Android 12(S) MultiMedia Learning（八）NuPlayer Renderer

NuPlayer的AVSync由Renderer实现，接下来主要来看AVSync的工作原理

相关代码位置：

NuPlayerRenderer.cpp - OpenGrok cross reference for /frameworks/av/media/libmediaplayerservice/nuplayer/NuPlayerRenderer.cpp (aospxref.com)

a. 构造函数

Renderer是在NuPlayer的start过程中被实例化，会传入一个AudioSink、MediaClock，之后还会调用setPlaybackSettings来设定播放模式（速率）

AudioSink：用于音频的播放，调用setDataSource时在MediaPlayerService::Client:中被创建

MediaClock：NuPlayer的AVSync是基于系统时间和Audio来同步的，Clock在NuPlayerDriver中实例化NuPlayer时创建，作为系统时间使用

Renderer对象创建完成之后会分别传入Audio和Video的Decoder中

void NuPlayer::onStart(int64_t startPositionUs, MediaPlayerSeekMode mode) {
    // ......
    sp<AMessage> notify = new AMessage(kWhatRendererNotify, this);
    ++mRendererGeneration;
    notify->setInt32("generation", mRendererGeneration);
    mRenderer = new Renderer(mAudioSink, mMediaClock, notify, flags);
    mRendererLooper = new ALooper;
    mRendererLooper->setName("NuPlayerRenderer");
    mRendererLooper->start(false, false, ANDROID_PRIORITY_AUDIO);
    mRendererLooper->registerHandler(mRenderer);

    status_t err = mRenderer->setPlaybackSettings(mPlaybackSettings);
    if (err != OK) {
        mSource->stop();
        mSourceStarted = false;
        notifyListener(MEDIA_ERROR, MEDIA_ERROR_UNKNOWN, err);
        return;
    }

    float rate = getFrameRate();
    if (rate > 0) {
　　　　 // 设定播放速度
        mRenderer->setVideoFrameRate(rate);
    }

    if (mVideoDecoder != NULL) {
        mVideoDecoder->setRenderer(mRenderer);
    }
    if (mAudioDecoder != NULL) {
        mAudioDecoder->setRenderer(mRenderer);
    }
}

status_t NuPlayer::instantiateDecoder(
        bool audio, sp<DecoderBase> *decoder, bool checkAudioModeChange) {
      // ......
        *decoder = new Decoder(
                notify, mSource, mPID, mUID, mRenderer, mSurface, mCCDecoder);
      // ......
}

b. Renderer的数据写入

NuPlayerDecoder收到mediacodec送上来的解码后的数据，会调用queueBuffer写入到Renderer队列当中去

bool NuPlayer::Decoder::handleAnOutputBuffer(
        size_t index,
        size_t offset,
        size_t size,
        int64_t timeUs,
        int32_t flags) {
    // ......
    sp<MediaCodecBuffer> buffer;
    mCodec->getOutputBuffer(index, &buffer);
    // ......
    int64_t frameIndex;
    bool frameIndexFound = buffer->meta()->findInt64("frameIndex", &frameIndex);

    buffer->setRange(offset, size);
    buffer->meta()->clear();
    buffer->meta()->setInt64("timeUs", timeUs);
    if (frameIndexFound) {
        buffer->meta()->setInt64("frameIndex", frameIndex);
    }
    // ......

     if (mRenderer != NULL) {
       // send the buffer to renderer.
　　　　// 通过queueBuffer做数据写入
       mRenderer->queueBuffer(mIsAudio, buffer, reply);
       if (eos && !isDiscontinuityPending()) {
           mRenderer->queueEOS(mIsAudio, ERROR_END_OF_STREAM);
       }
    }
}

c. Renderer::queueBuffer

Renderer会对audio和video做不同的处理，先看看对audio是怎么处理的

 Audio

光看调用方法的数量就很头疼，下面贴代码解释解释：

void NuPlayer::Renderer::onQueueBuffer(const sp<AMessage> &msg) {
    int32_t audio;
    CHECK(msg->findInt32("audio", &audio));

