Android 4.4 音量调节流程分析(一)

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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：

 1 public void handleKeyDown(KeyEvent event, int stream) {  2         int keyCode = event.getKeyCode(); 3         switch (keyCode) { 4             case KeyEvent.KEYCODE_VOLUME_UP:        /*KeyEvent 在KeyEvent.java中定义*/ 5             case KeyEvent.KEYCODE_VOLUME_DOWN: 6  7                 int flags = FLAG_SHOW_UI | FLAG_VIBRATE; 8  9                 if (mUseMasterVolume) {10                     adjustMasterVolume(11                             keyCode == KeyEvent.KEYCODE_VOLUME_UP12                                     ? ADJUST_RAISE13                                     : ADJUST_LOWER,14                             flags);15                 } else {16                     adjustSuggestedStreamVolume(17                             keyCode == KeyEvent.KEYCODE_VOLUME_UP18                                     ? ADJUST_RAISE19                                     : ADJUST_LOWER,20                             stream,21                             flags);22                 }23                 break;24     　　　　　　　... ... 25         }26     }

　　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。

 1     public void adjustSuggestedStreamVolume(int direction, int suggestedStreamType, int flags) { 2         IAudioService service = getService(); 3         try { 4             if (mUseMasterVolume) { 5                 service.adjustMasterVolume(direction, flags, mContext.getOpPackageName()); 6             } else { 7                 service.adjustSuggestedStreamVolume(direction, suggestedStreamType, flags, 8                         mContext.getOpPackageName()); 9             }10          　　... ...11　　　　　　}12     }

　　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。

 1     public void adjustStreamVolume(int streamType, int direction, int flags, 2             String callingPackage) { 3     ... ... 4     /*音量调大时，若要超过SafeMediaVolume时，系统会弹出对话框给予确认*/ 5             if ((direction == AudioManager.ADJUST_RAISE) && 6                     !checkSafeMediaVolume(streamTypeAlias, aliasIndex + step, device)) { 7                 Log.e(TAG, "adjustStreamVolume() safe volume index = "+oldIndex); 8                 mVolumePanel.postDisplaySafeVolumeWarning(flags); 9             } else if (streamState.adjustIndex(direction * step, device)) {10                 sendMsg(mAudioHandler,11                         MSG_SET_DEVICE_VOLUME,             /*需要处理的Message值*/12                         SENDMSG_QUEUE,13                         device,14                         0,15                         streamState,16                         0);17             }18         }19         int index = mStreamStates[streamType].getIndex(device);20         sendVolumeUpdate(streamType, oldIndex, index, flags);       /*通知上层更新Volume*/21     }

　　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。

 1        public void handleMessage(Message msg) { 2  3             switch (msg.what) { 4  5                 case MSG_SET_DEVICE_VOLUME: 6                     setDeviceVolume((VolumeStreamState) msg.obj, msg.arg1); 7                     break; 8  9                 case MSG_SET_ALL_VOLUMES:10                     setAllVolumes((VolumeStreamState) msg.obj);11                     break;12 13             　　... ... 14             }15         }

　　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：

 1         private void setDeviceVolume(VolumeStreamState streamState, int device) { 2  3             // Apply volume 4             streamState.applyDeviceVolume(device); 5  6             // Apply change to all streams using this one as alias 7     　　　　 ... ... 8  9             // Post a persist volume msg10     　　　　 ... ...     11         }

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

 1         public void applyDeviceVolume(int device) { 2             int index; 3             if (isMuted()) { 4                 index = 0; 5             } else if ((device & AudioSystem.DEVICE_OUT_ALL_A2DP) != 0 && 6                        mAvrcpAbsVolSupported) { 7                 index = (mIndexMax + 5)/10; 8             } else { 9                 index = (getIndex(device) + 5)/10;10             }11             AudioSystem.setStreamVolumeIndex(mStreamType, index, device);12         }

　　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。

1 status_t AudioSystem::setStreamVolumeIndex(audio_stream_type_t stream,2                                            int index,3                                            audio_devices_t device)4 {5     const sp<IAudioPolicyService>& aps = AudioSystem::get_audio_policy_service();6     if (aps == 0) return PERMISSION_DENIED;7     return aps->setStreamVolumeIndex(stream, index, device);8 }

　　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：

 1 status_t AudioPolicyService::setStreamVolumeIndex(audio_stream_type_t stream, 2                                                   int index, 3                                                   audio_devices_t device) 4 { 5     ... ...  6     if (mpAudioPolicy->set_stream_volume_index_for_device) { 7         return mpAudioPolicy->set_stream_volume_index_for_device(mpAudioPolicy, 8                                                                 stream, 9                                                                 index,10                                                                 device);11     } else {12         return mpAudioPolicy->set_stream_volume_index(mpAudioPolicy, stream, index);13     }14 }

　　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：

 1 status_t AudioPolicyManagerBase::setStreamVolumeIndex(AudioSystem::stream_type stream, 2                                                       int index, 3                                                       audio_devices_t device) 4 { 5     ... ...   6     // compute and apply stream volume on all outputs according to connected device 7     status_t status = NO_ERROR; 8     for (size_t i = 0; i < mOutputs.size(); i++) { 9         audio_devices_t curDevice =10                 getDeviceForVolume(mOutputs.valueAt(i)->device());11         if ((device == AUDIO_DEVICE_OUT_DEFAULT) || (device == curDevice)) {12             status_t volStatus = checkAndSetVolume(stream, index, mOutputs.keyAt(i), curDevice);13             if (volStatus != NO_ERROR) {14                 status = volStatus;15             }16         }17     }18     return status;19 }

继续调用checkAndSetVolume函数：

 1 status_t AudioPolicyManagerBase::checkAndSetVolume(int stream, 2                                                    int index, 3                                                    audio_io_handle_t output, 4                                                    audio_devices_t device, 5                                                    int delayMs, 6                                                    bool force) 7 { 8  9     // do not change actual stream volume if the stream is muted10     ... ...11     // do not change in call volume if bluetooth is connected and vice versa12     ... ...13     audio_devices_t checkedDevice = (device == AUDIO_DEVICE_NONE) ? mOutputs.valueFor(output)->device() : device;14     float volume = computeVolume(stream, index, checkedDevice);15 16     ... ...17 　　mpClientInterface->setStreamVolume((AudioSystem::stream_type)stream, volume, output, delayMs);/*将得到的volume应用到对应的output中*/18 }

　　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 = 1 + 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 = 0;    if (volIdx < curve[VOLMIN].mIndex) {         // out of bounds        return 0.0f;    } else if (volIdx < curve[VOLKNEE1].mIndex) {        segment = 0;    } else if (volIdx < curve[VOLKNEE2].mIndex) {        segment = 1;    } else if (volIdx <= curve[VOLMAX].mIndex) {        segment = 2;    } 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+1].mDBAttenuation -                        curve[segment].mDBAttenuation) /                    ((float)(curve[segment+1].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。

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