IJKPLAYER源码分析-OpenSL ES播放

1 前言

    与IJKPLAYER处理AudioTrack播放类似，OpenSL ES的接入需要满足SDL_Aout的接口规范，所不同的是OpenSL ES播放是在native完成的，调用的是NDK接口OpenSL ES的播放能力。关于OpenSL ES的详细介绍，请参考官方文档 OpenSL ES 一文。

    Pipeline及SDL_Aout结构体及相关创建，与AudioTrack一致，请参考前文IJKPLAYER源码分析-AudioTrack播放-优快云博客

2 接口

2.1 创建SDL_Aout

    创建OpenSL ES的SDL_Aout对象，调用链如下：

ijkmp_android_create() => ffpipeline_create_from_android() => func_open_audio_output() => SDL_AoutAndroid_CreateForOpenSLES()

    使能opensles选项，缺省为0，即使用AudioTrack播放： 

    { "opensles",                           "OpenSL ES: enable",
        OPTION_OFFSET(opensles),            OPTION_INT(0, 0, 1) },

     若使能了opensles选项，走OpenSL ES播放，相反则走AudioTrack播放：

static SDL_Aout *func_open_audio_output(IJKFF_Pipeline *pipeline, FFPlayer *ffp)
{
    SDL_Aout *aout = NULL;
    if (ffp->opensles) {
        aout = SDL_AoutAndroid_CreateForOpenSLES();
    } else {
        aout = SDL_AoutAndroid_CreateForAudioTrack();
    }
    if (aout)
        SDL_AoutSetStereoVolume(aout, pipeline->opaque->left_volume, pipeline->opaque->right_volume);
    return aout;
}

     OpenSL ES的SDL_Aout对象创建具体在此，遵循SDL_Aout接口规范：

SDL_Aout *SDL_AoutAndroid_CreateForOpenSLES()
{
    SDLTRACE("%s\n", __func__);
    SDL_Aout *aout = SDL_Aout_CreateInternal(sizeof(SDL_Aout_Opaque));
    if (!aout)
        return NULL;

    SDL_Aout_Opaque *opaque = aout->opaque;
    opaque->wakeup_cond = SDL_CreateCond();
    opaque->wakeup_mutex = SDL_CreateMutex();

    int ret = 0;

    SLObjectItf slObject = NULL;
    ret = slCreateEngine(&slObject, 0, NULL, 0, NULL, NULL);
    CHECK_OPENSL_ERROR(ret, "%s: slCreateEngine() failed", __func__);
    opaque->slObject = slObject;

    ret = (*slObject)->Realize(slObject, SL_BOOLEAN_FALSE);
    CHECK_OPENSL_ERROR(ret, "%s: slObject->Realize() failed", __func__);

    SLEngineItf slEngine = NULL;
    ret = (*slObject)->GetInterface(slObject, SL_IID_ENGINE, &slEngine);
    CHECK_OPENSL_ERROR(ret, "%s: slObject->GetInterface() failed", __func__);
    opaque->slEngine = slEngine;

    SLObjectItf slOutputMixObject = NULL;
    const SLInterfaceID ids1[] = {SL_IID_VOLUME};
    const SLboolean req1[] = {SL_BOOLEAN_FALSE};
    ret = (*slEngine)->CreateOutputMix(slEngine, &slOutputMixObject, 1, ids1, req1);
    CHECK_OPENSL_ERROR(ret, "%s: slEngine->CreateOutputMix() failed", __func__);
    opaque->slOutputMixObject = slOutputMixObject;

    ret = (*slOutputMixObject)->Realize(slOutputMixObject, SL_BOOLEAN_FALSE);
    CHECK_OPENSL_ERROR(ret, "%s: slOutputMixObject->Realize() failed", __func__);

    aout->free_l       = aout_free_l;
    aout->opaque_class = &g_opensles_class;
    aout->open_audio   = aout_open_audio;
    aout->pause_audio  = aout_pause_audio;
    aout->flush_audio  = aout_flush_audio;
    aout->close_audio  = aout_close_audio;
    aout->set_volume   = aout_set_volume;
    aout->func_get_latency_seconds = aout_get_latency_seconds;

    return aout;
fail:
    aout_free_l(aout);
    return NULL;
}

2.2 func_get_latency_seconds接口

• 此接口是OpenSL ES相比于AudioTrack新增的1个接口；
• 作用是：用来计算OpenSL ES底层buffer缓存了多少ms的音频数据，音视频同步时用来纠正音频的时钟；

    与AudioTrack相比，OpenSL ES增加了func_get_latency_seconds接口： 

    aout->func_get_latency_seconds = aout_get_latency_seconds;

    此接口的具体实现： 

static double aout_get_latency_seconds(SDL_Aout *aout)
{
    SDL_Aout_Opaque *opaque = aout->opaque;

    SLAndroidSim