    // ......

    sp<RefBase> obj;
    CHECK(msg->findObject("buffer", &obj));
    sp<MediaCodecBuffer> buffer = static_cast<MediaCodecBuffer *>(obj.get());

    sp<AMessage> notifyConsumed;
    CHECK(msg->findMessage("notifyConsumed", &notifyConsumed));

    // 将传进来的buffer封装为一个QueueEntry 加入到队列当中
    QueueEntry entry;
    entry.mBuffer = buffer;
    entry.mNotifyConsumed = notifyConsumed;
    entry.mOffset = 0;
    entry.mFinalResult = OK;
    entry.mBufferOrdinal = ++mTotalBuffersQueued;

    if (audio) {
        Mutex::Autolock autoLock(mLock);
        mAudioQueue.push_back(entry);
        postDrainAudioQueue_l();
    } else {
        mVideoQueue.push_back(entry);
        postDrainVideoQueue();
    }

    // ......
}

先看onQueueBuffer的前半部分，先判断送进来的buffer是audio还是video，接着封装为QueueEntry加入到队列当中，如果是Audio则调用postDrainAudioQueue_l处理，如果是video则调用postDrainVideoQueue来处理。

        case kWhatDrainAudioQueue:
        {
            mDrainAudioQueuePending = false;

            int32_t generation;
            CHECK(msg->findInt32("drainGeneration", &generation));
            if (generation != getDrainGeneration(true /* audio */)) {
                break;
            }

            // 如果onDrainAudioQueue return true会再次调用到该方法
            if (onDrainAudioQueue()) {
                // ......
                postDrainAudioQueue_l(delayUs);
            }
            break;
        }

postDrainAudioQueue_l发出一条消息，最终会调用到onDrainAudioQueue方法

bool NuPlayer::Renderer::onDrainAudioQueue() {

    // ......
    // 获取到Audio已经播放了多少帧
    uint32_t numFramesPlayed;
    if (mAudioSink->getPosition(&numFramesPlayed) != OK) {
        drainAudioQueueUntilLastEOS();
        ALOGW("onDrainAudioQueue(): audio sink is not ready");
        return false;
    }

    // 记录本次数据处理之前已经写入了多少帧
    uint32_t prevFramesWritten = mNumFramesWritten;
    // 循环处理加入到AudioQueue中的QueueEntry
    while (!mAudioQueue.empty()) {
        QueueEntry *entry = &*mAudioQueue.begin();

        // ......

        mLastAudioBufferDrained = entry->mBufferOrdinal;

        if (entry->mOffset == 0 && entry->mBuffer->size() > 0) {
            int64_t mediaTimeUs;
            // 获取到数据中带有的时间戳
            CHECK(entry->mBuffer->meta()->findInt64("timeUs", &mediaTimeUs));
            ALOGV("onDrainAudioQueue: rendering audio at media time %.2f secs",
                    mediaTimeUs / 1E6);
            // 调用onNewAudioMediaTime去更新信息
            onNewAudioMediaTime(mediaTimeUs);
        }
        // 计算buffer中数据长度
        size_t copy = entry->mBuffer->size() - entry->mOffset;
        // 将buffer写入到AudioSink中
        ssize_t written = mAudioSink->write(entry->mBuffer->data() + entry->mOffset,
                                            copy, false /* blocking */);
        // ......

        entry->mOffset += written;
        size_t remainder = entry->mBuffer->size() - entry->mOffset;
        if ((ssize_t)remainder < mAudioSink->frameSize()) {
            if (remainder > 0) {
                ALOGW("Corrupted audio buffer has fractional frames, discarding %zu bytes.",
                        remainder);
                entry->mOffset += remainder;
                copy -= remainder;
            }
            // 通知mediacodec释放buffer
            entry->mNotifyConsumed->post();
            mAudioQueue.erase(mAudioQueue.begin());

            entry = NULL;
        }
        // 计算写入的帧数
        size_t copiedFrames = written / mAudioSink->frameSize();
        mNumFramesWritten += copiedFrames;

