AudioTrack::createTrack()

status_t AudioTrack::createTrack(
        int streamType,
        uint32_t sampleRate,
        int format,
        int channelCount,
        int frameCount,
        uint32_t flags,
        const sp& sharedBuffer,
        audio_io_handle_t output,
        bool enforceFrameCount)
{
    status_t status;
    const sp& audioFlinger = AudioSystem::get_audio_flinger();
    if (audioFlinger == 0) {
       LOGE("Could not get audioflinger");
       return NO_INIT;
    }

    int afSampleRate;
    if (AudioSystem::getOutputSamplingRate(&afSampleRate, streamType) != NO_ERROR) {
        return NO_INIT;
    }
    int afFrameCount;
    if (AudioSystem::getOutputFrameCount(&afFrameCount, streamType) != NO_ERROR) {
        return NO_INIT;
    }
    uint32_t afLatency;
    if (AudioSystem::getOutputLatency(&afLatency, streamType) != NO_ERROR) {
        return NO_INIT;
    }

    mNotificationFramesAct = mNotificationFramesReq;
    if (!AudioSystem::isLinearPCM(format)) {
        if (sharedBuffer != 0) {
            frameCount = sharedBuffer->size();
        }
    } else {
        // Ensure that buffer depth covers at least audio hardware latency
        uint32_t minBufCount = afLatency / ((1000 * afFrameCount)/afSampleRate);
        if (minBufCount < 2) minBufCount = 2;

        int minFrameCount = (afFrameCount*sampleRate*minBufCount)/afSampleRate;

        if (sharedBuffer == 0) {
            if (frameCount == 0) {
                frameCount = minFrameCount;
            }
            if (mNotificationFramesAct == 0) {
                mNotificationFramesAct = frameCount/2;
            }
            // Make sure that application is notified with sufficient margin
            // before underrun
            if (mNotificationFramesAct > (uint32_t)frameCount/2) {
                mNotificationFramesAct = frameCount/2;
            }
            if (frameCount < minFrameCount) {
                if (enforceFrameCount) {
                    LOGE("Invalid buffer size: minFrameCount %d, frameCount %d", minFrameCount, frameCount);
                    return BAD_VALUE;
                } else {
                    frameCount = minFrameCount;
                }
            }
        } else {
            // Ensure that buffer alignment matches channelcount
            if (((uint32_t)sharedBuffer->pointer() & (channelCount | 1)) != 0) {
                LOGE("Invalid buffer alignement: address %p, channelCount %d", sharedBuffer->pointer(), channelCount);
                return BAD_VALUE;
            }
            frameCount = sharedBuffer->size()/channelCount/sizeof(int16_t);
        }
    }

    sp track = audioFlinger->createTrack(getpid(),
                                                      streamType,
                                                      sampleRate,
                                                      format,
                                                      channelCount,
                                                      frameCount,
                                                      ((uint16_t)flags) << 16,
                                                      sharedBuffer,
                                                      output,
                                                      &mSessionId,
                                                      &status);

    if (track == 0) {
        LOGE("AudioFlinger could not create track, status: %d", status);
        return status;
    }
    sp cblk = track->getCblk();
    if (cblk == 0) {
        LOGE("Could not get control block");
        return NO_INIT;
    }
    mAudioTrack.clear();
    mAudioTrack = track;
    mCblkMemory.clear();
    mCblkMemory = cblk;
    mCblk = static_cast(cblk->pointer());
    mCblk->flags |= CBLK_DIRECTION_OUT;
    if (sharedBuffer == 0) {
        mCblk->buffers = (char*)mCblk + sizeof(audio_track_cblk_t);
    } else {
        mCblk->buffers = sharedBuffer->pointer();
         // Force buffer full condition as data is already present in shared memory
        mCblk->stepUser(mCblk->frameCount);
    }

    mCblk->volumeLR = (uint32_t(uint16_t(mVolume[RIGHT] * 0x1000)) << 16) | uint16_t(mVolume[LEFT] * 0x1000);
    mCblk->sendLevel = uint16_t(mSendLevel * 0x1000);
    mAudioTrack->attachAuxEffect(mAuxEffectId);
    mCblk->bufferTimeoutMs = MAX_STARTUP_TIMEOUT_MS;
    mCblk->waitTimeMs = 0;
    mRemainingFrames = mNotificationFramesAct;
    mLatency = afLatency + (1000*mCblk->frameCount) / sampleRate;
    return NO_ERROR;
}


//--> client buffer 
//audio flinger client move data from client buffer to hardware buffer
//--> hardware buffer
//hardware playback porcess extract data from hardware buffer

//what is "frame"
//-----------------------------------------------------------------------------------------------------
//A frame is a set of samples, one per channel, at a particular instant in time. For stereophonic audio, 
//a frame consists of two samples. For Dolby 5.1 Surround Sound, a frame would consist of six samples 
//(left channel, center channel, right channel, rear right, rear left, and the low frequency channel). 
//For monophonic audio, a frame is equivalent to one sample.
//Retrieved from "http://alsa.opensrc.org/Frame"
//1个采样点只针对一个声道，而实际上可能会有一或多个声道.
//由于不能用一个独立的单位来表示全部声道一次采样的数据量，也就引出了Frame的概念.
//Frame的大小，就是一个采样点的字节数×声道数.
//另外，在目前的声卡驱动程序中，其内部缓冲区也是采用Frame作为单位来分配和管理的.

//what is "bufCount"
//-------------------------------------------------------------------------------------------------------
//bufCount 表示一个frame里面的字节数,用来描述一个时域的采样点所占的字节宽度,(注意:不等同于channel数.)

//what is "frame count"
//-------------------------------------------------------------------------------------------------------
//frameFount 表示一个buffer所能够缓冲的frame的个数,也就是时域的点数 

//所以:
//afFrameCount -- 硬件buffer的大小,以frame为单位.
//afSampleRate -- 硬件采样率.
//afFrameCount/afSampleRate -- 硬件buffer保存的数据能够播放的时间是多少,换算成ms就是
//                             1000*(afFrameCount/afSampleRate)
//硬件buffer的大小是既定的,如果硬件要求的延迟硬件buffer不能提供,则需要多开几个软件buffer,以缓冲更多的数据,提供更长的播放时间.
//如果硬件要求的延迟为afLatency(ms),
//那么硬件延迟要求对应的最小的软件buffer的数目为:
//minBufCount=afLatency/1000*(afFrameCount/afSampleRate)

//但这里并未给出每个软件buffer的大小.
//暂记每个软件buffer的大小(以frame为单位)为bufsize,采样率转换之前的采样率为sampleRate
//由于每个软件buffer和硬件buffer中的数据所对应的播放时长是一样的.所以:
//bufsize/sampleRate=afFrameCount/afSampleRate;
//bufsize=afFrameCount*sampleRate/afSampleRate;
//则所有软件buffer加在一起大小为
//minFrameCount=minBufCount*afFrameCount*sampleRate/afSampleRate;