, msg)) {
return;
}
int32_t finalResult;
CHECK(msg->findInt32("finalResult", &finalResult));
QueueEntry entry;
entry.mOffset = 0;
entry.mFinalResult = finalResult;
if (audio) { // 音频EOS
Mutex::Autolock autoLock(mLock);
if (mAudioQueue.empty() && mSyncQueues) {
syncQueuesDone_l();
}
mAudioQueue.push_back(entry);
postDrainAudioQueue_l();
} else { // 视频EOS
if (mVideoQueue.empty() && getSyncQueues()) {
Mutex::Autolock autoLock(mLock);
syncQueuesDone_l();
}
mVideoQueue.push_back(entry);
postDrainVideoQueue();
}
}
音视频原始数据输入——queueBuffer
在NuPlayer中没看到这个函数调用，但总体来说这个应该由音视频解码器调用，主要将解码之后的音视频原始数据通知显示端并作缓存和同步。主要实现代码如下：（有删减）
void NuPlayer::Renderer::onQueueBuffer(const sp<AMessage> &msg) {
int32_t audio;
CHECK(msg->findInt32("audio", &audio));
if (dropBufferIfStale(audio, msg)) {
return;
}
sp<ABuffer> buffer;
CHECK(msg->findBuffer("buffer", &buffer)); // 传入的数据存储在这里
QueueEntry entry;
entry.mBuffer = buffer;
entry.mNotifyConsumed = notifyConsumed;
entry.mOffset = 0;
entry.mFinalResult = OK;
entry.mBufferOrdinal = ++mTotalBuffersQueued;
// 将数据放到音频或者视频缓冲队列中
if (audio) {
Mutex::Autolock autoLock(mLock);
mAudioQueue.push_back(entry);
postDrainAudioQueue_l();
} else {
mVideoQueue.push_back(entry);
postDrainVideoQueue();
}
// 后续代码是做同步的
Mutex::Autolock autoLock(mLock);
if (!mSyncQueues || mAudioQueue.empty() || mVideoQueue.empty()) {
return;
}
sp<ABuffer> firstAudioBuffer = (*mAudioQueue.begin()).mBuffer;
sp<ABuffer> firstVideoBuffer = (*mVideoQueue.begin()).mBuffer;
if (firstAudioBuffer == NULL || firstVideoBuffer == NULL) {
// EOS signalled on either queue.
syncQueuesDone_l();
return;
}
int64_t firstAudioTimeUs;
int64_t firstVideoTimeUs;
CHECK(firstAudioBuffer->meta()
->findInt64("timeUs", &firstAudioTimeUs));
CHECK(firstVideoBuffer->meta()
->findInt64("timeUs", &firstVideoTimeUs));
int64_t diff = firstVideoTimeUs - firstAudioTimeUs;
ALOGV("queueDiff = %.2f secs", diff / 1E6);
if (diff > 100000ll) { //
// Audio data starts More than 0.1 secs before video.
// Drop some audio.