        {
            Mutex::Autolock autoLock(mLock);
            int64_t maxTimeMedia;
            // 计算更新最大媒体时间，更新给mediaclock
            maxTimeMedia =
                mAnchorTimeMediaUs +
                        (int64_t)(max((long long)mNumFramesWritten - mAnchorNumFramesWritten, 0LL)
                                * 1000LL * mAudioSink->msecsPerFrame());
            mMediaClock->updateMaxTimeMedia(maxTimeMedia);

            notifyIfMediaRenderingStarted_l();
        }
        // ......
    }

    // calculate whether we need to reschedule another write.
    bool reschedule = !mAudioQueue.empty()
            && (!mPaused
                || prevFramesWritten != mNumFramesWritten); // permit pause to fill buffers
    return reschedule;
}

onDrainAudioQueue这个方法很长，有几个关键点

1. onNewAudioMediaTime：拿到新的audio buffer之后会调用该方法去更新用于同步的锚点pts等信息

2. mAudioSink->write：没有去研究audiosink的工作原理，不过似乎有足够的数据写入之后就会自动开始工作做audio的渲染？

3. mMediaClock->updateMaxTimeMedia：更新最大媒体时间

计算方式通过：mAnchorTimeMediaUs  +  （mNumFramesWritten - mAnchorNumFramesWritten）* mAudioSink->msecsPerFrame * 100

= 本次写入数据的pts + 写入帧数 * 每帧播放时长

mAnchorTimeMediaUs 和 mAnchorNumFramesWritten 会在onNewAudioMediaTime中更新，但是mAnchorNumFramesWritten记录的是本    次数据写入之前的所有帧数，mNumFramesWritten记录的是数据写入之后的总帧数

接下来看看onNewAudioMediaTime这个方法

void NuPlayer::Renderer::onNewAudioMediaTime(int64_t mediaTimeUs) {
    Mutex::Autolock autoLock(mLock);
    if (mediaTimeUs == mAnchorTimeMediaUs) {
        return;
    }
    // 记录第一笔有效数据的pts
    setAudioFirstAnchorTimeIfNeeded_l(mediaTimeUs);

    // mNextAudioClockUpdateTimeUs is -1 if we're waiting for audio sink to start
    if (mNextAudioClockUpdateTimeUs == -1) {
        AudioTimestamp ts;
        // 获取当前audiosink渲染的时间戳，如果大于0就去开始更新mNextAudioClockUpdateTimeUs
        if (mAudioSink->getTimestamp(ts) == OK && ts.mPosition > 0) {
            mNextAudioClockUpdateTimeUs = 0; // start our clock updates
        }
    }
    int64_t nowUs = ALooper::GetNowUs();
    if (mNextAudioClockUpdateTimeUs >= 0) {
        if (nowUs >= mNextAudioClockUpdateTimeUs) {
            // 当前媒体播放时间
            int64_t nowMediaUs = mediaTimeUs - getPendingAudioPlayoutDurationUs(nowUs);
            // abc
            mMediaClock->updateAnchor(nowMediaUs, nowUs, mediaTimeUs);
            mUseVirtualAudioSink = false;
            mNextAudioClockUpdateTimeUs = nowUs + kMinimumAudioClockUpdatePeriodUs;
        }
    } else {
        int64_t unused;
        // 当getMediaTime发送错误 或者audiosink发送错误会进入到 if
        if ((mMediaClock->getMediaTime(nowUs, &unused) != OK)
                && (getDurationUsIfPlayedAtSampleRate(mNumFramesWritten)
                        > kMaxAllowedAudioSinkDelayUs)) {
            // Enough data has been sent to AudioSink, but AudioSink has not rendered
            // any data yet. Something is wrong with AudioSink, e.g., the device is not
            // connected to audio out.
            // Switch to system clock. This essentially creates a virtual AudioSink with
            // initial latenty of getDurationUsIfPlayedAtSampleRate(mNumFramesWritten).
            // This virtual AudioSink renders audio data starting from the very first sample
            // and it's paced by system clock.
            ALOGW("AudioSink stuck. ARE YOU CONNECTED TO AUDIO OUT? Switching to system clock.");
            mMediaClock->updateAnchor(mAudioFirstAnchorTimeMediaUs, nowUs, mediaTimeUs);
            mUseVirtualAudioSink = true;
        }
    }
    // 更新本次之前已写入帧数
    mAnchorNumFramesWritten = mNumFramesWritten;
    // 更新本次写入数据的开始时间戳
    mAnchorTimeMediaUs = mediaTimeUs;
}

