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_UP

12 ? ADJUST_RAISE

13 : ADJUST_LOWER,

14 flags);

15 } else {

16 adjustSuggestedStreamVolume(

17 keyCode == KeyEvent.KEYCODE_VOLUME_UP

18 ? ADJUST_RAISE

19 : 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

false

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 msg

10 　　　　 ... ...

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& 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 muted

10 ... ...

11 // do not change in call volume if bluetooth is connected and vice versa

12 ... ...

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。