(*mAudioQueue.begin()).mNotifyConsumed->post();
mAudioQueue.erase(mAudioQueue.begin());
return;
}
syncQueuesDone_l();
}
4 MediaClock简介
看名字，MediaClock有点时钟同步的感觉，说白了就是一个多媒体时钟，是libstagefright提供的一个公共类。具体接口如下：
struct MediaClock : public RefBase {
MediaClock();
void setStartingTimeMedia(int64_t startingTimeMediaUs);
void clearAnchor();
void updateAnchor( int64_t anchorTimeMediaUs,
int64_t anchorTimeRealUs, int64_t maxTimeMediaUs = INT64_MAX);
void updateMaxTimeMedia(int64_t maxTimeMediaUs);
void setPlaybackRate(float rate);
float getPlaybackRate() const;
// 查询与实际时间|realUs|对应的多媒体时间，并将结果保存在|outMediaUs|中
status_t getMediaTime( int64_t realUs, int64_t *outMediaUs,
bool allowPastMaxTime = false) const;
// query real time corresponding to media time 查询与多媒体时间|targetMediaUs|对应的实际时间，结果保存在|outRealUs|中
status_t getRealTimeFor(int64_t targetMediaUs, int64_t *outRealUs) const;
private:
int64_t mAnchorTimeMediaUs;
int64_t mAnchorTimeRealUs;
int64_t mMaxTimeMediaUs;
int64_t mStartingTimeMediaUs;
float mPlaybackRate;
};
我把这个类的实现分为两部分，不需要逻辑判断的赋值或返回代码，需要额外计算的代码。先看简单的部分，函数功能主要是赋值和返回参数。
// code from ~/frameworks/av/media/libstagefright/MediaClock.cpp
MediaClock::MediaClock() : mAnchorTimeMediaUs(-1), mAnchorTimeRealUs(-1),
mMaxTimeMediaUs(INT64_MAX), mStartingTimeMediaUs(-1), mPlaybackRate(1.0) {}
MediaClock::~MediaClock() {}
void MediaClock::setStartingTimeMedia(int64_t startingTimeMediaUs) {
mStartingTimeMediaUs = startingTimeMediaUs;
}
void MediaClock::clearAnchor() {
mAnchorTimeMediaUs = -1;
mAnchorTimeRealUs = -1;
}
void MediaClock::updateMaxTimeMedia(int64_t maxTimeMediaUs) {
mMaxTimeMediaUs = maxTimeMediaUs;
}
float MediaClock::getPlaybackRate() const {
Mutex::Autolock autoLock(mLock);
return mPlaybackRate;
}
这部分代码实现了时钟的主要功能，对多媒体时间和实际时间做了对应关系。（注意代码部分有删减，仅保留核心逻辑）
void MediaClock::updateAnchor(
int64_t anchorTimeMediaUs, // 锚点的播放时间戳
int64_t anchorTimeRealUs, // 锚点的实际时间
int64_t maxTimeMediaUs) {
int64_t nowUs = ALooper::GetNowUs(); // 当前系统时钟
int64_t nowMediaUs = anchorTimeMediaUs + (nowUs - anchorTimeRealUs) * (double)mPlaybackRate; // 转换为当前值，误差低
if (maxTimeMediaUs != -1) {
mMaxTimeMediaUs = maxTimeMediaUs;
}
mAnchorTimeRealUs = nowUs;
mAnchorTimeMediaUs = nowMediaUs;
}
void MediaClock::setPlaybackRate(float rate) {
CHECK_GE(rate, 0.0);
if (mAnchorTimeRealUs == -1) {
mPlaybackRate = rate;
return;
}
int64_t nowUs = ALooper::GetNowUs();
mAnchorTimeMediaUs += (nowUs - mAnchorTimeRealUs) * (double)mPlaybackRate;
mAnchorTimeRealUs = nowUs;
mPlaybackRate = rate;
}
// 以下两个函数完成MediaTime <-->realTime的映射，具体原理还是来自updateAnchor
status_t MediaClock::getMediaTime(int64_t realUs, int64_t *outMediaUs, bool allowPastMaxTime) const {
return getMediaTime_l(realUs, outMediaUs, allowPastMaxTime);
}
status_t MediaClock::getMediaTime_l(int64_t realUs, int64_t *outMediaUs, bool allowPastMaxTime) const {
if (mAnchorTimeRealUs == -1) {
return NO_INIT;
}
int64_t mediaUs = mAnchorTimeMediaUs
+ (realUs - mAnchorTimeRealUs) * (double)mPlaybackRate;
if (mediaUs > mMaxTimeMediaUs && !allowPastMaxTime) {
mediaUs = mMaxTimeMediaUs;
}
if (mediaUs < mStartingTimeMediaUs) {
mediaUs = mStartingTimeMediaUs;
