前言
当用户点击Android智能设备的按钮的时候，如果伴随有按键音效的话，会给用户更好的交互体验。本期我们将会结合Android系统源码来具体分析一下控件是如何发出按键音效的。
一、系统加载按键音效资源
1、在TV版的Android智能设备中，我们可以通过调节设置页面的开关来控制按键音效的有无，该设置页面对应的系统源码如下所示。
packages/apps/TvSettings/Settings/src/com/android/tv/settings/device/sound/SoundFragment.java
java
public class SoundFragment extends PreferenceControllerFragment implements
Preference.OnPreferenceChangeListener {
private AudioManager mAudioManager;
private Map<Integer, Boolean> mFormats;
public static SoundFragment newInstance() {
return new SoundFragment();
}
@Override
public void onAttach(Context context) {
mAudioManager = context.getSystemService(AudioManager.class);
mFormats = mAudioManager.getSurroundFormats();
super.onAttach(context);
}
//用户的点击行为首先触发此方法
@Override
public boolean onPreferenceTreeClick(Preference preference) {
if (TextUtils.equals(preference.getKey(), KEY_SOUND_EFFECTS)) {
final TwoStatePreference soundPref = (TwoStatePreference) preference;
//调用setSoundEffectsEnabled来设置按键音的开启与关闭
setSoundEffectsEnabled(soundPref.isChecked());
}
return super.onPreferenceTreeClick(preference);
}
//获取按键音效是否开启
public static boolean getSoundEffectsEnabled(ContentResolver contentResolver) {
return Settings.System.getInt(contentResolver, Settings.System.SOUND_EFFECTS_ENABLED, 1)
!= 0;
}
//设置是否开启按键音效
private void setSoundEffectsEnabled(boolean enabled) {
if (enabled) {
//如果开启按键音，则调用AudioManager的loadSoundEffects方法来加载按键音效资源
mAudioManager.loadSoundEffects();
} else {
mAudioManager.unloadSoundEffects();
}
Settings.System.putInt(getActivity().getContentResolver(),
Settings.System.SOUND_EFFECTS_ENABLED, enabled ? 1 : 0);
}
}我们在设置页面点击按键音效开关按钮，最终会触发SoundFragment的setSoundEffectsEnabled方法，该方法会判断是否开启按键音，如果开启，则调用AudioManager的loadSoundEffects方法来加载按键音效资源，反之则会调用unloadSoundEffects方法不加载音效资源。
2、AudioManager的loadSoundEffects方法如下所示。
frameworks/base/media/java/android/media/AudioManager.java
java
public class AudioManager {
//获取AudioService的代理对象
private static IAudioService getService()
{
if (sService != null) {
return sService;
}
IBinder b = ServiceManager.getService(Context.AUDIO_SERVICE);
sService = IAudioService.Stub.asInterface(b);
return sService;
}
public void loadSoundEffects() {
final IAudioService service = getService();
try {
//调用AudioService服务的loadSoundEffects方法
service.loadSoundEffects();
} catch (RemoteException e) {
throw e.rethrowFromSystemServer();
}
}
public void unloadSoundEffects() {
final IAudioService service = getService();
try {
service.unloadSoundEffects();
} catch (RemoteException e) {
throw e.rethrowFromSystemServer();
}
}
}AudioManager的首先通过getService方法获取了音频服务AudioService的代理对象，然后调用该对象的具体方法。
3、AudioService的loadSoundEffects方法如下所示。
java
public class AudioService extends IAudioService.Stub
implements AccessibilityManager.TouchExplorationStateChangeListener,
AccessibilityManager.AccessibilityServicesStateChangeListener {
private AudioHandler mAudioHandler;
//加载音效资源
public boolean loadSoundEffects() {
int attempts = 3;
