Android Audio代码分析4

前面看过的代码中，经常会调用到AudioSystem类中的getOutputSamplingRate函数，getOutputFrameCount函数，getOutputLatency函数等。

这些函数的实现基本类似，今天就细细品味下AudioSystem::getOutputSamplingRate函数。

*****************************************源码*************************************************

status_t AudioSystem::getOutputSamplingRate(int* samplingRate, int streamType)

{

    OutputDescriptor *outputDesc;

    audio_io_handle_t output;

    if (streamType == DEFAULT) {

        streamType = MUSIC;

    }

    output = getOutput((stream_type)streamType);

    if (output == 0) {

        return PERMISSION_DENIED;

    }

    gLock.lock();

    outputDesc = AudioSystem::gOutputs.valueFor(output);

    if (outputDesc == 0) {

        LOGV("getOutputSamplingRate() no output descriptor for output %d in gOutputs", output);

        gLock.unlock();

        const sp<IAudioFlinger>& af = AudioSystem::get_audio_flinger();

        if (af == 0) return PERMISSION_DENIED;

        *samplingRate = af->sampleRate(output);

    } else {

        LOGV("getOutputSamplingRate() reading from output desc");

        *samplingRate = outputDesc->samplingRate;

        gLock.unlock();

    }

    LOGV("getOutputSamplingRate() streamType %d, output %d, sampling rate %d", streamType, output, *samplingRate);

    return NO_ERROR;

}

**********************************************************************************************

源码路径：

frameworks\base\media\libmedia\AudioSystem.cpp

###########################################说明################################################

status_t AudioSystem::getOutputSamplingRate(int* samplingRate, int streamType)

{

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

    // audio output descritor used to cache output configurations in client process to avoid frequent calls

    // through IAudioFlinger

    class OutputDescriptor {

    public:

        OutputDescriptor()

        : samplingRate(0), format(0), channels(0), frameCount(0), latency(0)  {}

        uint32_t samplingRate;

        int32_t format;

        int32_t channels;

        size_t frameCount;

        uint32_t latency;

    };

----------------------------------------------------------------

    OutputDescriptor *outputDesc;

// typedef int audio_io_handle_t;

    audio_io_handle_t output;

// Default类型的stream，强制转为music

    if (streamType == DEFAULT) {

        streamType = MUSIC;

    }