1. mNextAudioClockUpdateTimeUs 是用来记录锚点更新时间的，意思就是每隔kMinimumAudioClockUpdatePeriodUs就去更新锚点，防止sync误差滚雪球，当Audiosink开始工作之后会触发锚点更新

2. 更新锚点前会先用公式计算 nowMediaUs =mediaTimeUs - getPendingAudioPlayoutDurationUs(nowUs)，那这个nowMeidaUs指的是什么呢？

先看一下getPendingAudioPlayoutDurationUs，计算的是当前剩余可播时长；

所以nowMediaUs = 当前传入的pts（也可以理解为当前写入帧播完时的pts） -  剩余可以播时长 = 当前媒体播放时长

// Calculate duration of pending samples if played at normal rate (i.e., 1.0).
int64_t NuPlayer::Renderer::getPendingAudioPlayoutDurationUs(int64_t nowUs) {
    // 计算当前写入帧总共可以播放时间
    int64_t writtenAudioDurationUs = getDurationUsIfPlayedAtSampleRate(mNumFramesWritten);
    if (mUseVirtualAudioSink) {
        int64_t nowUs = ALooper::GetNowUs();
        int64_t mediaUs;
        if (mMediaClock->getMediaTime(nowUs, &mediaUs) != OK) {
            return 0LL;
        } else {
            return writtenAudioDurationUs - (mediaUs - mAudioFirstAnchorTimeMediaUs);
        }
    }
    // 通过audiosink获取当前已经播放的时间
    const int64_t audioSinkPlayedUs = mAudioSink->getPlayedOutDurationUs(nowUs);
    // pending意为当前剩余可播时长
    int64_t pendingUs = writtenAudioDurationUs - audioSinkPlayedUs;
    if (pendingUs < 0) {
        // This shouldn't happen unless the timestamp is stale.
        ALOGW("%s: pendingUs %lld < 0, clamping to zero, potential resume after pause "
                "writtenAudioDurationUs: %lld, audioSinkPlayedUs: %lld",
                __func__, (long long)pendingUs,
                (long long)writtenAudioDurationUs, (long long)audioSinkPlayedUs);
        pendingUs = 0;
    }
    return pendingUs;
}

int64_t NuPlayer::Renderer::getDurationUsIfPlayedAtSampleRate(uint32_t numFrames) {
    int32_t sampleRate = offloadingAudio() ?
            mCurrentOffloadInfo.sample_rate : mCurrentPcmInfo.mSampleRate;
    if (sampleRate == 0) {
        ALOGE("sampleRate is 0 in %s mode", offloadingAudio() ? "offload" : "non-offload");
        return 0;
    }
    // 音频帧的播放时间 = 音频帧的样本个数 / 采样频率 （单位为s）
    return (int64_t)(numFrames * 1000000LL / sampleRate);
}

