Android系列——AudioTrack如何与AudioFlinger交换音频数据3

IMemory接口

在createTrack的过程中，AudioFlinger会根据传入的frameCount参数，申请一块内存，AudioTrack可以通过IAudioTrack接口的getCblk()函数获得指向该内存块的IMemory接口，然后AudioTrack通过该IMemory接口的pointer()函数获得指向该内存块的指针，这块内存的开始部分就是audio_track_cblk_t结构，紧接着是大小为frameSize的FIFO内存。

IMemory->pointer() ---->|_______________________________________________________

                                     |__audio_track_cblk_t__|_______buffer of FIFO(size==frameCount)____|

看看AudioTrack的createTrack()的代码就明白了：

sp track = audioFlinger->createTrack(getpid(),
                                                      streamType,
                                                      sampleRate,
                                                      format,
                                                      channelCount,
                                                      frameCount,
                                                      ((uint16_t)flags) << 16,
                                                      sharedBuffer,
                                                      output,
                                                      &status);
    // 得到IMemory接口
    sp cblk = track->getCblk();
    mAudioTrack.clear();
    mAudioTrack = track;
    mCblkMemory.clear();
    mCblkMemory = cblk;
    // 得到audio_track_cblk_t结构
    mCblk = static_cast(cblk->pointer());
    // 该FIFO用于输出
    mCblk->out = 1;
    // Update buffer size in case it has been limited by AudioFlinger during track creation
    mFrameCount = mCblk->frameCount;
    if (sharedBuffer == 0) {
       // 给FIFO的起始地址赋值
        mCblk->buffers = (char*)mCblk + sizeof(audio_track_cblk_t);
    } else {
        ..........