Android学习（十）AudioFlinger(2)

AudioFlinger（2）

查看Track的构造函数：

AudioFlinger::PlaybackThread::Track::Track(             const wp<ThreadBase>& thread,             const sp<Client>& client,             int streamType,             uint32_t sampleRate,             int format,             int channelCount,             int frameCount,             const sp<IMemory>& sharedBuffer,             int sessionId)     :   TrackBase(thread, client, sampleRate, format, channelCount, frameCount, 0, sharedBuffer, sessionId),     mMute(false), mSharedBuffer(sharedBuffer), mName(-1), mMainBuffer(NULL), mAuxBuffer(NULL),     mAuxEffectId(0), mHasVolumeController(false) {     if (mCblk != NULL) {à在父类TrackBase中创建         sp<ThreadBase> baseThread = thread.promote();         if (baseThread != 0) {             PlaybackThread *playbackThread = (PlaybackThread *)baseThread.get();             mName = playbackThread->getTrackName_l();             mMainBuffer = playbackThread->mixBuffer();         }         mVolume[0] = 1.0f;         mVolume[1] = 1.0f;         mStreamType = streamType;         // NOTE: audio_track_cblk_t::frameSize for 8 bit PCM data is based on a sample size of 16 bit because data is converted to 16 bit before being stored in buffer by AudioTrack         mCblk->frameSize = AudioSystem::isLinearPCM(format) ? channelCount * sizeof(int16_t) : sizeof(int8_t);     } }

查看父类TrackBase的构造函数：

AudioFlinger::ThreadBase::TrackBase::TrackBase(             const wp<ThreadBase>& thread,             const sp<Client>& client,             uint32_t sampleRate,             int format,             int channelCount,             int frameCount,             uint32_t flags,             const sp<IMemory>& sharedBuffer,             int sessionId)     :   RefBase(),         mThread(thread),         mClient(client),         mCblk(0),         mFrameCount(0),         mState(IDLE),         mClientTid(-1),         mFormat(format),         mFlags(flags & ~SYSTEM_FLAGS_MASK),         mSessionId(sessionId) {    size_t size = sizeof(audio_track_cblk_t);à得到audio_track_cblk_t的大小    size_t bufferSize = frameCount*channelCount*sizeof(int16_t);    if (sharedBuffer == 0) {        size += bufferSize;à共享内存中前面为audio_track_cblk_t，后面为数据    }      if (client != NULL) {         mCblkMemory = client->heap()->allocate(size);à根据size大小创建共享内存         if (mCblkMemory != 0) {             mCblk = static_cast<audio_track_cblk_t *>(mCblkMemory->pointer());àpointer返回共享内存首地址，强制转换为audio_track_cblk_t类型             if (mCblk) { // construct the shared structure in-place.                 new(mCblk) audio_track_cblk_t();                 // clear all buffers                 mCblk->frameCount = frameCount;                 mCblk->sampleRate = sampleRate;                 mCblk->channelCount = (uint8_t)channelCount;                 if (sharedBuffer == 0) {                     mBuffer = (char*)mCblk + sizeof(audio_track_cblk_t);                     memset(mBuffer, 0, frameCount*channelCount*sizeof(int16_t));à清空数据区                     // Force underrun condition to avoid false underrun callback until first data is                     // written to buffer (other flags are cleared)                     mCblk->flags = CBLK_UNDERRUN_ON;à初始化flag                 } …

new(mCblk) audio_track_cblk_t();是一个placement new操作，目的是在已经分配的内存中创建一个对象（普通的new是无法完成的）。这里采用placement new将使audio_track_cblk_t创建在共享内存上，从而能使多个进程看见并使用。

上面分析可知：

• Track创建了共享内存

• audio_track_cblk_t对象通过placement new创建在共享内存中

查看AudioFlinger::createTrack：

sp<IAudioTrack> AudioFlinger::createTrack(..) { sp<TrackHandle> trackHandle; track = thread->createTrack_l(client, streamType, sampleRate, format,                 channelCount, frameCount, sharedBuffer, lSessionId, &lStatus); trackHandle = new TrackHandle(track); return trackHandle; }

可见AudioFlinger::createTrack返回的是一个TrackHandle对象，它是Track的代理模式，完成Track本身不具备的Binder通信的功能。