播放PCM步骤

• 初始化SDL SDL_Init(SDL_INIT_AUDIO)
• 设置SDL音频参数 SDL_AudioSpec
• 设置PCM拉取回调
• SDL打开音频 SDL_OpenAudio
• 开始拉取 SDL_PauseAudio(0);
• 回调监听
  • 设置音频流内存大小，播放器的内存数据在这个 stream 中
  • 传入PCM数据进行混音
  • 移动缓存指针，进行下轮的拉取
• 释放资源

void pulAudioData(void *userData, Uint8 *stream, int len) {
    AudioBuffer *buffer = (AudioBuffer*)userData;
    SDL_memset(stream, 0, len);
    if (buffer->len <= 0) {
        return;
    }
    buffer->pullLen = len > buffer->len ? buffer->len : len;
    NSLog(@"before-buffer->len: %d, buffer->pullLen %d, len: %d", buffer->len, buffer->pullLen, len);
    SDL_MixAudio(stream, (UInt8 *)buffer->data, buffer->pullLen, SDL_MIX_MAXVOLUME);
    buffer->data += buffer->pullLen;
    buffer->len -= buffer->pullLen;
    NSLog(@"buffer->len: %d, buffer->pullLen %d", buffer->len, buffer->pullLen);
}

- (void)playPCM:(NSString*)filename {
    if (SDL_Init(SDL_INIT_AUDIO)) {
        NSLog(@"SDL_INIT Error: %s", SDL_GetError());
        return;
    }
    SDL_AudioSpec spec;
    spec.freq = SAMPLE_RATE;
    spec.format = SAMPLE_FORMAT;
    spec.channels = CHANNELS;
    spec.samples = SAMPLES;
    spec.callback = pulAudioData;
    AudioBuffer *buffer = new AudioBuffer();
    spec.userdata = buffer;
    if (SDL_OpenAudio(&spec, nullptr)) {
        SDL_Quit();
        NSLog(@"SDL_OpenAudio Error: %s", SDL_GetError());
        return;
    }
    SDL_PauseAudio(0);
    NSFileHandle *filehandle = [NSFileHandle fileHandleForReadingAtPath:filename];
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        while (!self.isInterruptionRequested) {
            if (buffer->len > 0) continue;
            NSData *data = [filehandle readDataOfLength:BUFFER_SIZE];
            buffer->len = (int)data.length;
            if (buffer->len <= 0) {
                // 剩余样本数量
                // BYTES_PER_SAMPLE 每个样本的大小 = 采样率 * 通道数 >> 3
                // 这样做的目的是推迟线程结束的时间，让剩余的音频播放完毕
                int samples = buffer->pullLen / BYTES_PER_SAMPLE;
                int ms = samples * 1000 / SAMPLE_RATE;
                SDL_Delay(ms);
                break;
            }
            buffer->data =  (Uint8 *)[data bytes];
        }
        [filehandle closeFile];
        SDL_CloseAudio();
        SDL_Quit();
        dispatch_async(dispatch_get_main_queue(), ^{
            [self playEnd];
        });
    });
    
}

播放WAV步骤

• 初始化SDL SDL_Init(SDL_INIT_AUDIO)
• 设置SDL音频参数 SDL_AudioSpec
• 直接加载WAV文件 SDL_LoadWAV
• 设置拉取回调
• SDL打开音频 SDL_OpenAudio
• 开始拉取 SDL_PauseAudio(0);
• 回调监听
  • 设置音频流内存大小，播放器的内存数据在这个 stream 中
  • 传入PCM数据进行混音
  • 移动缓存指针，进行下轮的拉取
• 释放资源

// 等待音频设备回调(会回调多次)
void pull_audio_data(void *userdata,
                     // 需要往stream中填充PCM数据
                     Uint8 *stream,
                     // 希望填充的大小(samples * format * channels / 8)
                     int len
                     ) {
    AudioBuffer *buffer = (AudioBuffer*)userdata;
    SDL_memset(stream, 0, len);
    if (buffer->len <= 0) {
        return;
    }
    buffer->pullLen = len > buffer->len ? buffer->len : len;
    NSLog(@"before-buffer->len: %d, buffer->pullLen %d, len: %d", buffer->len, buffer->pullLen, len);
    SDL_MixAudio(stream, (UInt8 *)buffer->data, buffer->pullLen, SDL_MIX_MAXVOLUME);
    buffer->data += buffer->pullLen;
    buffer->len -= buffer->pullLen;
    NSLog(@"buffer->len: %d, buffer->pullLen %d", buffer->len, buffer->pullLen);
}

+ (void)initialize {
    SDL_SetMainReady();
}

- (void)playWithFile:(NSString*)wavFile {
    dispatch_async(self.queue, ^{
        if (SDL_Init(SDL_INIT_AUDIO)) {
            return;
        }
        self.interruptionRequested = false;
        SDL_AudioSpec spec;
        Uint8 *data = nullptr;
        UInt32 len = 0;
        if (!SDL_LoadWAV([wavFile UTF8String], &spec, &data, &len)) {
            NSLog(@"SDL_LoadWAV Error: %s", SDL_GetError());
            SDL_Quit();
            return;
        }
        spec.samples = 1024;
        spec.callback = pull_audio_data;
        
        AudioBuffer buffer;
        buffer.data = data;
        buffer.len = len;
        spec.userdata = &buffer;
        if (SDL_OpenAudio(&spec, nullptr)) {
            NSLog(@"SDL_OpenAudio Error: %s", SDL_GetError());
            SDL_Quit();
            return;
        }
        
        int sampleSize = SDL_AUDIO_BITSIZE(spec.format);
        int bytesPerSample = (sampleSize * spec.channels) >> 3;
        SDL_PauseAudio(0);
        while (!self.interruptionRequested) {
            if (buffer.len > 0) {
                continue;
            }
            if (buffer.len <= 0) {
                int samples = buffer.pullLen / bytesPerSample;
                int ms = samples * 1000 / spec.freq;
                SDL_Delay(ms);
                break;
            }
        }
        SDL_FreeWAV(data);
        SDL_CloseAudio();
        SDL_Quit();
    });
    
}