3.  调用updateAnchor来更新锚点 mMediaClock->updateAnchor(nowMediaUs, nowUs, mediaTimeUs)，锚点到底是什么？

void MediaClock::updateAnchor(
        int64_t anchorTimeMediaUs,
        int64_t anchorTimeRealUs,
        int64_t maxTimeMediaUs) {
    if (anchorTimeMediaUs < 0 || anchorTimeRealUs < 0) {
        ALOGW("reject anchor time since it is negative.");
        return;
    }

    Mutex::Autolock autoLock(mLock);
    int64_t nowUs = ALooper::GetNowUs();
    // 重新校准一下当前媒体播放时间，减少函数调用时间引起的误差
    int64_t nowMediaUs =
        anchorTimeMediaUs + (nowUs - anchorTimeRealUs) * (double)mPlaybackRate;
    if (nowMediaUs < 0) {
        ALOGW("reject anchor time since it leads to negative media time.");
        return;
    }

    if (maxTimeMediaUs != -1) {
        mMaxTimeMediaUs = maxTimeMediaUs;
    }
    if (mAnchorTimeRealUs != -1) {
        // 用锚点时间来算一次当前媒体播放时间，如果和之前计算的时间差距在误差区间内，就不会去更新锚点
        int64_t oldNowMediaUs =
            mAnchorTimeMediaUs + (nowUs - mAnchorTimeRealUs) * (double)mPlaybackRate;
        if (nowMediaUs < oldNowMediaUs + kAnchorFluctuationAllowedUs
                && nowMediaUs > oldNowMediaUs - kAnchorFluctuationAllowedUs) {
            return;
        }
    }
    // 更新锚点
    updateAnchorTimesAndPlaybackRate_l(nowMediaUs, nowUs, mPlaybackRate);

    ++mGeneration;
    processTimers_l();
}

updateAnchor方法给传进来的参数换了个名字：

anchorTimeMediaUs -> 当前媒体播放时间

anchorTimeRealUs  -> 当前媒体媒体播放时间对应的真实系统时间

maxTimeMediaUs -> 写入数据帧的pts

该方法做了三件事情：

（一）先调整了下当前媒体播放时间，公式为 nowMediaUs = anchorTimeMediaUs + (nowUs - anchorTimeRealUs) * (double)mPlaybackRate

当前媒体播放时间 = 传入时的媒体播放时间   +（当前系统时间 - 传入时的系统时间 = 语句执行时间）* 播放速率

所以播放速率 * 播放时间也可以获得真实的视频走过的时间

（二）同样使用上面的公式来计算当前媒体播放时间，不一样的是mAnchorTimeMediaUs记录的是锚点媒体播放时间，时间跨度是当前时间和锚点系统时间之间的长度

oldNowMediaUs = mAnchorTimeMediaUs + (nowUs - mAnchorTimeRealUs) * (double)mPlaybackRate

当前媒体播放时间 = 锚点媒体播放时间  + （当前系统时间 - 锚点系统时间）* 播放速率

从这里可以看出来，锚点其实记录的是一组 媒体播放时间 以及其对应的 真实的系统时间，通过锚点就可以计算任意系统时间的媒体播放时间了

（三）调用 updateAnchorTimesAndPlaybackRate_l 去更新锚点时间，误差过大时就会更新了

void MediaClock::updateAnchorTimesAndPlaybackRate_l(int64_t anchorTimeMediaUs,
        int64_t anchorTimeRealUs, float playbackRate) {
    if (mAnchorTimeMediaUs != anchorTimeMediaUs
            || mAnchorTimeRealUs != anchorTimeRealUs
            || mPlaybackRate != playbackRate) {
        mAnchorTimeMediaUs = anchorTimeMediaUs;
        mAnchorTimeRealUs = anchorTimeRealUs;
        mPlaybackRate = playbackRate;
        notifyDiscontinuity_l();
    }
}

