Android audio 五 AudioTrack 分析下

Android audio 一 源码路径

Android audio 二 AudioRecord 分析上

Android audio 三 AudioRecord 分析下

Android audio 四 AudioTrack 分析上

Android audio 五 AudioTrack 分析下

Android audio 六 AudioRecord AudiTrack 拾音放音例子

该小节简要分析 AudioTrack.cpp 

文件：

frameworks/av/media/libmedia/AudioTrack.cpp

AudioTrack native 方法实现的播放音频的逻辑和 java 层类似，详情参考上小节 AudioTrack 播放音频的步骤。

先看下 AudioTrack 本地方法的构造和析构函数 ：

在构造函数中调用 set(......)函数完成 AudioTrack 的初始化。

AudioTrack::AudioTrack(): mStatus(NO_INIT),mState(STATE_STOPPED),mPreviousPriority(ANDROID_PRIORITY_NORMAL),mPreviousSchedulingGroup(SP_DEFAULT),mPausedPosition(0),mSelectedDeviceId(AUDIO_PORT_HANDLE_NONE)
{mAttributes.content_type = AUDIO_CONTENT_TYPE_UNKNOWN;mAttributes.usage = AUDIO_USAGE_UNKNOWN;mAttributes.flags = 0x0;strcpy(mAttributes.tags, "");
}AudioTrack::AudioTrack(audio_stream_type_t streamType,uint32_t sampleRate,audio_format_t format,audio_channel_mask_t channelMask,size_t frameCount,audio_output_flags_t flags,callback_t cbf,void* user,int32_t notificationFrames,audio_session_t sessionId,transfer_type transferType,const audio_offload_info_t *offloadInfo,int uid,pid_t pid,const audio_attributes_t* pAttributes,bool doNotReconnect,float maxRequiredSpeed): mStatus(NO_INIT),mState(STATE_STOPPED),mPreviousPriority(ANDROID_PRIORITY_NORMAL),mPreviousSchedulingGroup(SP_DEFAULT),mPausedPosition(0),mSelectedDeviceId(AUDIO_PORT_HANDLE_NONE)
{mStatus = set(streamType, sampleRate, format, channelMask,frameCount, flags, cbf, user, notificationFrames,0 /*sharedBuffer*/, false /*threadCanCallJava*/, sessionId, transferType,offloadInfo, uid, pid, pAttributes, doNotReconnect, maxRequiredSpeed);
}AudioTrack::AudioTrack(audio_stream_type_t streamType,uint32_t sampleRate,audio_format_t format,audio_channel_mask_t channelMask,const sp<IMemory>& sharedBuffer,audio_output_flags_t flags,callback_t cbf,void* user,int32_t notificationFrames,audio_session_t sessionId,transfer_type transferType,const audio_offload_info_t *offloadInfo,int uid,pid_t pid,const audio_attributes_t* pAttributes,bool doNotReconnect,float maxRequiredSpeed): mStatus(NO_INIT),mState(STATE_STOPPED),mPreviousPriority(ANDROID_PRIORITY_NORMAL),mPreviousSchedulingGroup(SP_DEFAULT),mPausedPosition(0),mSelectedDeviceId(AUDIO_PORT_HANDLE_NONE)
{mStatus = set(streamType, sampleRate, format, channelMask,0 /*frameCount*/, flags, cbf, user, notificationFrames,sharedBuffer, false /*threadCanCallJava*/, sessionId, transferType, offloadInfo,uid, pid, pAttributes, doNotReconnect, maxRequiredSpeed);
}AudioTrack::~AudioTrack()
{if (mStatus == NO_ERROR) {// Make sure that callback function exits in the case where// it is looping on buffer full condition in obtainBuffer().// Otherwise the callback thread will never exit.stop();if (mAudioTrackThread != 0) {mProxy->interrupt();mAudioTrackThread->requestExit();   // see comment in AudioTrack.hmAudioTrackThread->requestExitAndWait();mAudioTrackThread.clear();}// No lock here: worst case we remove a NULL callback which will be a nopif (mDeviceCallback != 0 && mOutput != AUDIO_IO_HANDLE_NONE) {AudioSystem::removeAudioDeviceCallback(mDeviceCallback, mOutput);}IInterface::asBinder(mAudioTrack)->unlinkToDeath(mDeathNotifier, this);mAudioTrack.clear();mCblkMemory.clear();mSharedBuffer.clear();IPCThreadState::self()->flushCommands();ALOGV("~AudioTrack, releasing session id %d from %d on behalf of %d",mSessionId, IPCThreadState::self()->getCallingPid(), mClientPid);AudioSystem::releaseAudioSessionId(mSessionId, mClientPid);}
}

