前言
本文是流程分析的第二篇，分析ijkPlayer中的音频播放流程，在aout_thread中，如下流程图中所示。
SDL_Aout结构体
SDL_Aout是对音频播放的抽象，实现是使用OpenGLES播放和使用AudioTrack播放。
// ijksdl_aout.h
typedef struct SDL_Aout_Opaque SDL_Aout_Opaque; // 在各自实现类有不同的定义
typedef struct SDL_Aout SDL_Aout;
struct SDL_Aout {
SDL_mutex *mutex;
double minimal_latency_seconds;
SDL_Class *opaque_class;
SDL_Aout_Opaque *opaque;
void (*free_l)(SDL_Aout *vout);
int (*open_audio)(SDL_Aout *aout, const SDL_AudioSpec *desired, SDL_AudioSpec *obtained);
void (*pause_audio)(SDL_Aout *aout, int pause_on);
void (*flush_audio)(SDL_Aout *aout);
void (*set_volume)(SDL_Aout *aout, float left, float right);
void (*close_audio)(SDL_Aout *aout);
double (*func_get_latency_seconds)(SDL_Aout *aout);
void (*func_set_default_latency_seconds)(SDL_Aout *aout, double latency);
// optional
void (*func_set_playback_rate)(SDL_Aout *aout, float playbackRate);
void (*func_set_playback_volume)(SDL_Aout *aout, float playbackVolume);
int (*func_get_audio_persecond_callbacks)(SDL_Aout *aout);
// Android only
int (*func_get_audio_session_id)(SDL_Aout *aout);
};
- 实现为
ijksdl_aout_android_opensles.c
ijksdl_aout_android_audiotrack.c
代码流程
1. 通过什么方法播放的，入口是什么
在单独线程aout_thread中，获取音频解码数据进行播放
static int aout_thread_n(JNIEnv *env, SDL_Aout *aout)
{
SDL_Android_AudioTrack *atrack = opaque->atrack; // 获取AudioTrack实例
SDL_AudioCallback audio_cblk = opaque->spec.callback; // callback，获取解码后音频数据
SDL_SetThreadPriority(SDL_THREAD_PRIORITY_HIGH);
while (!opaque->abort_request) {
// 循环读取
SDL_LockMutex(opaque->wakeup_mutex);
if (opaque->need_flush) {
opaque->need_flush = 0;
SDL_Android_AudioTrack_flush(env, atrack);
}
SDL_UnlockMutex(opaque->wakeup_mutex);
audio_cblk(userdata, buffer, copy_size); // 获取解码数据
if (opaque->need_flush) {
opaque->need_flush = 0;
SDL_Android_AudioTrack_flush(env, atrack);
} else {
SDL_Android_AudioTrack_write(env, atrack, buffer, copy_size); // 写数据，即播放
}
}
SDL_Android_AudioTrack_free(env, atrack);
return 0;
}
2. aout_thread_n怎么被调用的
-
在stream_component_open函数中调用audio_open打开创建音频播放器
ff_ffplay.c#stream_component_open
-> audio_open
-> SDL_AoutOpenAudio // 去找SDL_Aout的实现 -
ijksdl_aout_android_audiotrack.c实现
-> ijksdl_aout_android_audiotrack.c#aout_open_audio
-> ijksdl_aout_android_audiotrack.c#aout_open_audio_n，创建aout_thread执行aout_thread_n
audio_open执行完，即已经创建了AudioTrack并打开，等待读取解码数据并播放。 -
ijksdl_aout_android_opensles.c实现
-> ijksdl_aout_android_opensles.c#aout_open_audio，创建aout_thread执行aout_thread_n
audio_open执行完，即已经创建了SLObjectItf并打开，等待读取解码数据并播放。
以下用ijksdl_aout_android_audiotrack.c为示例梳理代码：
// ff_ffplay.c，stream_compoenent_open调用
static int audio_open(FFPlayer *opaque, int64_t wanted_channel_layout, int wanted_nb_channels,
int wanted_sample_rate, struct AudioParams *audio_hw_params) {
SDL_AudioSpec wanted_spec, spec;
wanted_spec.callback = sdl_audio_callback; // 设置回调函数
SDL_AoutOpenAudio(ffp->aout, &wanted_spec, &spec); // 打开音频
}
// ijksdl_aout.c
int SDL_AoutOpenAudio(SDL_Aout *aout, const SDL_AudioSpec *desired, SDL_AudioSpec *obtained)
{
if (aout && desired && aout->open_audio)
return aout->open_audio(aout, desired, obtained);
return -1;
}
// ijksdl_aout_android_audiotrack.c
static int aout_open_audio(SDL_Aout *aout, const SDL_AudioSpec *desired, SDL_AudioSpec *obtained) {
return aout_open_audio_n(env, aout, desired, obtained);
}
// ijksdl_aout_android_audiotrack.c
static int aout_open_audio_n(JNIEnv *env, SDL_Aout *aout, const SDL_AudioSpec *desired, SDL_AudioSpec *obtained) {
SDL_Aout_Opaque *opaque = aout->opaque;
// 创建AudioTrack
opaque->atrack = SDL_Android_AudioTrack_new_from_sdl_spec(env, desired);
// 开启线程aout_thread，进行播放流程
opaque->audio_tid = SDL_CreateThreadEx(&opaque->_audio_tid, aout_thread, aout, "ff_aout_android");
}
// ijksdl_aout_android_audiotrack.c
static int aout_thread(void *arg) {
return aout_thread_n(env, aout);
}
3. ffp->aout在哪赋值，即SDK_Aout何时初始化的
- 初始化赋值SDK_Aout流程：
ijkplayer.c#ijkmp_prepare_async
-> ijkplayer.c#ijkmp_prepare_async_l
->ff_play.c#ffp_prepare_async_l
-> ff_ffpipeline.c#ffpipeline_open_audio_output
-> ffpipeline_android.c#func_open_audio_output // 创建SDL_Aout实现
// ffpipeline_android.c
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;
}
- 初始化赋值ffpipeline流程：
IjkMediaPlayer_native_setup
-> ijkmp_android_create
-> ffpipeline_android.c#ffpipeline_create_from_android // 创建 ffpipeline
4. audio_cblk具体流程
// ff_ffplay.c, stream是buffer，向里填充数据，write到AudioTrack里去
static void sdl_audio_callback(void *opaque, Uint8 *stream, int len) {
int audio_size, len1;
while (len > 0) {
if (is->audio_buf_index >= is->audio_buf_size) {
// audio_buf消耗完了，调用audio_decode_frame重新填充audio_buf
audio_size = audio_decode_frame(ffp);
is->audio_buf_size = audio_size;
is->audio_buf_index = 0;
}
len1 = is->audio_buf_size - is->audio_buf_index;
if (len1 > len)
len1 = len;
// 读取数据到stream中
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);
}
// 向后移动，len>0则继续读取数据
len -= len1;
stream += len1;
is->audio_buf_index += len1;
}
}
static int audio_decode_frame(FFPlayer *ffp) {
Frame *af;
do {
// 获取一个可读结点，读取新的音频帧
if (!(af = frame_queue_peek_readable(&is->sampq))) {
return -1;
}
// 移动指针指向下一个
frame_queue_next(&is->sampq);
} while (af->serial != is->audioq.serial);
if () {
// 重采样
} else {
// 把这一帧数据赋值给audio_buf
is->audio_buf = af->frame->data[0];
resampled_data_size = data_size;
}
// 更新audio_clock
is->audio_clock = af->pts + (double) af->frame->nb_samples / af->frame->sample_rate;
return resampled_data_size;
}