首先简单介绍下预备知识：

1.Android的audio流的类型有以下12种：

[java] view plain copy

1. /*Theaudiostreamforphonecalls*/
2. publicstaticfinalintSTREAM_VOICE_CALL=0;//通话连接时的音频流（通话声）
3. /*Theaudiostreamforsystemsounds*/
4. publicstaticfinalintSTREAM_SYSTEM=1;//系统音频流
5. /*Theaudiostreamforthephoneringandmessagealerts*/
6. publicstaticfinalintSTREAM_RING=2;//来电铃声
7. /*Theaudiostreamformusicplayback*/
8. publicstaticfinalintSTREAM_MUSIC=3;//媒体音频流
9. /*Theaudiostreamforalarms*/
10. publicstaticfinalintSTREAM_ALARM=4;//闹钟音频流
11. /*Theaudiostreamfornotifications*/
12. publicstaticfinalintSTREAM_NOTIFICATION=5;//通知音频流
13. /*@hideTheaudiostreamforphonecallswhenconnectedonbluetooth*/
14. publicstaticfinalintSTREAM_BLUETOOTH_SCO=6;//从注释上看时使用蓝牙耳机通话的音频流
15. /*@hideTheaudiostreamforenforcedsystemsoundsincertaincountries(e.gcamerainJapan)*/
16. publicstaticfinalintSTREAM_SYSTEM_ENFORCED=7;//一些国家强制使用的音频流？？不太明白
17. /*@hideTheaudiostreamforDTMFtones*/
18. publicstaticfinalintSTREAM_DTMF=8;//DTMF音频流
19. /*@hideTheaudiostreamfortexttospeech(TTS)*/
20. publicstaticfinalintSTREAM_TTS=9;//TTS:TexttoSpeech:文件到语言的音频流，即机器说话
21. /*@hideTheaudiostreamforFm*/
22. publicstaticfinalintSTREAM_FM=10;//FM的音频流
23. /*@hideTheaudiostreamforMATV*/
24. publicstaticfinalintSTREAM_MATV=11;//TV的音频流

每种音频流所规定的最大值：

[java] view plain copy

1. /**@hideMaximumvolumeindexvaluesforaudiostreams*/
2. privateint[]MAX_STREAM_VOLUME=newint[]{
3. 6,//STREAM_VOICE_CALL
4. 7,//STREAM_SYSTEM
5. 7,//STREAM_RING
6. 12,//STREAM_MUSIC
7. 7,//STREAM_ALARM
8. 7,//STREAM_NOTIFICATION
9. 15,//STREAM_BLUETOOTH_SCO
10. 7,//STREAM_SYSTEM_ENFORCED
11. 15,//STREAM_DTMF
12. 15,//STREAM_TTS
13. 13,//STREAM_FM
14. 13//stream_MATV
15. };

2.所有的按键事件都是touch事件，这部分我会另外开篇博文介绍。

开始本文正文，Anndroid系统中所有View带有按键音，用户可以通过Settings>Sound>勾选Audible Selection即可开启按键音。但是有个奇怪的地方：此按键音是与媒体音量（即STREAM_MUSIC）绑定的，难道按键音的STREAM TYPE就是STREAM_MUSIC吗？我们从代码中寻找一下。

首先所有的View点击的时候都有按键音，我们从View.java的点击事件找起，在view的响应的onTouchEvent（）方法中有如下代码：

[java] view plain copy

1. switch(event.getAction()){
2. caseMotionEvent.ACTION_UP:
3. booleanprepressed=(mPrivateFlags&PREPRESSED)!=0;
4. if((mPrivateFlags&PRESSED)!=0||prepressed){
5. //takefocusifwedon'thaveitalreadyandweshouldin
6. //touchmode.
7. booleanfocusTaken=false;
8. if(isFocusable()&&isFocusableInTouchMode()&&!isFocused()){
9. focusTaken=requestFocus();
10. }
12. if(!mHasPerformedLongPress){
13. //Thisisatap,soremovethelongpresscheck
14. removeLongPressCallback();
16. //Onlyperformtakeclickactionsifwewereinthepressedstate
17. if(!focusTaken){
18. //UseaRunnableandpostthisratherthancalling
19. //performClickdirectly.Thisletsothervisualstate
20. //oftheviewupdatebeforeclickactionsstart.
21. if(mPerformClick==null){
22. mPerformClick=newPerformClick();
23. }
24. if(!post(mPerformClick)){
25. performClick();//这里响应click事件
26. }
27. }
28. }
30. if(mUnsetPressedState==null){
31. mUnsetPressedState=newUnsetPressedState();
32. }
34. if(prepressed){
35. mPrivateFlags|=PRESSED;
36. refreshDrawableState();
37. postDelayed(mUnsetPressedState,
38. ViewConfiguration.getPressedStateDuration());
39. }elseif(!post(mUnsetPressedState)){
40. //Ifthepostfailed,unpressrightnow
41. mUnsetPressedState.run();
42. }
43. removeTapCallback();
44. }
45. break;