}
if (mediaUs < 0) {
mediaUs = 0;
}
*outMediaUs = mediaUs;
return OK;
}
status_t MediaClock::getRealTimeFor(int64_t targetMediaUs, int64_t *outRealUs) const {
if (outRealUs == NULL) {
return BAD_VALUE;
}
if (mPlaybackRate == 0.0) {
return NO_INIT;
}
int64_t nowUs = ALooper::GetNowUs();
int64_t nowMediaUs;
status_t status =
getMediaTime_l(nowUs, &nowMediaUs, true /* allowPastMaxTime */);
if (status != OK) {
return status;
}
*outRealUs = (targetMediaUs - nowMediaUs) / (double)mPlaybackRate + nowUs;
return OK;
}
还记得在前面解释Renderer::pause实现的时候把mPlaybackRate设置成0嘛，看到上面的计算代码基本上就可以明白了。
比较有意思的是针对mPlaybackRate的处理及Renderer调用的逻辑。下面是获得当前播放位置的函数实现
status_t NuPlayer::Renderer::getCurrentPosition(int64_t *mediaUs) {
// 注意是直接调用的MediaClock::getMediaTime()
status_t result = mMediaClock->getMediaTime(ALooper::GetNowUs(), mediaUs);
if (result == OK) {
return result;
}
// MediaClock未初始化，尝试初始化之
{
AudioTimestamp ts;// 另一种时钟计算方法
status_t res = mAudioSink->getTimestamp(ts);
if (res != OK) {
return result;
}
// AudioSink has rendered some frames.
int64_t nowUs = ALooper::GetNowUs();
int64_t nowMediaUs = mAudioSink->getPlayedOutDurationUs(nowUs)
+ mAudioFirstAnchorTimeMediaUs;
mMediaClock->updateAnchor(nowMediaUs, nowUs, -1);
}
return mMediaClock->getMediaTime(ALooper::GetNowUs(), mediaUs);
}
到这里基本解释清楚MediaClock是做什么的，但是疑问还在，音视频同步在哪里，怎么做到的？
5 AudioSink简介
以下资料来在Google group，内容如下：
AudioTrack is the hardware audio sink. AudioSink is used for in-memory
decode and potentially other applications where output doesn't go
straight to hardware.
翻译过来就是AudioTrack是一种特殊的AudioSink，与硬件对应；而AudioSink是用于内存解码的，所得数据不直接输出到音频设备上。
在之前文章MediaPlayer Interface&State中可以看到MediaPlayerBase里面有一个抽象类定义，AudioSink。下面是具体的接口：
class AudioSink : public RefBase {
public:
enum cb_event_t {
CB_EVENT_FILL_BUFFER, // Request to write more data to buffer.
CB_EVENT_STREAM_END, // Sent after all the buffers queued in AF and HW are played
// back (after stop is called)
CB_EVENT_TEAR_DOWN // The AudioTrack was invalidated due to use case change:
// Need to re-evaluate offloading options
};
// Callback returns the number of bytes actually written to the buffer.
typedef size_t (*AudioCallback)(
AudioSink *audioSink, void *buffer, size_t size, void *cookie, cb_event_t event);
virtual ~AudioSink() {}
virtual bool ready() const = 0; // audio output is open and ready
virtual ssize_t bufferSize() const = 0;
virtual ssize_t frameCount() const = 0;
virtual ssize_t channelCount() const = 0;
virtual ssize_t frameSize() const = 0;
virtual uint32_t latency() const = 0;
virtual float msecsPerFrame() const = 0;
virtual status_t getPosition(uint32_t *position) const = 0;
virtual status_t getTimestamp(AudioTimestamp &ts) const = 0;
virtual int64_t getPlayedOutDurationUs(int64_t nowUs) const = 0;
virtual status_t getFramesWritten(uint32_t *frameswritten) const = 0;
virtual audio_session_t getSessionId() const = 0;
virtual audio_stream_type_t getAudioStreamType() const = 0;
virtual uint32_t getSampleRate() const = 0;
virtual int64_t getBufferDurationInUs() const = 0;
// If no callback is specified, use the "write" API below to submit audio data.
