Android 4.4 音量调节流程分析(一)

　　最近在做Android Audio方面的工作，有需求是在调节Volume_Up_Key & Volume_Down_key时，Spearker or Headset每音阶的衰减变为3db左右。所以利用Source Insight分析Android源码中音量控制的流程，如有错误，欢迎指正，谢谢！　　

　　以下是调节音量的流程：

　　Step_1.首先在调节机台Volume_Up_Key & Volume_Down_Key操作时，系统会调用到AudioManager.java中handleKeyUp & handleKeyDown函数，以 handleKeyDown函数为例：

 public void handleKeyDown(KeyEvent event, int stream) {
         int keyCode = event.getKeyCode();
         switch (keyCode) {
             case KeyEvent.KEYCODE_VOLUME_UP:        /*KeyEvent 在KeyEvent.java中定义*/
             case KeyEvent.KEYCODE_VOLUME_DOWN:

                 int flags = FLAG_SHOW_UI | FLAG_VIBRATE;

                 if (mUseMasterVolume) {
                     adjustMasterVolume(
                             keyCode == KeyEvent.KEYCODE_VOLUME_UP
                                     ? ADJUST_RAISE
                                     : ADJUST_LOWER,
                             flags);
                 } else {
                     adjustSuggestedStreamVolume(
                             keyCode == KeyEvent.KEYCODE_VOLUME_UP
                                     ? ADJUST_RAISE
                                     : ADJUST_LOWER,
                             stream,
                             flags);
                 }
                 break;
     　　　　　　　... ...
         }
     }

　　其中是否进入adjustMasterVolume 函数是通过mUseMasterVolume的值判断的，而mUseMasterVolume的值是在AudioManager的构造函数中定义，其值的大小如下：mUseMasterVolume = mContext.getResources().getBoolean(com.android.internal.R.bool.config_useMasterVolume)，所以首先从系统的配置文件config.xml中查找config_useMasterVolume值的大小

　　<bool name="config_useMasterVolume">false</bool>

　　所以handleKeyDown中 switch语句中会选择进入adjustSuggestedStreamVolume函数。

     public void adjustSuggestedStreamVolume(int direction, int suggestedStreamType, int flags) {
         IAudioService service = getService();
         try {
             if (mUseMasterVolume) {
                 service.adjustMasterVolume(direction, flags, mContext.getOpPackageName());
             } else {
                 service.adjustSuggestedStreamVolume(direction, suggestedStreamType, flags,
                         mContext.getOpPackageName());
             }
          　　... ...
　　　　　　}
     }

　　Step_2.在adjustSuggestedStreamVolume函数中首先会通过binder机制得到AudioService，并将音量控制过程转入到AudioService.java中。

     public void adjustStreamVolume(int streamType, int direction, int flags,
             String callingPackage) {
     ... ...
     /*音量调大时，若要超过SafeMediaVolume时，系统会弹出对话框给予确认*/
             if ((direction == AudioManager.ADJUST_RAISE) &&
                     !checkSafeMediaVolume(streamTypeAlias, aliasIndex + step, device)) {
                 Log.e(TAG, "adjustStreamVolume() safe volume index = "+oldIndex);
                 mVolumePanel.postDisplaySafeVolumeWarning(flags);
             } else if (streamState.adjustIndex(direction * step, device)) {
                 sendMsg(mAudioHandler,
                         MSG_SET_DEVICE_VOLUME,             /*需要处理的Message值*/
                         SENDMSG_QUEUE,
                         device,
                         0,
                         streamState,
                         0);
             }
         }
         int index = mStreamStates[streamType].getIndex(device);
         sendVolumeUpdate(streamType, oldIndex, index, flags);       /*通知上层更新Volume*/
     }

