Android Audio代码分析12 - stream type续

前几天在看stream type的时候，调用函数AudioSystem::getOutput的地方并没有继续往下看。
今天深入看看。
*****************************************源码*************************************************
status_t AudioTrack::set(
        int streamType,
        uint32_t sampleRate,
        int format,
        int channels,
        int frameCount,
        uint32_t flags,
        callback_t cbf,
        void* user,
        int notificationFrames,
        const sp<IMemory>& sharedBuffer,
        bool threadCanCallJava,
        int sessionId)
{

LOGV_IF(sharedBuffer != 0, "sharedBuffer: %p, size: %d", sharedBuffer->pointer(), sharedBuffer->size());

    if (mAudioTrack != 0) {
        LOGE("Track already in use");
        return INVALID_OPERATION;
    }

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

    // handle default values first.
    if (streamType == AudioSystem::DEFAULT) {
        streamType = AudioSystem::MUSIC;
    }
    if (sampleRate == 0) {
        sampleRate = afSampleRate;
    }
    // these below should probably come from the audioFlinger too...
    if (format == 0) {
        format = AudioSystem::PCM_16_BIT;
    }
    if (channels == 0) {
        channels = AudioSystem::CHANNEL_OUT_STEREO;
    }

    // validate parameters
    if (!AudioSystem::isValidFormat(format)) {
        LOGE("Invalid format");
        return BAD_VALUE;
    }

    // force direct flag if format is not linear PCM
    if (!AudioSystem::isLinearPCM(format)) {
        flags |= AudioSystem::OUTPUT_FLAG_DIRECT;
    }

    if (!AudioSystem::isOutputChannel(channels)) {
        LOGE("Invalid channel mask");
        return BAD_VALUE;
    }
    uint32_t channelCount = AudioSystem::popCount(channels);

audio_io_handle_t output = AudioSystem::getOutput((AudioSystem::stream_type)streamType,
sampleRate, format, channels, (AudioSystem::output_flags)flags);

    if (output == 0) {
        LOGE("Could not get audio output for stream type %d", streamType);
        return BAD_VALUE;
    }

    mVolume[LEFT] = 1.0f;
    mVolume[RIGHT] = 1.0f;
    mSendLevel = 0;
    mFrameCount = frameCount;
    mNotificationFramesReq = notificationFrames;
    mSessionId = sessionId;
    mAuxEffectId = 0;

    // create the IAudioTrack
    status_t status = createTrack(streamType, sampleRate, format, channelCount,
                                  frameCount, flags, sharedBuffer, output, true);

    if (status != NO_ERROR) {
        return status;
    }

    if (cbf != 0) {
        mAudioTrackThread = new AudioTrackThread(*this, threadCanCallJava);
        if (mAudioTrackThread == 0) {
          LOGE("Could not create callback thread");
          return NO_INIT;
        }
    }

mStatus = NO_ERROR;

    mStreamType = streamType;
    mFormat = format;
    mChannels = channels;
    mChannelCount = channelCount;
    mSharedBuffer = sharedBuffer;
    mMuted = false;
    mActive = 0;
    mCbf = cbf;
    mUserData = user;
    mLoopCount = 0;
    mMarkerPosition = 0;
    mMarkerReached = false;
    mNewPosition = 0;
    mUpdatePeriod = 0;
    mFlags = flags;

return NO_ERROR;
}
**********************************************************************************************

#######################说明################################
+++++++++++++++++++++++++AudioSystem::getOutput+++++++++++++++++++++++++++++++++++++++
audio_io_handle_t AudioSystem::getOutput(stream_type stream,
                                    uint32_t samplingRate,
                                    uint32_t format,
                                    uint32_t channels,
                                    output_flags flags)
{
    audio_io_handle_t output = 0;
    // Do not use stream to output map cache if the

补充：移动开发 , Android ,