virtual status_t open(
uint32_t sampleRate, int channelCount, audio_channel_mask_t channelMask,
audio_format_t format=AUDIO_FORMAT_PCM_16_BIT,
int bufferCount=DEFAULT_AUDIOSINK_BUFFERCOUNT,
AudioCallback cb = NULL,
void *cookie = NULL,
audio_output_flags_t flags = AUDIO_OUTPUT_FLAG_NONE,
const audio_offload_info_t *offloadInfo = NULL,
bool doNotReconnect = false,
uint32_t suggestedFrameCount = 0) = 0;
virtual status_t start() = 0;
/* Input parameter |size| is in byte units stored in |buffer|.
* Data is copied over and actual number of bytes written (>= 0)
* is returned, or no data is copied and a negative status code
* is returned (even when |blocking| is true).
* When |blocking| is false, AudioSink will immediately return after
* part of or full |buffer| is copied over.
* When |blocking| is true, AudioSink will wait to copy the entire
* buffer, unless an error occurs or the copy operation is
* prematurely stopped.
*/
virtual ssize_t write(const void* buffer, size_t size, bool blocking = true) = 0;
virtual void stop() = 0;
virtual void flush() = 0;
virtual void pause() = 0;
virtual void close() = 0;
virtual status_t setPlaybackRate(const AudioPlaybackRate& rate) = 0;
virtual status_t getPlaybackRate(AudioPlaybackRate* rate /* nonnull */) = 0;
virtual bool needsTrailingPadding() { return true; }
virtual status_t setParameters(const String8& /* keyValuePairs */) { return NO_ERROR; }
virtual String8 getParameters(const String8& /* keys */) { return String8::empty(); }
};
在Renderer的构造函数中可以看到AudioSink是由NuPlayer传递过来的。明显的这仅仅是通过抽象实现了在Renderer中操作AudioSink及其子类的逻辑。当然在实际使用中，AudioSink也可以作为播放时间的参考，比如上面的getCurrentPosition的实现。这里面的open/close/start/stop/flush/pause/write接口均在Renderer中调用过，后续针对同步的解释会详细说明的。
6 VideoFrameScheduler简介
看名字，感觉这个功能跟MediaClock类似，只是专门针对视频帧的处理逻辑，这也是libstagefright提供的一个公共类，实际上是做视频渲染调整的，以保证视频渲染时间在VSYNC时间之后，防止出现画面撕裂的情况。其对外接口如下：
struct VideoFrameScheduler : public RefBase {
VideoFrameScheduler();
// (re)initialize scheduler 初始化，给定帧率
void init(float videoFps = -1);
// 仅在视频渲染时间不连续的情况下使用，比如seek
void restart();
// 通过renderTime计算视频帧的调整时间（单位纳秒）
nsecs_t schedule(nsecs_t renderTime);
// 返回主屏的垂直同步间隔
nsecs_t getVsyncPeriod();
// 返回帧率
float getFrameRate();
void release();
}
内部实现我就不做解释了，基本意思还是从Renderer的调用中说起。Renderer中主要调用了VideoFrameScheduler的以下接口：
mVideoScheduler = new VideoFrameScheduler();
mVideoScheduler->init(fps);
mVideoScheduler->restart(); // 以下调用都在postDrainVideoQueue中
realTimeUs = mVideoScheduler->schedule(realTimeUs * 1000) / 1000;
int64_t twoVsyncsUs = 2 * (mVideoScheduler->getVsyncPeriod() / 1000);
7 音视频同步时如何实现的？
从Renderer接口层来看，没有任何关于同步处理的接口，仅有有限的几个控制接口flush/pause/resume，以及queueBuffer/queueEOS接口。同步问题的核心就在于ALooper-AHandler机制。其实真正的同步都是在消息循环的响应函数里实现的。先看音频。
Renderer中的音频同步机制
起始位置从音频PCM数据进入开始，处理在Renderer::queueBuffer()中，最终发送了kWhatQueueBuffer消息。这个消息的实际处理函数是Renderer::onQueueBuffer()。实际代码在“音视频原始数据输入——queueBuffer”中有，这里仅针对音频流程解释下。 基本逻辑很简单，保存传入的buffer参数，并通知输出下AudioQueue。
QueueEntry entry;
Mutex::Autolock autoLock(mLock);
mAudioQueue.push_back(entry);
postDrainAudioQueue_l();
下面看看postDrainAudioQueue_l的实现，内部实现逻辑基本上就是边界判断加上发送kWhatDrainAudioQueue消息。