　　在adjustStreamVolume 中会通过sendMsg的方式来将调节音量的事件加入到消息列队SENDMSG_QUENE中，当轮寻到该Message时，系统会调用handleMessage函数来处理该Message，此时该处对应的Message为MSG_SET_DEVICE_VOLUME。

        public void handleMessage(Message msg) {

             switch (msg.what) {

                 case MSG_SET_DEVICE_VOLUME:
                     setDeviceVolume((VolumeStreamState) msg.obj, msg.arg1);
                     break;

                 case MSG_SET_ALL_VOLUMES:
                     setAllVolumes((VolumeStreamState) msg.obj);
                     break;

             　　... ...
             }
         }

　　可以发现当msg.what =  MSG_SET_DEVICE_VOLUME时，会进到setDeviceVolume函数中，继续往下分析：

         private void setDeviceVolume(VolumeStreamState streamState, int device) {

             // Apply volume
             streamState.applyDeviceVolume(device);

             // Apply change to all streams using this one as alias
     　　　　 ... ...

             // Post a persist volume msg
     　　　　 ... ...
         }

　　applyDeviceVolume就是将音量Volume设置到对应的设备Device上，继续往下分析:

         public void applyDeviceVolume(int device) {
             int index;
             if (isMuted()) {
                 index = 0;
             } else if ((device & AudioSystem.DEVICE_OUT_ALL_A2DP) != 0 &&
                        mAvrcpAbsVolSupported) {
                 index = (mIndexMax + 5)/10;
             } else {
                 index = (getIndex(device) + 5)/10;
             }
             AudioSystem.setStreamVolumeIndex(mStreamType, index, device);
         }

　　此处VolumeIndex就是对应UI界面调节音量时，音量所处在的位置下标。在AudioService.java中定义了每种音频流对应的Max-Index，在AudioManager.java中定义了每种音频流在第一次刷机后默认的Index。

　　Step_3.此时得到音量的下标Index后，会调用AudioSystem.java中的setStreamVolumeIndex函数中来得到此时音量的放大倍数。通过JNI层调用到AudioSystem.cpp文件中的 setStreamVolumeIndex中。

 status_t AudioSystem::setStreamVolumeIndex(audio_stream_type_t stream,
                                            int index,
                                            audio_devices_t device)
 {
     const sp<IAudioPolicyService>& aps = AudioSystem::get_audio_policy_service();
     if (aps == ) return PERMISSION_DENIED;
     return aps->setStreamVolumeIndex(stream, index, device);
 }

　　setStreamVolumeIndex函数中比较简单，通过StrongPointer来与AudioPolicyService建立联系，将AudioSystem中的setStreamVolumeIndex操作移到aps中完成。下面进入到AudioPolicyService.cpp文件中的setStreamVolumeIndex继续分析：

 status_t AudioPolicyService::setStreamVolumeIndex(audio_stream_type_t stream,
                                                   int index,
                                                   audio_devices_t device)
 {
     ... ...
     if (mpAudioPolicy->set_stream_volume_index_for_device) {
         return mpAudioPolicy->set_stream_volume_index_for_device(mpAudioPolicy,
                                                                 stream,
                                                                 index,
                                                                 device);
     } else {
         return mpAudioPolicy->set_stream_volume_index(mpAudioPolicy, stream, index);
     }
 }

　　Step_4.AudioPolicyService.cpp作为bn端，其对应的bp端为AudioPolicyManagerBase.cpp。在当前函数的if语句中判断AudioPolicyManagerBase.cpp文件中是否存在setStreamVolumeIndexForDevice函数，条件成立则会选择setStreamVolumeIndexForDevice作为函数入口端；否则选择setStreamVolumeIndex作为函数入口。现在进入AudioPolicyManagerBase.cpp中文件中完成最后的分析：

 status_t AudioPolicyManagerBase::setStreamVolumeIndex(AudioSystem::stream_type stream,
                                                       int index,
                                                       audio_devices_t device)
 {
     ... ...
     // compute and apply stream volume on all outputs according to connected device
     status_t status = NO_ERROR;
     for (size_t i = ; i < mOutputs.size(); i++) {
         audio_devices_t curDevice =
                 getDeviceForVolume(mOutputs.valueAt(i)->device());
         if ((device == AUDIO_DEVICE_OUT_DEFAULT) || (device == curDevice)) {
             status_t volStatus = checkAndSetVolume(stream, index, mOutputs.keyAt(i), curDevice);
             if (volStatus != NO_ERROR) {
                 status = volStatus;
             }
         }
     }
     return status;
 }