到这里audio部分就结束了，接下来看看怎么处理video的

video

video数据的处理入口是postDrainVideoQueue方法

void NuPlayer::Renderer::postDrainVideoQueue() {
    if (mDrainVideoQueuePending
            || getSyncQueues()
            || (mPaused && mVideoSampleReceived)) {
        return;
    }

    if (mVideoQueue.empty()) {
        return;
    }

    QueueEntry &entry = *mVideoQueue.begin();

    sp<AMessage> msg = new AMessage(kWhatDrainVideoQueue, this);
    msg->setInt32("drainGeneration", getDrainGeneration(false /* audio */));

    if (entry.mBuffer == NULL) {
        // EOS doesn't carry a timestamp.
        msg->post();
        mDrainVideoQueuePending = true;
        return;
    }

    int64_t nowUs = ALooper::GetNowUs();
    // 如果数据中带的是真实的播放时间，那么会走到这个if中来，可能是与tunnel mode相关
    if (mFlags & FLAG_REAL_TIME) {
        int64_t realTimeUs;
        CHECK(entry.mBuffer->meta()->findInt64("timeUs", &realTimeUs));

        realTimeUs = mVideoScheduler->schedule(realTimeUs * 1000) / 1000;

        int64_t twoVsyncsUs = 2 * (mVideoScheduler->getVsyncPeriod() / 1000);

        int64_t delayUs = realTimeUs - nowUs;

        ALOGW_IF(delayUs > 500000, "unusually high delayUs: %lld", (long long)delayUs);
        // post 2 display refreshes before rendering is due
        msg->post(delayUs > twoVsyncsUs ? delayUs - twoVsyncsUs : 0);

        mDrainVideoQueuePending = true;
        return;
    }

    // 获取pts
    int64_t mediaTimeUs;
    CHECK(entry.mBuffer->meta()->findInt64("timeUs", &mediaTimeUs));

    {
        Mutex::Autolock autoLock(mLock);
        if (mAnchorTimeMediaUs < 0) {
            // 如果video数据先到来，会用video pts去更新anchor
            mMediaClock->updateAnchor(mediaTimeUs, nowUs, mediaTimeUs);
            mAnchorTimeMediaUs = mediaTimeUs;
        }
    }
    mNextVideoTimeMediaUs = mediaTimeUs;
    if (!mHasAudio) {
        // smooth out videos >= 10fps
        // 如果没有音频就用视频更新最大媒体时间
        mMediaClock->updateMaxTimeMedia(mediaTimeUs + kDefaultVideoFrameIntervalUs);
    }

    if (!mVideoSampleReceived || mediaTimeUs < mAudioFirstAnchorTimeMediaUs) {
        // 如果第一帧时间小于音频第一帧时间，立即渲染
        msg->post();
    } else {
        // Vsync查了资料是垂直同步时间
        int64_t twoVsyncsUs = 2 * (mVideoScheduler->getVsyncPeriod() / 1000);
        // 定时在2倍vsync时间之前 发送消息处理本次avsync
        // post 2 display refreshes before rendering is due
        mMediaClock->addTimer(msg, mediaTimeUs, -twoVsyncsUs);
    }

    mDrainVideoQueuePending = true;
}

mMediaClock->addTimer 这个方法用做定时，在到达应播时间前2vsync时发送kWhatDrainVideoQueue事件给Renderer，进入到onDrainVideoQueue方法中进一步处理

void NuPlayer::Renderer::onDrainVideoQueue() {
    if (mVideoQueue.empty()) {
        return;
    }

    QueueEntry *entry = &*mVideoQueue.begin();

    if (entry->mBuffer == NULL) {
        // EOS

        notifyEOS(false /* audio */, entry->mFinalResult);

        mVideoQueue.erase(mVideoQueue.begin());
        entry = NULL;

        setVideoLateByUs(0);
        return;
    }

    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));
        // 获取应该播放的系统时间
        realTimeUs = getRealTimeUs(mediaTimeUs, nowUs);
    }
    // 做一次矫正
    realTimeUs = mVideoScheduler->schedule(realTimeUs * 1000) / 1000;