void NuPlayer::Renderer::postDrainAudioQueue_l(int64_t delayUs) {
if (mAudioQueue.empty()) return;
mDrainAudioQueuePending = true;
sp<AMessage> msg = new AMessage(kWhatDrainAudioQueue, this);
msg->setInt32("drainGeneration", mAudioDrainGeneration);
msg->post(delayUs);
}
那就继续查看下这个消息如何处理的。
case kWhatDrainAudioQueue:
{
mDrainAudioQueuePending = false;
if (onDrainAudioQueue()) {
uint32_t numFramesPlayed;
uint32_t numFramesPendingPlayout = mNumFramesWritten - numFramesPlayed;
// 这里是audio sink中缓存了多长的可用于播放的数据
int64_t delayUs = mAudioSink->msecsPerFrame() * numFramesPendingPlayout * 1000ll;
if (mPlaybackRate > 1.0f) {
delayUs /= mPlaybackRate;
}
// 利用一半的延时来保证下次刷新时间（注意时间上有重叠）
delayUs /= 2;
// 参考buffer大小来估计最大的延时时间
const int64_t maxDrainDelayUs = std::max(
mAudioSink->getBufferDurationInUs(), (int64_t)500000 /* half second */);
ALOGD_IF(delayUs > maxDrainDelayUs, "postDrainAudioQueue long delay: %lld > %lld",
(long long)delayUs, (long long)maxDrainDelayUs);
Mutex::Autolock autoLock(mLock);
postDrainAudioQueue_l(delayUs); // 这里同一个消息重发了
}
break;
}
到这里，貌似还是没有同步的机制，不过我们已经知道这个音频播放消息的触发机制了，在queueBuffer和消息处理函数中都会触发，基本上就是定时器。还有最后一个函数onDrainAudioQueue()。下面是代码：
bool NuPlayer::Renderer::onDrainAudioQueue() {
uint32_t numFramesPlayed;
if (mAudioSink->getPosition(&numFramesPlayed) != OK) {
drainAudioQueueUntilLastEOS();
ALOGW("onDrainAudioQueue(): audio sink is not ready");
return false;
}
uint32_t prevFramesWritten = mNumFramesWritten;
while (!mAudioQueue.empty()) {
QueueEntry *entry = &*mAudioQueue.begin();
mLastAudioBufferDrained = entry->mBufferOrdinal;
if (entry->mBuffer == NULL) {
// 删除针对EOS的处理代码
}
// ignore 0-sized buffer which could be EOS marker with no data
if (entry->mOffset == 0 && entry->mBuffer->size() > 0) {
int64_t mediaTimeUs;
CHECK(entry->mBuffer->meta()->findInt64("timeUs", &mediaTimeUs));
ALOGV("onDrainAudioQueue: rendering audio at media time %.2f secs",
mediaTimeUs / 1E6);
onNewAudioMediaTime(mediaTimeUs);
}
size_t copy = entry->mBuffer->size() - entry->mOffset;
ssize_t written = mAudioSink->write(entry->mBuffer->data() + entry->mOffset,
copy, false /* blocking */);
if (written < 0) {/* ...忽略异常处理部分代码 */}
entry->mOffset += written;
size_t remainder = entry->mBuffer->size() - entry->mOffset;
if ((ssize_t)remainder < mAudioSink->frameSize()) {
if (remainder > 0) {// 这是直接凑成完整的一帧音频
ALOGW("Corrupted audio buffer has fractional frames, discarding %zu bytes.", remainder);
entry->mOffset += remainder;
copy -= remainder;
}
entry->mNotifyConsumed->post();
mAudioQueue.erase(mAudioQueue.begin());
entry = NULL;
}
size_t copiedFrames = written / mAudioSink->frameSize();
mNumFramesWritten += copiedFrames;
{
Mutex::Autolock autoLock(mLock);
int64_t maxTimeMedia;
maxTimeMedia = mAnchorTimeMediaUs +
(int64_t)(max((long long)mNumFramesWritten - mAnchorNumFramesWritten, 0LL)
* 1000LL * mAudioSink->msecsPerFrame());
mMediaClock->updateMaxTimeMedia(maxTimeMedia);
notifyIfMediaRenderingStarted_l();
}
if (written != (ssize_t)copy) {
// A short count was received from AudioSink::write()
//
// AudioSink write is called in non-blocking mode.
