Android OpenSL ES播放pcm音频

OpenSL ES是一个嵌入式、跨平台、免费的、音频处理库，android ndk也引入了它，在android-ndk-r14b/platforms/android-21/arch-arm/usr/lib/libOpenSLES.so的目录下，libOpenSLES.so就是适用于Android平台的开发库。OpenELES对于android平台的支持只有部分API，因此，对于 OpenSL ES API 的使用，我们还需要特别留意哪些是 Android 支持的，哪些是不支持的，具体相关文档的地址位于 NDK docs 目录下：
NDK根目录/docs/Additional_library_docs/opensles/index.html
NDK根目录/docs/Additional_library_docs/opensles/OpenSL_ES_Specification_1.0.1.pdf
OpenSLES详情请参考Google官网介绍：https://developer.android.com/ndk/guides/audio/opensl-for-android?hl=es

接口解释：

类接口：SLObjectltf
通过SLObjectltf接口类，可以创建所需要的各种类型的类接口，比如：
创建引擎接口对象：SLObjectltf engineObject
创建混音器接口对象：SLObjectltf outputMixObject
创建播放器接口对象：SLObjectltf playerObject
具体接口类
引擎：SLEngineltf
播放器：SLPlayltf
声音控制器：SLVolumeltf等

创建三部曲

以创建引擎接口对象为例(create->realize->get)

SLObjectltf engineObject = NULL;//用SLObjectltf声明引擎接口对象
SLEngineltf engineEngine = NULL;//声明具体的引擎对象实例
void createEnngine()
{
SLresult result;//返回结果
//第一步：创建引擎
result=slCreateEngine(&engineObject,0,NULL,0,NULL,NULL);
//第二步：实现(Realize)engineObject,SL_BOOLEAN_FALSE);
result=(*engineObject)->Realize(engineObject,SL_BOOLEAN_FALSE);
//第三步：通过engineObject的GetInterface方法初始化enngineEngine
result=(*engineObject)->GetInterface(engineObject,SL_IID_ENGINNE,&engineEngine);
}
//销毁
(*engineObject)->Destroy(engineObject);

OpenEL ES使用流程：
1、创建接口对象；

 // TODO
    //读取pcm文件
    pcmFile = fopen(url, "r");
    if(pcmFile == NULL)
    {
        LOGE("%s", "fopen file error");
        return;
    }
    out_buffer = (uint8_t *) malloc(44100 * 2 * 2);
    SLresult result;
    // 创建引擎对象
   slCreateEngine(&engineObject, 0, 0, 0, 0, 0);
    (*engineObject)->Realize(engineObject, SL_BOOLEAN_FALSE);
    (*engineObject)->GetInterface(engineObject, SL_IID_ENGINE, &engineEngine);

2、设置混音器；

//设置混音器；
  const SLInterfaceID mids[1] = {SL_IID_ENVIRONMENTALREVERB};
    const SLboolean mreq[1] = {SL_BOOLEAN_FALSE};
    result = (*engineEngine)->CreateOutputMix(engineEngine, &outputMixObject, 1, mids, mreq);
    (void)result;
    result = (*outputMixObject)->Realize(outputMixObject, SL_BOOLEAN_FALSE);
    (void)result;
    result = (*outputMixObject)->GetInterface(outputMixObject, SL_IID_ENVIRONMENTALREVERB, &outputMixEnvironmentalReverb);
    if (SL_RESULT_SUCCESS == result) {
        result = (*outputMixEnvironmentalReverb)->SetEnvironmentalReverbProperties(
                outputMixEnvironmentalReverb, &reverbSettings);
        (void)result;
    }
    SLDataLocator_OutputMix outputMix = {SL_DATALOCATOR_OUTPUTMIX, outputMixObject};

3、创建播放器（录音器）；

 // 创建播放器
 SLDataLocator_AndroidSimpleBufferQueue android_queue={SL_DATALOCATOR_ANDROIDSIMPLEBUFFERQUEUE,2};
    SLDataFormat_PCM pcm={
            SL_DATAFORMAT_PCM,//播放pcm格式的数据
            2,//2个声道（立体声）
            SL_SAMPLINGRATE_44_1,//44100hz的频率
            SL_PCMSAMPLEFORMAT_FIXED_16,//位数 16位
            SL_PCMSAMPLEFORMAT_FIXED_16,//和位数一致就行
            SL_SPEAKER_FRONT_LEFT | SL_SPEAKER_FRONT_RIGHT,//立体声（前左前右）
            SL_BYTEORDER_LITTLEENDIAN//结束标志
    };

    SLDataSource slDataSource = {&android_queue, &pcm};
    SLDataSink audioSnk = {&outputMix, NULL};
    const SLInterfaceID ids[3] = {SL_IID_BUFFERQUEUE, SL_IID_EFFECTSEND, SL_IID_VOLUME};
    const SLboolean req[3] = {SL_BOOLEAN_TRUE, SL_BOOLEAN_TRUE, SL_BOOLEAN_TRUE};

    result = (*engineEngine)->CreateAudioPlayer(engineEngine, &pcmPlayerObject, &slDataSource, &audioSnk, 3, ids, req);
    // 初始化播放器
    (*pcmPlayerObject)->Realize(pcmPlayerObject, SL_BOOLEAN_FALSE);
    //得到接口后调用  获取Player接口
    (*pcmPlayerObject)->GetInterface(pcmPlayerObject, SL_IID_PLAY, &pcmPlayerPlay);

4、设置缓冲队列和回调函数；

  // 创建缓冲区和回调函数
    (*pcmPlayerObject)->GetInterface(pcmPlayerObject, SL_IID_BUFFERQUEUE, &pcmBufferQueue);

    //缓冲接口回调
    (*pcmBufferQueue)->RegisterCallback(pcmBufferQueue, pcmBufferCallBack, NULL);
    //获取音量接口
    (*pcmPlayerObject)->GetInterface(pcmPlayerObject, SL_IID_VOLUME, &pcmPlayerVolume);
    //初始化pcm文件数据
    void getPcmData(void **pcm)
{
    while(!feof(pcmFile))
    {
        fread(out_buffer, 44100 * 2 * 2, 1, pcmFile);
        if(out_buffer == NULL)
        {
            LOGI("%s", "read end");
            break;
        } else{
            LOGI("%s", "reading");
        }
        *pcm = out_buffer;
        break;
    }
}
//取出缓冲数据
void pcmBufferCallBack(SLAndroidSimpleBufferQueueItf bf, void * context)
{
    //assert(NULL == context);
    getPcmData(&buffer);
    // for streaming playback, replace this test by logic to find and fill the next buffer
    if (NULL != buffer) {
        SLresult result;
        // enqueue another buffer
        result = (*pcmBufferQueue)->Enqueue(pcmBufferQueue, buffer, 44100 * 2 * 2);
        // the most likely other result is SL_RESULT_BUFFER_INSUFFICIENT,
        // which for this code example would indicate a programming error
    }
}

5、设置播放状态；

// 设置播放状态
    (*pcmPlayerPlay)->SetPlayState(pcmPlayerPlay, SL_PLAYSTATE_PLAYING);

6、启动回到函数；

   // 主动调用回调函数开始工作
    pcmBufferCallBack(pcmBufferQueue, NULL);