AudioTrack的start方法用于实现Android的音频输出，start究竟做了什么？回顾一下上一小节createTrack_l的最后部分，通过binder返回了一个Track的句柄，并以被保存了下来
status_t AudioTrack::createTrack_l(...)
{
sp<IAudioTrack> track = audioFlinger->createTrack(...);
mAudioTrack = track;
}
start主要就是调用这个track的start方法实现音频输出功能的
// -------------------------------------------------------------------------
status_t AudioTrack::start()
{
AutoMutex lock(mLock);
//如果该AudioTrack已经是start状态，直接返回
if (mState == STATE_ACTIVE) {
return INVALID_OPERATION;
}
mInUnderrun = true;
//保存上一次的状态
State previousState = mState;
//设置当前状态
if (previousState == STATE_PAUSED_STOPPING) {
mState = STATE_STOPPING;
} else {
mState = STATE_ACTIVE;
}
//如果上一状态是停止状态，表明需要重新把position设置为0，从头播放
if (previousState == STATE_STOPPED || previousState == STATE_FLUSHED) {
// reset current position as seen by client to 0
mProxy->setEpoch(mProxy->getEpoch() - mProxy->getPosition());
// force refresh of remaining frames by processAudioBuffer() as last
// write before stop could be partial.
mRefreshRemaining = true;
}
//当前位置
mNewPosition = mProxy->getPosition() + mUpdatePeriod;
//获取share buffer的flag，原子操作
int32_t flags = android_atomic_and(~CBLK_DISABLED, &mCblk->mFlags);
//是否有回调线程，一般如果我们在apk端独立调用AudioTrack，是不会设置回调线程的，但是AudioPlayer这种系统播放器则会设置回调线程
//这样做是为了设置优先级，否则Audio可能会由于得不到时间片，而卡顿
//如果是AudioPlayer，会有自己定义的优先级，AudioTrack后面新创建的线程则会继承它的优先级
//如果是Apk调用，优先级一般都是固定的，那么我们需要在这里设置一个ANDROID_PRIORITY_AUDIO的优先级来保证Audio的流畅输出
sp<AudioTrackThread> t = mAudioTrackThread;
if (t != 0) {
if (previousState == STATE_STOPPING) {
//中断
mProxy->interrupt();
} else {
//恢复播放
t->resume();
}
} else {
//保存当前线程优先级，在后面停止的时候设置回来
mPreviousPriority = getpriority(PRIO_PROCESS, 0);
get_sched_policy(0, &mPreviousSchedulingGroup);
//设置线程优先级为ANDROID_PRIORITY_AUDIO
androidSetThreadPriority(0, ANDROID_PRIORITY_AUDIO);
}
status_t status = NO_ERROR;
if (!(flags & CBLK_INVALID)) {
//如果share buffer可用，则调用track的start方法
status = mAudioTrack->start();
if (status == DEAD_OBJECT) {
flags |= CBLK_INVALID;
}
}
if (flags & CBLK_INVALID) {
status = restoreTrack_l("start");
}
if (status != NO_ERROR) {
//start出错后的处理
ALOGE("start() status %d", status);
mState = previousState;
if (t != 0) {
if (previousState != STATE_STOPPING) {
t->pause();
}
} else {
setpriority(PRIO_PROCESS, 0, mPreviousPriority);
set_sched_policy(0, mPreviousSchedulingGroup);
}
}
return status;
}
由于mAudioTrack是binder的proxy对象，因此start会调用到BBinder对象的start方法，即
status_t AudioFlinger::TrackHandle::start() {
return mTrack->start();
}
由于我们是在PlaybackThread下进行音频输出的，因此会进一步调用到PlaybackThread::Track:: start方法，其中最主要的是下面两个步骤：
status_t AudioFlinger::PlaybackThread::Track::start(
PlaybackThread *playbackThread = (PlaybackThread *)thread.get();
status = playbackThread->addTrack_l(this);
}
还记得我们在getOutput的时候创建了一个MixerThread吗，而且在createTrack_l的时候把这个Thread加入了mPlaybackThreads进行管理，现在我们要把它取出来，调用它的addTrack_l方法了
audio_io_handle_t AudioFlinger::openOutput(audio_module_handle_t module,...)
{ thread = new MixerThread(this, output, id, *pDevices); return id;
}
AudioFlinger::PlaybackThread *AudioFlinger::checkPlaybackThread_l(audio_io_handle_t output) const
{ return mPlaybackThreads.valueFor(output).get();
}
在addTrack_l方法内，主要步骤有三个：
- 如果该track（share buffer）是新增track，则需要调用startOutput进行初始化
- 把该track加入mActiveTracks
- 发送广播，通知MixerThread开始工作
// addTrack_l() must be called with ThreadBase::mLock held
status_t AudioFlinger::PlaybackThread::addTrack_l(const sp<Track>& track)
{
if (mActiveTracks.indexOf(track) < 0) {
status = AudioSystem::startOutput(mId, track->streamType(), track->sessionId());
}
mActiveTracks.add(track);
broadcast_l();
}
1. track初始化
在分析getOutput的时候，我们已经知道Audio接口的调用流程，即AudioSystem->AudioPolicyService->Audio_policy_hal->AudioPolicyManagerBase，现在我们来看一下AudioPolicyManagerBase:: startOutput做了什么
status_t AudioPolicyManagerBase::startOutput(audio_io_handle_t output,
AudioSystem::stream_type stream,
int session)
{
checkAndSetVolume(stream,
mStreams[stream].getVolumeIndex(newDevice),
output,
newDevice);
}
checkAndSetVolume其实只是设置了stream volume.
2. track加入mAudioTrack
mAudioTrack即当前MixerThread所包含的Track集合，在后面就是对这些Track集合进行混音
3. broadcast_l
void AudioFlinger::PlaybackThread::broadcast_l()
{
// Thread could be blocked waiting for async
// so signal it to handle state changes immediately
// If threadLoop is currently unlocked a signal of mWaitWorkCV will
// be lost so we also flag to prevent it blocking on mWaitWorkCV
mSignalPending = true;
mWaitWorkCV.broadcast();
}
我们已经有了MixerThread，由于MixerThread继承与PlaybackThread，因此跑的是PlaybackThread::threadLoop，在threadLoop内，如果mActiveTrack为空的话，表明没有音频数据等待输出，那么threadLoop会进入睡眠，等待唤醒，这里的broadcast就是做了这个唤醒的工作
bool AudioFlinger::PlaybackThread::threadLoop()
{
if ((!mActiveTracks.size() && systemTime() > standbyTime) ||
isSuspended())
mWaitWorkCV.wait(mLock);
}
...
}
下面是start的总体流程