// It may return with a short count when:
//
// 1) Size to be copied is not a multiple of the frame size. Fractional frames are
// discarded.
// 2) The data to be copied exceeds the available buffer in AudioSink.
// 3) An error occurs and data has been partially copied to the buffer in AudioSink.
// 4) AudioSink is an AudioCache for data retrieval, and the AudioCache is exceeded.
// (Case 1)
// Must be a multiple of the frame size. If it is not a multiple of a frame size, it
// needs to fail, as we should not carry over fractional frames between calls.
CHECK_EQ(copy % mAudioSink->frameSize(), 0);
// (Case 2, 3, 4)
// Return early to the caller.
// Beware of calling immediately again as this may busy-loop if you are not careful.
ALOGV("AudioSink write short frame count %zd < %zu", written, copy);
break;
}
}
// calculate whether we need to reschedule another write.
bool reschedule = !mAudioQueue.empty()
&& (!mPaused
|| prevFramesWritten != mNumFramesWritten); // permit pause to fill buffers
//ALOGD("reschedule:%d empty:%d mPaused:%d prevFramesWritten:%u mNumFramesWritten:%u",
// reschedule, mAudioQueue.empty(), mPaused, prevFramesWritten, mNumFramesWritten);
return reschedule;
}
这里面比较主要的更新是onNewAudioMediaTime和mNumFramesWritten字段。
剩下的一部分代码是关于异常边界情况下的音视频处理逻辑：
sp<ABuffer> firstAudioBuffer = (*mAudioQueue.begin()).mBuffer;
sp<ABuffer> firstVideoBuffer = (*mVideoQueue.begin()).mBuffer;
if (firstAudioBuffer == NULL || firstVideoBuffer == NULL) {
// 对于一个队列为空的情况，通知另个一队列EOS
syncQueuesDone_l();
return;
}
int64_t firstAudioTimeUs;
int64_t firstVideoTimeUs;
CHECK(firstAudioBuffer->meta()
->findInt64("timeUs", &firstAudioTimeUs));
CHECK(firstVideoBuffer->meta()
->findInt64("timeUs", &firstVideoTimeUs));
int64_t diff = firstVideoTimeUs - firstAudioTimeUs;
if (diff > 100000ll) {
// 音频数据时间戳比视频数据早0.1s，
(*mAudioQueue.begin()).mNotifyConsumed->post();
mAudioQueue.erase(mAudioQueue.begin());
return;
}
syncQueuesDone_l();
Renderer中的视频同步部分
和音频同步类似，入口在在Renderer::queueBuffer()，主要区分在Renderer::onQueueBuffer()中，代码如下：
// 如果是视频，则将数据存放到视频队列，然后安排刷新
mVideoQueue.push_back(entry);
postDrainVideoQueue();
下面按照之前的思路继续分析，接下来是postDrainVideoQueue实现，主要音视频同步逻辑位于这里。
void NuPlayer::Renderer::postDrainVideoQueue() {
if (mVideoQueue.empty()) {
return;
}
QueueEntry &entry = *mVideoQueue.begin();
sp<AMessage> msg = new AMessage(kWhatDrainVideoQueue, this); //这是实际处理视频缓冲区和显示的消息
msg->setInt32("drainGeneration", getDrainGeneration(false /* audio */));
if (entry.mBuffer == NULL) {
// EOS doesn't carry a timestamp.