处理click事件就写在performClick()函数当中，继续看该函数具体做了什么：

[java] view plain copy

1. publicbooleanperformClick(){
2. sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);
4. if(mOnClickListener!=null){
5. playSoundEffect(SoundEffectConstants.CLICK);
6. mOnClickListener.onClick(this);
7. returntrue;
8. }
10. returnfalse;
11. }

从这里可以看到与用户接口onClickListener结合起来了，当用户注册了clickListener，则调用发出按键音函数playSoundEffect ()和响应用户写好的clickListener的onClick()方法。这里playSoundEffect函数传的参数SoundEffectContants.CLICK为多少呢，从SoundEffectConstants.java可知SoundEffectConstants.CLICK = 0:

[java] view plain copy

1. publicclassSoundEffectConstants
2. {
3. SoundEffectConstants(){thrownewRuntimeException("Stub!");}
4. publicstaticintgetContantForFocusDirection(intdirection){thrownewRuntimeException("Stub!");}
5. publicstaticfinalintCLICK=0;
6. publicstaticfinalintNAVIGATION_LEFT=1;
7. publicstaticfinalintNAVIGATION_UP=2;
8. publicstaticfinalintNAVIGATION_RIGHT=3;
9. publicstaticfinalintNAVIGATION_DOWN=4;
10. }

playSoundEffect ()的具体内容如下：

[java] view plain copy

1. publicvoidplaySoundEffect(intsoundConstant){
2. if(mAttachInfo==null||mAttachInfo.mRootCallbacks==null||!isSoundEffectsEnabled()){
3. return;
4. }
5. mAttachInfo.mRootCallbacks.playSoundEffect(soundConstant);
6. }

真正调用的是AttachInfo Callbacks接口的playSoundEffect()函数：

[java] view plain copy

1. /**
2. *Asetofinformationgiventoaviewwhenitisattachedtoitsparent
3. *window.
4. */
5. staticclassAttachInfo{
6. interfaceCallbacks{
7. voidplaySoundEffect(inteffectId);
8. booleanperformHapticFeedback(inteffectId,booleanalways);
9. }

看注释可知其真正的方法写在parent window中，那parent window是哪个呢？ViewRoot的实现该回调接口：

[java] view plain copy

1. publicfinalclassViewRootextendsHandlerimplementsViewParent,
2. View.AttachInfo.Callbacks{

具体的playSoundEffect()函数内容：

[java] view plain copy

1. publicvoidplaySoundEffect(inteffectId){
2. checkThread();
4. try{
5. finalAudioManageraudioManager=getAudioManager();
7. switch(effectId){
8. caseSoundEffectConstants.CLICK:
9. audioManager.playSoundEffect(AudioManager.FX_KEY_CLICK);
10. return;
11. caseSoundEffectConstants.NAVIGATION_DOWN:
12. audioManager.playSoundEffect(AudioManager.FX_FOCUS_NAVIGATION_DOWN);
13. return;
14. caseSoundEffectConstants.NAVIGATION_LEFT:
15. audioManager.playSoundEffect(AudioManager.FX_FOCUS_NAVIGATION_LEFT);
16. return;
17. caseSoundEffectConstants.NAVIGATION_RIGHT:
18. audioManager.playSoundEffect(AudioManager.FX_FOCUS_NAVIGATION_RIGHT);
19. return;
20. caseSoundEffectConstants.NAVIGATION_UP:
21. audioManager.playSoundEffect(AudioManager.FX_FOCUS_NAVIGATION_UP);
22. return;
23. default:
24. thrownewIllegalArgumentException("unknowneffectid"+effectId+
25. "notdefinedin"+SoundEffectConstants.class.getCanonicalName());
26. }
27. }catch(IllegalStateExceptione){
28. //ExceptionthrownbygetAudioManager()whenmViewisnull
29. Log.e(TAG,"FATALEXCEPTIONwhenattemptingtoplaysoundeffect:"+e);
30. e.printStackTrace();
31. }
32. }