AudioTrack::set

AudioTrack::set的代码很长，这里简要叙述下，不做贴出源码。

1. 根据 transferType 选择传输模式 TRANSFER_SHARED/TRANSFER_SYNC/TRANSFER_CALLBACK 
   默认使用 AudioTrack::TRANSFER_SYNC ；
2. 检查输出音频的格式和音频的通道数是否合法；
3. 创建并运行放音线程 AudioTrackThread ；
4. 调用 AudioTrack::createTrack_l()；
   1. 检查传输的音频参数和 AudioSystem 的参数是否匹配；
   2. 调用 audioFlinger->createTrack(......);   Android 的音频离不开 AudioFlinger ，
      AudioFlinger 是承上启下的作用，承上(为上层提供访问接口)，启下(通过HAL来管理音频设备)。是整个音频系统的核心与难点。
5. 使用 binder 创建一个链接 Service/Client ： 

       mDeathNotifier = new DeathNotifier(this);IInterface::asBinder(mAudioTrack)->linkToDeath(mDeathNotifier, this);

AudioTrack::write（播放音频）

write(......) 调用 AudioTrack::obtainBuffer(......) 发送数据

AudioTrack::obtainBuffer(......) 中使用 AudioTrackClientProxy::obtainBuffer(......) 发送音频数据。

virtual status_t AudioTrackClientProxy::obtainBuffer(Buffer* buffer, bool ackFlush = false);

这里用到了 virtual 虚函数，放音服务的成员实现 obtainBuffer(......) 的业务逻辑，即播放音频。

ssize_t AudioTrack::write(const void* buffer, size_t userSize, bool blocking)
{if (mTransfer != TRANSFER_SYNC) {return INVALID_OPERATION;}if (isDirect()) {AutoMutex lock(mLock);int32_t flags = android_atomic_and(~(CBLK_UNDERRUN | CBLK_LOOP_CYCLE | CBLK_LOOP_FINAL | CBLK_BUFFER_END),&mCblk->mFlags);if (flags & CBLK_INVALID) {return DEAD_OBJECT;}}if (ssize_t(userSize) < 0 || (buffer == NULL && userSize != 0)) {// Sanity-check: user is most-likely passing an error code, and it would// make the return value ambiguous (actualSize vs error).ALOGE("AudioTrack::write(buffer=%p, size=%zu (%zd)", buffer, userSize, userSize);return BAD_VALUE;}size_t written = 0;Buffer audioBuffer;while (userSize >= mFrameSize) {audioBuffer.frameCount = userSize / mFrameSize;// 发送数据status_t err = obtainBuffer(&audioBuffer,blocking ? &ClientProxy::kForever : &ClientProxy::kNonBlocking);if (err < 0) {if (written > 0) {break;}return ssize_t(err);}size_t toWrite = audioBuffer.size;memcpy(audioBuffer.i8, buffer, toWrite);buffer = ((const char *) buffer) + toWrite;userSize -= toWrite;written += toWrite;releaseBuffer(&audioBuffer);}if (written > 0) {mFramesWritten += written / mFrameSize;}return written;
}