msg->post();
mDrainVideoQueuePending = true;
return;
}
bool needRepostDrainVideoQueue = false;
int64_t delayUs;
int64_t nowUs = ALooper::GetNowUs();
int64_t realTimeUs;
int64_t mediaTimeUs;
CHECK(entry.mBuffer->meta()->findInt64("timeUs", &mediaTimeUs));
if (mFlags & FLAG_REAL_TIME) {
realTimeUs = mediaTimeUs;
} else {
{
Mutex::Autolock autoLock(mLock);
if (mAnchorTimeMediaUs < 0) { // 同步基准未设置的情况下，直接显示
mMediaClock->updateAnchor(mediaTimeUs, nowUs, mediaTimeUs);
mAnchorTimeMediaUs = mediaTimeUs;
realTimeUs = nowUs;
} else if (!mVideoSampleReceived) { // 第一帧未显示前，直接显示
// Always render the first video frame.
realTimeUs = nowUs;
} else if (mAudioFirstAnchorTimeMediaUs < 0 // 音频未播放之前，以视频为准
|| mMediaClock->getRealTimeFor(mediaTimeUs, &realTimeUs) == OK) {
realTimeUs = getRealTimeUs(mediaTimeUs, nowUs);
} else if (mediaTimeUs - mAudioFirstAnchorTimeMediaUs >= 0) { // 视频超前的情况下，等待
needRepostDrainVideoQueue = true;
realTimeUs = nowUs;
} else {
realTimeUs = nowUs;
}
}
// Heuristics to handle situation when media time changed without a
// discontinuity. If we have not drained an audio buffer that was
// received after this buffer, repost in 10 msec. Otherwise repost
// in 500 msec.
delayUs = realTimeUs - nowUs;
int64_t postDelayUs = -1;
if (delayUs > 500000) {
postDelayUs = 500000;
if (mHasAudio && (mLastAudioBufferDrained - entry.mBufferOrdinal) <= 0) {
postDelayUs = 10000;
}
} else if (needRepostDrainVideoQueue) {
// CHECK(mPlaybackRate > 0);
// CHECK(mAudioFirstAnchorTimeMediaUs >= 0);
// CHECK(mediaTimeUs - mAudioFirstAnchorTimeMediaUs >= 0);
postDelayUs = mediaTimeUs - mAudioFirstAnchorTimeMediaUs;
postDelayUs /= mPlaybackRate;
}
if (postDelayUs >= 0) {
msg->setWhat(kWhatPostDrainVideoQueue);
msg->post(postDelayUs);
mVideoScheduler->restart();
ALOGI("possible video time jump of %dms or uninitialized media clock, retrying in %dms",
(int)(delayUs / 1000), (int)(postDelayUs / 1000));
mDrainVideoQueuePending = true;
return;
}
}
realTimeUs = mVideoScheduler->schedule(realTimeUs * 1000) / 1000;
int64_t twoVsyncsUs = 2 * (mVideoScheduler->getVsyncPeriod() / 1000);
delayUs = realTimeUs - nowUs;
// 上面代码的主要目的是计算这个延时
ALOGW_IF(delayUs > 500000, "unusually high delayUs: %" PRId64, delayUs);
// post 2 display refreshes before rendering is due
msg->post(delayUs > twoVsyncsUs ? delayUs - twoVsyncsUs : 0);
mDrainVideoQueuePending = true;
}
这里主要的是发送了一个延时消息kWhatDrainVideoQueue，下面是如何处理的代码：
case kWhatDrainVideoQueue:
{
int32_t generation;
CHECK(msg->findInt32("drainGeneration", &generation));
if (generation != getDrainGeneration(false /* audio */)) {
break;
}
mDrainVideoQueuePending = false;