我们传入的参数为SoundEffectContants.CLICK，调用AudioManager的playSoundEffect()方法，参数为AudioManger.FX_KEY_CLICK，继续往下看，在AudioManager.java中playSoundEffect()方法：

[java] view plain copy

1. publicvoidplaySoundEffect(inteffectType){
2. if(effectType<0||effectType>=NUM_SOUND_EFFECTS){
3. return;
4. }
6. if(!querySoundEffectsEnabled()){
7. return;
8. }
10. IAudioServiceservice=getService();
11. try{
12. service.playSoundEffect(effectType);
13. }catch(RemoteExceptione){
14. Log.e(TAG,"DeadobjectinplaySoundEffect"+e);
15. }
16. }

调用了IAudioService的playSoundEffect() 方法，IAudioService方法是使用aidl生成的接口，aidl源文件：frameworks/base/media/java/android/media/IAudioService.aidl，真正响应的地方在AudioService.java中：

[java] view plain copy

1. /**@seeAudioManager#playSoundEffect(int)*/
2. publicvoidplaySoundEffect(inteffectType){
3. sendMsg(mAudioHandler,MSG_PLAY_SOUND_EFFECT,SHARED_MSG,SENDMSG_NOOP,
4. effectType,-1,null,0);
5. }

该方法调用sendMsg()方法，传入一下参数：（mAudioHandler, 7, -1, 1, 0, -1, null, 0）,sendMsg()方法如下:

[java] view plain copy

1. privatestaticvoidsendMsg(Handlerhandler,intbaseMsg,intstreamType,
2. intexistingMsgPolicy,intarg1,intarg2,Objectobj,intdelay){
3. intmsg=(streamType==SHARED_MSG)?baseMsg:getMsg(baseMsg,streamType);
5. if(existingMsgPolicy==SENDMSG_REPLACE){
6. handler.removeMessages(msg);
7. }elseif(existingMsgPolicy==SENDMSG_NOOP&&handler.hasMessages(msg)){
8. Log.d(TAG,"sendMsg:Msg"+msg+"existed!");
9. return;
10. }
12. handler
13. .sendMessageDelayed(handler.obtainMessage(msg,arg1,arg2,obj),delay);
14. }

该方法就是将传入的参数经过计算，obtain一个message，message的what = 7 arg1 = 0 arg2 = -1 object = null; 处理该消息的地方handleMessage （）：

[java] view plain copy

1. @Override
2. publicvoidhandleMessage(Messagemsg){
3. intbaseMsgWhat=getMsgBase(msg.what);
5. switch(baseMsgWhat){
7. ...
9. caseMSG_PLAY_SOUND_EFFECT:
10. playSoundEffect(msg.arg1,msg.arg2);
11. break;
12. ...
14. }