继续调用checkAndSetVolume函数：

 status_t AudioPolicyManagerBase::checkAndSetVolume(int stream,
                                                    int index,
                                                    audio_io_handle_t output,
                                                    audio_devices_t device,
                                                    int delayMs,
                                                    bool force)
 {

     // do not change actual stream volume if the stream is muted
     ... ...
     // do not change in call volume if bluetooth is connected and vice versa
     ... ...
     audio_devices_t checkedDevice = (device == AUDIO_DEVICE_NONE) ? mOutputs.valueFor(output)->device() : device;
     float volume = computeVolume(stream, index, checkedDevice);

     ... ...
 　　mpClientInterface->setStreamVolume((AudioSystem::stream_type)stream, volume, output, delayMs);/*将得到的volume应用到对应的output中*/
 }

　　在checkAndSetVolume中可以知道volume是通过computeVolume得到的。继续向下分析：

float AudioPolicyManagerBase::computeVolume(int stream,
                                            int index,
                                            audio_devices_t device)
{
    ... ...
    volume = volIndexToAmpl(device, streamDesc, index);
    ... ...
    return volume;
}

　　终于到了最后volIndexToAmpl，从函数名就可以知道该函数的作用是通过volIndex得到音量放大倍数。

float AudioPolicyManagerBase::volIndexToAmpl(audio_devices_t device, const StreamDescriptor& streamDesc,
        int indexInUi)
{
    device_category deviceCategory = getDeviceCategory(device);
    const VolumeCurvePoint *curve = streamDesc.mVolumeCurve[deviceCategory];

    // the volume index in the UI is relative to the min and max volume indices for this stream type
    int nbSteps =  + curve[VOLMAX].mIndex -
            curve[VOLMIN].mIndex;
    int volIdx = (nbSteps * (indexInUi - streamDesc.mIndexMin)) /
            (streamDesc.mIndexMax - streamDesc.mIndexMin);

    // find what part of the curve this index volume belongs to, or if it's out of bounds
    int segment = ;
    if (volIdx < curve[VOLMIN].mIndex) {         // out of bounds
        return 0.0f;
    } else if (volIdx < curve[VOLKNEE1].mIndex) {
        segment = ;
    } else if (volIdx < curve[VOLKNEE2].mIndex) {
        segment = ;
    } else if (volIdx <= curve[VOLMAX].mIndex) {
        segment = ;
    } else {                                                               // out of bounds
        return 1.0f;
    }

    // linear interpolation in the attenuation table in dB
    float decibels = curve[segment].mDBAttenuation +
            ((float)(volIdx - curve[segment].mIndex)) *
                ( (curve[segment+].mDBAttenuation -
                        curve[segment].mDBAttenuation) /
                    ((float)(curve[segment+].mIndex -
                            curve[segment].mIndex)) );

    float amplification = exp( decibels * 0.115129f); // exp( dB * ln(10) / 20 )

    return amplification;
}

　　其中curve代表在用设备（如SPEAKER、HEADSET & EARPIECE）播放音频流（如SYSTEM、MUSIC、ALARM、RING & TTS等）时的音量曲线，最后decibles & amplification就是我们需要求的值，其中decibles代表某一音节所对应的dB值，而amplification则是由dB值转化得到的音量放大倍数。这样整个音量调节过程到此就算完成了，具体的计算分析会放在后面继续分析。