onDrainVideoQueue();
postDrainVideoQueue(); // 注意这里相当于定时器的实现了
break;
}
直接调用onDrainVideoQueue函数，看看如何实现的：
void NuPlayer::Renderer::onDrainVideoQueue() {
if (mVideoQueue.empty()) {
return;
}
QueueEntry *entry = &*mVideoQueue.begin();
if (entry->mBuffer == NULL) {
// ...省略针对EOS 处理
}
int64_t nowUs = ALooper::GetNowUs();
int64_t realTimeUs;
int64_t mediaTimeUs = -1;
if (mFlags & FLAG_REAL_TIME) {
CHECK(entry->mBuffer->meta()->findInt64("timeUs", &realTimeUs));
} else {
CHECK(entry->mBuffer->meta()->findInt64("timeUs", &mediaTimeUs));
realTimeUs = getRealTimeUs(mediaTimeUs, nowUs);
}
bool tooLate = false;
if (!mPaused) {
setVideoLateByUs(nowUs - realTimeUs);
tooLate = (mVideoLateByUs > 40000);
if (tooLate) {
ALOGV("video late by %lld us (%.2f secs)",
(long long)mVideoLateByUs, mVideoLateByUs / 1E6);
} else {
int64_t mediaUs = 0;
mMediaClock->getMediaTime(realTimeUs, &mediaUs);
ALOGV("rendering video at media time %.2f secs",
(mFlags & FLAG_REAL_TIME ? realTimeUs :
mediaUs) / 1E6);
if (!(mFlags & FLAG_REAL_TIME)
&& mLastAudioMediaTimeUs != -1
&& mediaTimeUs > mLastAudioMediaTimeUs) {
// If audio ends before video, video continues to drive media clock.
// Also smooth out videos >= 10fps.
mMediaClock->updateMaxTimeMedia(mediaTimeUs + 100000);
}
}
} else {
setVideoLateByUs(0);
if (!mVideoSampleReceived && !mHasAudio) {
// This will ensure that the first frame after a flush won't be used as anchor
// when renderer is in paused state, because resume can happen any time after seek.
Mutex::Autolock autoLock(mLock);
clearAnchorTime_l();
}
}
// Always render the first video frame while keeping stats on A/V sync.
if (!mVideoSampleReceived) {
realTimeUs = nowUs;
tooLate = false;
}
entry->mNotifyConsumed->setInt64("timestampNs", realTimeUs * 1000ll); // 上面所有计算的参数在这里使用了
entry->mNotifyConsumed->setInt32("render", !tooLate);
entry->mNotifyConsumed->post(); // 注意这里，实际是向解码器发送消息，用于显示
mVideoQueue.erase(mVideoQueue.begin());
entry = NULL;
mVideoSampleReceived = true;
if (!mPaused) { // 这里是通知NuPlayer层渲染开始
if (!mVideoRenderingStarted) {
mVideoRenderingStarted = true;
notifyVideoRenderingStart();
}
Mutex::Autolock autoLock(mLock);
notifyIfMediaRenderingStarted_l();
}
}
到这里，小结下，读完这部分代码发现，NuPlayer::Renderer使用的以视频为基准的同步机制，音频晚了直接丢包，视频需要显示。同步主要位于视频缓冲区处理部分onDrainVideoQueue和音频缓冲区处理部分onDrainVideoQueue中。音视频的渲染都是采用类似定时器的机制，只不过视频显示需要依赖于实际解码器，音频播放需要依赖于AudioSink的接口。
8 总结
本文主要参考NuPlayer::Renderer的代码做的分析，持续时间比较长。我都怀疑自己具体写的对不对。
非常抱歉拖了这么久，文中代码比较多，如果诸位绝对不对胃口可以略过。
怎么说呢？ Renderer涉及部分比较多，包括NuPlayer、AudioSink、MediaClock、VideoScheduler等。细节还是有待分析，不过基本整理情况是什么了。
我到现在才认识到理解和整理出来的差距。还需要多历练下。