调用了带两个参数的playSoundEffect()函数，传入参数 0，-1：

[java] view plain copy

1. privatevoidplaySoundEffect(inteffectType,intvolume){
2. synchronized(mSoundEffectsLock){
4. if(mSoundPool==null){
5. return;
6. }
7. floatvolFloat;
8. //useSTREAM_MUSICvolumeattenuatedby3dBifvolumeisnotspecifiedbycaller
9. if(volume<0){
10. //以下计算播放的音量大小：
11. //SamelineartologconversionasinnativeAudioSystem::linearToLog()(AudioSystem.cpp)
12. floatdBPerStep=(float)((0.5*100)/MAX_STREAM_VOLUME[AudioSystem.STREAM_MUSIC]);
13. intmusicVolIndex=(mStreamStates[AudioSystem.STREAM_MUSIC].mIndex+5)/10;
14. floatmusicVoldB=dBPerStep*(musicVolIndex-MAX_STREAM_VOLUME[AudioSystem.STREAM_MUSIC]);
15. volFloat=(float)Math.pow(10,(musicVoldB-3)/20);
16. }else{
17. volFloat=(float)volume/1000.0f;
18. }
19. if(SOUND_EFFECT_FILES_MAP[effectType][1]>0){
21. mSoundPool.play(SOUND_EFFECT_FILES_MAP[effectType][1],volFloat,volFloat,0,0,1.0f);//调用该函数播放按键音
22. }else{
24. MediaPlayermediaPlayer=newMediaPlayer();
25. if(mediaPlayer!=null){
26. try{
27. StringfilePath=Environment.getRootDirectory()+SOUND_EFFECTS_PATH+SOUND_EFFECT_FILES[SOUND_EFFECT_FILES_MAP[effectType][0]];
28. mediaPlayer.setDataSource(filePath);
29. mediaPlayer.setAudioStreamType(AudioSystem.STREAM_RING);
30. mediaPlayer.prepare();
31. mediaPlayer.setVolume(volFloat,volFloat);
32. mediaPlayer.setOnCompletionListener(newOnCompletionListener(){
33. publicvoidonCompletion(MediaPlayermp){
34. cleanupPlayer(mp);
35. }
36. });
37. mediaPlayer.setOnErrorListener(newOnErrorListener(){
38. publicbooleanonError(MediaPlayermp,intwhat,intextra){
39. cleanupPlayer(mp);
40. returntrue;
41. }
42. });
43. mediaPlayer.start();
44. }catch(IOExceptionex){
45. Log.w(TAG,"MediaPlayerIOException:"+ex);
46. }catch(IllegalArgumentExceptionex){
47. Log.w(TAG,"MediaPlayerIllegalArgumentException:"+ex);
48. }catch(IllegalStateExceptionex){
49. Log.w(TAG,"MediaPlayerIllegalStateException:"+ex);
50. }
51. }
52. }
53. }
54. }

因为传入的参数volume为-1，按键音大小的值走if（volume < 0）内，函数中此部分计算的是按键音的大小volFloat，可以看出，整个计算过程都跟媒体音量STREAM_MUSIC有关，这里就看出，按键音的音量大小是与STREAM_MUSIC绑定的，那按键音的类型呢？继续看下去，函数mSoundPool.play(SOUND_EFFECT_FILES_MAP[effectType][1], volFloat, volFloat, 0, 0, 1.0f);中使用了SoundPool来播放按键音，我们看看该SoundPool初始化步骤，在AudioService初始化时，调用了loadSoundEffects()函数：

[java] view plain copy

1. publicAudioService(Contextcontext){
2. ......
3. loadSoundEffects();
4. .......
5. }

loadSoundEffects()函数的具体实现如下：

[java] view plain copy

1. publicbooleanloadSoundEffects(){
2. synchronized(mSoundEffectsLock){
3. if(mSoundPool!=null){
4. returntrue;
5. }
6. mSoundPool=newSoundPool(NUM_SOUNDPOOL_CHANNELS,AudioSystem.STREAM_SYSTEM,0);
8. ......
10. returntrue;
11. }

在此函数中，初始化了该SoundPool，类型为STREAM_SYSTEM。

到这里，结果出来了，android中，view的按键音类型为系统音频（STREAM_SYSTEM)，而音量的大小与媒体音量（STREAM_MUSIC）绑定了起来。

更多相关文章

1. Android(安卓)使用Visualizer获取播放音频的频率
2. android 显示系统
3. Android媒体扫描详细解析之二(MediaScanner & MediaProvider)
4. Android(安卓)Hook框架Xposed详解：从源代码分析到开发指南
5. android 背光驱动
6. android ViewTreeObserver详细讲解
7. cocos2dx中利用xcode 调用java中的函数
8. 初学Android,音频管理器之控制音频(六十六)
9. Android音频系统之AudioFlinger(二)

随机推荐

1. 解决异常报错org.mybatis.spring.MyBatis
2. mysql在渗透中的技巧总结
3. Sql Server网络配置协议不可用
4. mysql中MAX()函数MIN()函数
5. Oracle系统表v$session、v$sql字段说明（转
6. 项目连接MySQL数据库，提示异常
7. mysql 中 case when then .... else end
8. linux下安装2个mysql 失败
9. MySql生日闰月处理
10. sql*loader问题,我有几万条记录,为什么只