上篇里面提到了声音调整的两种操作，接下来就要具体分析下音量调节的大概步骤,分别涉及到两部分:
android\frameworks\base\media\java\android\media\AudioService.java
android\frameworks\base\media\java\android\media\IAudioService.aidl
其中adjustStreamVolume(int streamType, int direction, int flags)函数的声明是在AudioService.aidl中，而函数的实现是在AudioService.java中。
下面具体可以看源码:
/** @see AudioManager#adjustStreamVolume(int, int, int) */ public void adjustStreamVolume(int streamType, int direction, int flags) { if (DEBUG_VOL) Log.d(TAG, "adjustStreamVolume() stream="+streamType+", dir="+direction); ensureValidDirection(direction); ensureValidStreamType(streamType); // use stream type alias here so that streams with same alias have the same behavior, // including with regard to silent mode control (e.g the use of STREAM_RING below and in // checkForRingerModeChange() in place of STREAM_RING or STREAM_NOTIFICATION) int streamTypeAlias = mStreamVolumeAlias[streamType]; VolumeStreamState streamState = mStreamStates[streamTypeAlias]; final int device = getDeviceForStream(streamTypeAlias); // get last audible index if stream is muted, current index otherwise final int aliasIndex = streamState.getIndex(device, (streamState.muteCount() != 0) /* lastAudible */); boolean adjustVolume = true; // convert one UI step (+/-1) into a number of internal units on the stream alias int step = rescaleIndex(10, streamType, streamTypeAlias); // If either the client forces allowing ringer modes for this adjustment, // or the stream type is one that is affected by ringer modes if (((flags & AudioManager.FLAG_ALLOW_RINGER_MODES) != 0) || (streamTypeAlias == getMasterStreamType())) { int ringerMode = getRingerMode(); // do not vibrate if already in vibrate mode if (ringerMode == AudioManager.RINGER_MODE_VIBRATE) { flags &= ~AudioManager.FLAG_VIBRATE; } // Check if the ringer mode changes with this volume adjustment. If // it does, it will handle adjusting the volume, so we won't below adjustVolume = checkForRingerModeChange(aliasIndex, direction, step); if ((streamTypeAlias == getMasterStreamType()) && (mRingerMode == AudioManager.RINGER_MODE_SILENT)) { streamState.setLastAudibleIndex(0, device); } } // If stream is muted, adjust last audible index only int index; final int oldIndex = mStreamStates[streamType].getIndex(device, (mStreamStates[streamType].muteCount() != 0) /* lastAudible */); if (streamState.muteCount() != 0) { if (adjustVolume) { // Post a persist volume msg // no need to persist volume on all streams sharing the same alias streamState.adjustLastAudibleIndex(direction * step, device); sendMsg(mAudioHandler, MSG_PERSIST_VOLUME, SENDMSG_QUEUE, PERSIST_LAST_AUDIBLE, device, streamState, PERSIST_DELAY); } index = mStreamStates[streamType].getIndex(device, true /* lastAudible */); } else { if (adjustVolume && streamState.adjustIndex(direction * step, device)) { // Post message to set system volume (it in turn will post a message // to persist). Do not change volume if stream is muted. sendMsg(mAudioHandler, MSG_SET_DEVICE_VOLUME, SENDMSG_QUEUE, device, 0, streamState, 0); } index = mStreamStates[streamType].getIndex(device, false /* lastAudible */); } sendVolumeUpdate(streamType, oldIndex, index, flags); } 解析上面函数可知，函数最开始会有一个对于前两个参数有效性的检测，贴出具体的检测代码:
private void ensureValidDirection(int direction) { if (direction < AudioManager.ADJUST_LOWER || direction > AudioManager.ADJUST_RAISE) { throw new IllegalArgumentException("Bad direction " + direction); } } private void ensureValidSteps(int steps) { if (Math.abs(steps) > MAX_BATCH_VOLUME_ADJUST_STEPS) { throw new IllegalArgumentException("Bad volume adjust steps " + steps); } } private void ensureValidStreamType(int streamType) { if (streamType < 0 || streamType >= mStreamStates.length) { throw new IllegalArgumentException("Bad stream type " + streamType); } } 继续跟进源码，可以看到int step = rescaleIndex(10, streamType, streamTypeAlias);
此参数为调整步进数值的
再下面的判断条件if (streamState.muteCount() != 0)为判断是否是从静音状态发起的音量调整以便进行不同的操作。
在函数的最后
sendVolumeUpdate(streamType, oldIndex, index, flags);
此函数则是用于更新系统音量条的显示
// UI update and Broadcast Intent private void sendVolumeUpdate(int streamType, int oldIndex, int index, int flags) { if (!mVoiceCapable && (streamType == AudioSystem.STREAM_RING)) { streamType = AudioSystem.STREAM_NOTIFICATION; } mVolumePanel.postVolumeChanged(streamType, flags); oldIndex = (oldIndex + 5) / 10; index = (index + 5) / 10; Intent intent = new Intent(AudioManager.VOLUME_CHANGED_ACTION); if (streamType == AudioManager.STREAM_VOICE_CALL) streamType = AudioManager.STREAM_MUSIC; intent.putExtra(AudioManager.EXTRA_VOLUME_STREAM_TYPE, streamType); intent.putExtra(AudioManager.EXTRA_VOLUME_STREAM_VALUE, index); intent.putExtra(AudioManager.EXTRA_PREV_VOLUME_STREAM_VALUE, oldIndex); mContext.sendBroadcast(intent); } 可以看到源码中调用了 \android\frameworks\base\core\java\android\view\VolumePanel.java
其中VolumePanel.java则用于显示系统音量条的显示，其中具体如何显示可以参考代码。同时此部分代码中会涉及到系统所有音量相关的图标显示，需要进行修改美化等操作的可以具体找到资源文件进行修改。
具体路径可以在
android\frameworks\base\core\res\res
另一种设置音量的方法setStreamVolume(int streamType, int index, int flags)也是同样的可以在上述代码中找到，关于音量调整的部分解析到此基本结束，暂时可以明白的就上述的流程了。