    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 + kDefaultVideoFrameIntervalUs);
            }
        }
    } 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.
            clearAnchorTime();
        }
    }

    // Always render the first video frame while keeping stats on A/V sync.
    if (!mVideoSampleReceived) {
        realTimeUs = nowUs;
        tooLate = false;
    }
    // 给NuPlayerDecoder发送消息，渲染
    entry->mNotifyConsumed->setInt64("timestampNs", realTimeUs * 1000LL);
    entry->mNotifyConsumed->setInt32("render", !tooLate);
    entry->mNotifyConsumed->post();
    mVideoQueue.erase(mVideoQueue.begin());
    entry = NULL;

    mVideoSampleReceived = true;

    if (!mPaused) {
        if (!mVideoRenderingStarted) {
            mVideoRenderingStarted = true;
            notifyVideoRenderingStart();
        }
        Mutex::Autolock autoLock(mLock);
        notifyIfMediaRenderingStarted_l();
    }
}

这个方法首先调用getRealTimeUs获取应当播放的系统时间，后面就发消息给NuPlayerDecoder，让mediacodec释放buffer并渲染了

int64_t NuPlayer::Renderer::getRealTimeUs(int64_t mediaTimeUs, int64_t nowUs) {
    int64_t realUs;
    if (mMediaClock->getRealTimeFor(mediaTimeUs, &realUs) != OK) {
        // If failed to get current position, e.g. due to audio clock is
        // not ready, then just play out video immediately without delay.
        return nowUs;
    }
    return realUs;
}

status_t MediaClock::getRealTimeFor(
        int64_t targetMediaUs, int64_t *outRealUs) const {
    if (outRealUs == NULL) {
        return BAD_VALUE;
    }

    Mutex::Autolock autoLock(mLock);
    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;
}

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;
}

getRealTimeUs涉及到几个计算：

getMediaTime_l 获取当前的媒体时间：媒体播放锚点 + （当前系统时间 - 锚点系统时间） =  当前媒体播放位置

getRealTimeFor 获取目标pts应当播放的系统时间：本帧PTS  -  当前媒体播放位置   +  当前系统时间   =    应该播放的系统时间

void NuPlayer::Decoder::onRenderBuffer(const sp<AMessage> &msg) {
    status_t err;
    int32_t render;
    size_t bufferIx;
    int32_t eos;
    size_t size;
    CHECK(msg->findSize("buffer-ix", &bufferIx));

    if (!mIsAudio) {
        int64_t timeUs;
        sp<MediaCodecBuffer> buffer = mOutputBuffers[bufferIx];
        buffer->meta()->findInt64("timeUs", &timeUs);

        if (mCCDecoder != NULL && mCCDecoder->isSelected()) {
            mCCDecoder->display(timeUs);
        }
    }

    if (mCodec == NULL) {
        err = NO_INIT;
    } else if (msg->findInt32("render", &render) && render) {
        int64_t timestampNs;
        CHECK(msg->findInt64("timestampNs", &timestampNs));
        err = mCodec->renderOutputBufferAndRelease(bufferIx, timestampNs);
    } else {
        if (!msg->findInt32("eos", &eos) || !eos ||
                !msg->findSize("size", &size) || size) {
            mNumOutputFramesDropped += !mIsAudio;
        }
        err = mCodec->releaseOutputBuffer(bufferIx);
    }
    if (err != OK) {
        ALOGE("failed to release output buffer for [%s] (err=%d)",
                mComponentName.c_str(), err);
        handleError(err);
    }
    if (msg->findInt32("eos", &eos) && eos
            && isDiscontinuityPending()) {
        finishHandleDiscontinuity(true /* flushOnTimeChange */);
    }
}

调用renderOutputBufferAndRelease就可以做渲染啦，timestampNs应该就是应当播放的系统时间

到这里NuPlayer的AVsync就大致学习完了，如有不对的地方欢迎讨论和指正。