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
音频解码线程：
static int audio_thread(void *arg)
{
......
do {
// decode audio
if ((got_frame = decoder_decode_frame(ffp, &is->auddec, frame, NULL)) < 0)
goto the_end;
// filter audio
#if CONFIG_AVFILTER
if ((ret = av_buffersrc_add_frame(is->in_audio_filter, frame)) < 0)
goto the_end;
while ((ret = av_buffersink_get_frame_flags(is->out_audio_filter, frame, 0)) >= 0) {
tb = av_buffersink_get_time_base(is->out_audio_filter);
}
#endif
frame_queue_push(&is->sampq);
} while (ret >= 0 || ret == AVERROR(EAGAIN) || ret == AVERROR_EOF);
the_end:
av_frame_free(&frame);
return ret;
}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()进行音频转换：
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);
}
}