Android 音频系统：从 AudioTrack 到 AudioFlinger

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audiotrack和audioflinger交互 添加链接描述

3.3. AudioFlinger 服务接口
AudioFlinger 对外提供的主要的服务接口如下：

可以归纳出 AudioFlinger 响应的服务请求主要有：

获取硬件设备的配置信息 sampleRate、format、frameCount、latency
音量调节 setMasterVolume、setStreamVolume、setVoiceVolume
静音操作 setMasterMute、setStreamMute、setMicMute
音频模式切换 setMode Normal、Ringtone、Call、Communicatoin
音频参数设置 setParameters
输入输出流设备管理 openOutput closeOutput openInput closeInput openRecord
音频流管理 createTrack start/stop/pause

1. ThreadBase：PlaybackThread 和 RecordThread 的基类
2. RecordThread：录制线程类，由 ThreadBase 派生
3. PlaybackThread：回放线程基类，同由 ThreadBase 派生
4. MixerThread：混音回放线程类，由 PlaybackThread 派生，负责处理标识为
   AUDIO_OUTPUT_FLAG_PRIMARY、AUDIO_OUTPUT_FLAG_FAST、AUDIO_OUTPUT_FLAG_DEEP_BUFFER
   的音频流，MixerThread 可以把多个音轨的数据混音后再输出
5. DirectOutputThread：直输回放线程类，由 PlaybackThread 派生，负责处理标识为
   AUDIO_OUTPUT_FLAG_DIRECT 的音频流，这种音频流数据不需要软件混音，直接输出到音频设备即可
6. DuplicatingThread：复制回放线程类，由 MixerThread
   派生，负责复制音频流数据到其他输出设备，使用场景如主声卡设备、蓝牙耳机设备、USB 声卡设备同时输出
7. OffloadThread：硬解回放线程类，由 DirectOutputThread 派生，负责处理标识为
   AUDIO_OUTPUT_FLAG_COMPRESS_OFFLOAD 的音频流，这种音频流未经软件解码的（一般是 MP3、AAC
   等格式的数据），需要输出到硬件解码器，由硬件解码器解码成 PCM 数据

PlaybackThread 中有个极为重要的函数 threadLoop()，当 PlaybackThread 被强引用时，threadLoop() 会真正运行起来进入循环主体，处理音频流数据相关事务，threadLoop() 大致流程如下（以 MixerThread 为例）：

bool AudioFlinger::PlaybackThread::threadLoop()
{
    // ......

    while (!exitPending())
    {
        // ......

        { // scope for mLock

            Mutex::Autolock _l(mLock);
       processConfigEvents_l();

            // ......

            if ((!mActiveTracks.size() && systemTime() > mStandbyTimeNs) ||
                                   isSuspended()) {
                // put audio hardware into standby after short delay
                if (shouldStandby_l()) {

                    threadLoop_standby();

                    mStandby = true;
                }

                // ......
            }
            // mMixerStatusIgnoringFastTracks is also updated internally
            mMixerStatus = prepareTracks_l(&tracksToRemove);

            // ......
        } // mLock scope ends

        // ......

        if (mBytesRemaining == 0) {
            mCurrentWriteLength = 0;
            if (mMixerStatus == MIXER_TRACKS_READY) {
                // threadLoop_mix() sets mCurrentWriteLength
                threadLoop_mix();
            }
            // ......
        }

        // ......

        if (!waitingAsyncCallback()) {
            // mSleepTimeUs == 0 means we must write to audio hardware
            if (mSleepTimeUs == 0) {
                // ......
                if (mBytesRemaining) {
                    // FIXME rewrite to reduce number of system calls
                    ret = threadLoop_write();
                    lastWriteFinished = systemTime();
                    delta = lastWriteFinished - mLastWriteTime;
                    if (ret < 0) {
                        mBytesRemaining = 0;
                    } else {
                        mBytesWritten += ret;
                        mBytesRemaining -= ret;
                        mFramesWritten += ret / mFrameSize;
                    }
                }
                // ......
            }
            // ......
        }

        // Finally let go of removed track(s), without the lock held
        // since we can't guarantee the destructors won't acquire that
        // same lock.  This will also mutate and push a new fast mixer state.
        threadLoop_removeTracks(tracksToRemove);
        tracksToRemove.clear();

        // ......
    }

    threadLoop_exit();

    if (!mStandby) {
        threadLoop_standby();
        mStandby = true;
    }

    // ......
    return false;
}

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来源： https://blog.csdn.net/weixin_34323585/article/details/118695849