ijkplayer是一款跨平台播放器，支持Android与iOS，音频解码默认使用FFmpeg软解。Android端播放音频可以用OpenSL ES和AudioTrack，而iOS端播放音频默认使用AudioQueue。

一、iOS解码播放

采用pipeline形式创建音频播放组件，整体流水线如下：

1、创建IjkMediaPlayer

首先调用ijkplayer_ios.m创建IjkMediaPlayer，具体代码如下：

IjkMediaPlayer *ijkmp_ios_create(int (*msg_loop)(void*))
{
    IjkMediaPlayer *mp = ijkmp_create(msg_loop);
    if (!mp)
        goto fail;
    mp->ffplayer->vout = SDL_VoutIos_CreateForGLES2();
    if (!mp->ffplayer->vout)
        goto fail;
    mp->ffplayer->pipeline = ffpipeline_create_from_ios(mp->ffplayer);
    if (!mp->ffplayer->pipeline)
        goto fail;
    return mp;
fail:
    ijkmp_dec_ref_p(&mp);
    return NULL;
}

2、创建pipeline

在第一步调用ffpipeline_ios.c创建pipeline，对func_open_video_decoder和func_open_audio_output函数指针进行赋值：

IJKFF_Pipeline *ffpipeline_create_from_ios(FFPlayer *ffp)
{
    IJKFF_Pipeline *pipeline = ffpipeline_alloc(&g_pipeline_class, sizeof(IJKFF_Pipeline_Opaque));
    ......
    pipeline->func_open_video_decoder = func_open_video_decoder;
    pipeline->func_open_audio_output  = func_open_audio_output;
    return pipeline;
}

static SDL_Aout *func_open_audio_output(IJKFF_Pipeline *pipeline, FFPlayer *ffp)
{
    return SDL_AoutIos_CreateForAudioUnit();
}

3、创建SDL_Aout

在ff_play.c的ffp_prepare_async_l()方法调用ff_ffpipeline.c的ffpipeline_open_audio_output()方法：

int ffp_prepare_async_l(FFPlayer *ffp, const char *file_name)
{
    ......
    if (!ffp->aout) {
        ffp->aout = ffpipeline_open_audio_output(ffp->pipeline, ffp);
        if (!ffp->aout)
            return -1;
    }
}

然后调用ffpipeline_ios.c的func_open_audio_output()方法，最终调用ijksdl_aout_ios_audiounit.m创建SDL_Aout，分别为open_audio、pause_audio、flush_audio、close_audio函数指针进行赋值：

SDL_Aout *SDL_AoutIos_CreateForAudioUnit()
{
    SDL_Aout *aout = SDL_Aout_CreateInternal(sizeof(SDL_Aout_Opaque));
    ......
    aout->open_audio  = aout_open_audio;
    aout->pause_audio = aout_pause_audio;
    aout->flush_audio = aout_flush_audio;
    aout->close_audio = aout_close_audio;
    ......
    return aout;
}

4、打开音频解码器

在ff_ffplay.c的stream_component_open()方法寻找并且打开音频解码器：

static int stream_component_open(FFPlayer *ffp, int stream_index)
{
    // 寻找解码器
    codec = avcodec_find_decoder(avctx->codec_id);
    // 打开解码器
    if ((ret = avcodec_open2(avctx, codec, &opts)) < 0) {
        goto fail;
    }
    ......
    switch (avctx->codec_type) {
    case AVMEDIA_TYPE_AUDIO:
        // 打开音频输出设备
        if ((ret = audio_open(ffp, channel_layout, nb_channels, sample_rate, &is->audio_tgt)) < 0)
            goto fail;
        // 启动音频解码线程
        if ((ret = decoder_start(&is->auddec, audio_thread, ffp, "ff_audio_dec")) < 0)
            goto out;
        ......
    default:
        break;
    }
    goto out;
fail:
    avcodec_free_context(&avctx);
out:
    av_dict_free(&opts);
    return ret;
}

5、创建AudioQueue

在audio_open()方法里调用ijksdl_aout.c的SDL_AoutOpenAudio()方法打开音频输出设备，并且设置pcm数据回调函数：

static int audio_open(FFPlayer *opaque, int64_t wanted_channel_layout, int wanted_nb_channels, int wanted_sample_rate, struct AudioParams *audio_hw_params)
{
    // 设置sdl_audio_callback作为callback函数
    wanted_spec.callback = sdl_audio_callback;
    // 打开音频输出设备
    while (SDL_AoutOpenAudio(ffp->aout, &wanted_spec, &spec) < 0) {
        if (is->abort_request)
            return -1;
    }
}

ijksdl_aout.c内部调用ijksdl_aout_ios_audiounit.m的aout_open_audio()，最终调用IJKSDLAudioQueueController.m创建AudioQueue用于播放音频：

- (id)initWithAudioSpec:(const SDL_AudioSpec *)aSpec
{
    // 创建AudioQueue，设置回调函数
    OSStatus status = AudioQueueNewOutput(&streamDescription,
                                              IJKSDLAudioQueueOuptutCallback,
                                              (__bridge void *) self,
                                              NULL,
                                              kCFRunLoopCommonModes,
                                              0,
                                              &audioQueueRef);
    // 启动AudioQueue
    status = AudioQueueStart(audioQueueRef, NULL);
    return self;
}

AudioQueue内部会创建一个工作线程进行播放，如下图所示：

6、解码音频

sdl_audio_callback的代码如下：

static void sdl_audio_callback(void *opaque, Uint8 *stream, int len)
{
    ......
    while (len > 0) {
        if (is->audio_buf_index >= is->audio_buf_size) {
            // decode and convert frame
           audio_size = audio_decode_frame(ffp);
        }
        if (!is->muted && is->audio_buf && is->audio_volume == SDL_MIX_MAXVOLUME)
            memcpy(stream, (uint8_t *)is->audio_buf + is->audio_buf_index, len1);
        else {
            memset(stream, 0, len1);
            if (!is->muted && is->audio_buf)
                SDL_MixAudio(stream, (uint8_t *)is->audio_buf + is->audio_buf_index, len1, is->audio_volume);
        }
        len -= len1;
        stream += len1;
        is->audio_buf_index += len1;
    }
}

然后内部调用audio_decode_frame()进行解码：

static int audio_decode_frame(FFPlayer *ffp)
{
    ......
    do {
        // 获取下一个frame
        frame_queue_next(&is->sampq);
    } while (af->serial != is->audioq.serial);
    // 音频转换
    swr_convert(is->swr_ctx, out, out_count, in, af->frame->nb_samples);
    return resampled_data_size;
}

7、播放音频

在AudioQueue的回调函数会定时回调，然后调用ff_ffplay.c的sdl_audio_callback()获取pcm数据来播放：

static void IJKSDLAudioQueueOuptutCallback(void * inUserData, AudioQueueRef inAQ, AudioQueueBufferRef inBuffer) {
    @autoreleasepool {
        // 获取pcm数据
        (*aqController.spec.callback)(aqController.spec.userdata, inBuffer->mAudioData, inBuffer->mAudioDataByteSize);
        // pcm数据入队列
        AudioQueueEnqueueBuffer(inAQ, inBuffer, 0, NULL);
    }
}