Android Audio代码分析26 - Audio Strategy

在看示例代码 testPlaybackHeadPositionIncrease 的时候，我们对 play 函数进行了研究。
不过，当时对 Android 中声音竞争策略相关的内容，并没有详细分析。
今天就以 AudioTrack 的播放为引子，来仔细看看 Anroid 中各种声音是以什么样的策略来竞争的。

从 Java 侧类 AudioTrack 的 play 函数到函数 AudioFlinger::PlaybackThread::Track::start 之间的调用关系就不再叙述了。
在看示例代码 testPlaybackHeadPositionIncrease 的时候已经说明过了。
今天就从函数 AudioFlinger::PlaybackThread::Track::start 开始分析。

*****************************************源码*************************************************
status_t AudioFlinger::PlaybackThread::Track::start()
{
    status_t status = NO_ERROR;
    LOGV("start(%d), calling thread %d session %d",
            mName, IPCThreadState::self()->getCallingPid(), mSessionId);
    sp<ThreadBase> thread = mThread.promote();
    if (thread != 0) {
        Mutex::Autolock _l(thread->mLock);
        int state = mState;
        // here the track could be either new, or restarted
        // in both cases "unstop" the track
        if (mState == PAUSED) {
            mState = TrackBase::RESUMING;
            LOGV("PAUSED => RESUMING (%d) on thread %p", mName, this);
        } else {
            mState = TrackBase::ACTIVE;
            LOGV("? => ACTIVE (%d) on thread %p", mName, this);
        }

        if (!isOutputTrack() && state != ACTIVE && state != RESUMING) {
            thread->mLock.unlock();
            status = AudioSystem::startOutput(thread->id(),
                                              (AudioSystem::stream_type)mStreamType,
                                              mSessionId);
            thread->mLock.lock();
        }
        if (status == NO_ERROR) {
            PlaybackThread *playbackThread = (PlaybackThread *)thread.get();
            playbackThread->addTrack_l(this);
        } else {
            mState = state;
        }
    } else {
        status = BAD_VALUE;
    }
    return status;
}
**********************************************************************************************
源码路径：
frameworks\base\services\AudioFlinger.cpp

#######################说明################################
status_t AudioFlinger::PlaybackThread::Track::start()
{
    status_t status = NO_ERROR;
    LOGV("start(%d), calling thread %d session %d",
            mName, IPCThreadState::self()->getCallingPid(), mSessionId);
    sp<ThreadBase> thread = mThread.promote();
    if (thread != 0) {
        Mutex::Autolock _l(thread->mLock);
        int state = mState;
        // here the track could be either new, or restarted
        // in both cases "unstop" the track
        if (mState == PAUSED) {
            mState = TrackBase::RESUMING;
            LOGV("PAUSED => RESUMING (%d) on thread %p", mName, this);
        } else {
            mState = TrackBase::ACTIVE;
            LOGV("? => ACTIVE (%d) on thread %p", mName, this);
        }

        if (!isOutputTrack() && state != ACTIVE && state != RESUMING) {
            thread->mLock.unlock();
            status = AudioSystem::startOutput(thread->id(),
                                              (AudioSystem::stream_type)mStreamType,
                                              mSessionId);
++++++++++++++++++++++++++++AudioSystem::startOutput++++++++++++++++++++++++++++++++++++
从函数 AudioFlinger::PlaybackThread::Track::start 进入：
status_t AudioSystem::startOutput(audio_io_handle_t output,
                                  AudioSystem::stream_type stream,
                                  int session)
{
    const sp<IAudioPolicyService>& aps = AudioSystem::get_audio_policy_service();
    if (aps == 0) return PERMISSION_DENIED;
    return aps->startOutput(output, stream, session);
+++++++++++++++++++++++++++++AudioPolicyService::startOutput+++++++++++++++++++++++++++++++++++
从函数 AudioSystem::startOutput 进入：
status_t AudioPolicyService::startOutput(audio_io_handle_t output,
                   &n

补充：移动开发 , Android ,