LoadSoundEffectReply reply = new LoadSoundEffectReply();
synchronized (reply) {
//调用sendMsg发送消息给mAudioHandler。
sendMsg(mAudioHandler, MSG_LOAD_SOUND_EFFECTS, SENDMSG_QUEUE, 0, 0, reply, 0);
while ((reply.mStatus == 1) && (attempts-- > 0)) {
try {
reply.wait(SOUND_EFFECTS_LOAD_TIMEOUT_MS);
} catch (InterruptedException e) {
Log.w(TAG, "loadSoundEffects Interrupted while waiting sound pool loaded.");
}
}
}
return (reply.mStatus == 0);
}
//不加载音效资源
public void unloadSoundEffects() {
sendMsg(mAudioHandler, MSG_UNLOAD_SOUND_EFFECTS, SENDMSG_QUEUE, 0, 0, null, 0);
}
private static void sendMsg(Handler handler, int msg,
int existingMsgPolicy, int arg1, int arg2, Object obj, int delay) {
if (existingMsgPolicy == SENDMSG_REPLACE) {
handler.removeMessages(msg);
} else if (existingMsgPolicy == SENDMSG_NOOP && handler.hasMessages(msg)) {
return;
}
synchronized (mLastDeviceConnectMsgTime) {
long time = SystemClock.uptimeMillis() + delay;
if (msg == MSG_SET_A2DP_SRC_CONNECTION_STATE ||
msg == MSG_SET_A2DP_SINK_CONNECTION_STATE ||
msg == MSG_SET_HEARING_AID_CONNECTION_STATE ||
msg == MSG_SET_WIRED_DEVICE_CONNECTION_STATE ||
msg == MSG_A2DP_DEVICE_CONFIG_CHANGE ||
msg == MSG_BTA2DP_DOCK_TIMEOUT) {
if (mLastDeviceConnectMsgTime >= time) {
// add a little delay to make sure messages are ordered as expected
time = mLastDeviceConnectMsgTime + 30;
}
mLastDeviceConnectMsgTime = time;
}
//调用handler的sendMessageAtTime方法
handler.sendMessageAtTime(handler.obtainMessage(msg, arg1, arg2, obj), time);
}
}
private class AudioHandler extends Handler {
//加载音效
private boolean onLoadSoundEffects() {
...代码暂时省略...
}
@Override
public void handleMessage(Message msg) {
switch (msg.what) {
...代码省略...
case MSG_UNLOAD_SOUND_EFFECTS:
onUnloadSoundEffects();//不加载音效
break;
case MSG_LOAD_SOUND_EFFECTS://加载音效
boolean loaded = onLoadSoundEffects();//调用onLoadSoundEffects加载音效，并将加载结果赋值给loaded
if (msg.obj != null) {
LoadSoundEffectReply reply = (LoadSoundEffectReply)msg.obj;
synchronized (reply) {
reply.mStatus = loaded ? 0 : -1;
reply.notify();
}
}
break;
...代码省略...
}
}
}这里我们只要分析一下AudioService的加载音效资源的loadSoundEffects方法，该方法会调用sendMsg，发送类型为MSG_UNLOAD_SOUND_EFFECTS的msg给mAudioHandler。然后会进一步触发AudioHandler的handleMessage方法，该消息最终会触发onLoadSoundEffects方法。
4、AudioHandler的onLoadSoundEffects方法如下所示。
java
public class AudioService extends IAudioService.Stub
implements AccessibilityManager.TouchExplorationStateChangeListener,
AccessibilityManager.AccessibilityServicesStateChangeListener {
//音效资源文件名称
private static final List<String> SOUND_EFFECT_FILES = new ArrayList<String>();
private class AudioHandler extends Handler {
//加载音效
private boolean onLoadSoundEffects() {
int status;
synchronized (mSoundEffectsLock) {
//如果系统未启动完毕直接返回
if (!mSystemReady) {
Log.w(TAG, "onLoadSoundEffects() called before boot complete");
return false;
}
//如果mSoundPool不为空直接返回
if (mSoundPool != null) {
return true;
}
//加载触摸音效
loadTouchSoundAssets();
...代码暂时省略...
}
//加载按键音效资源
private void loadTouchSoundAssets() {
XmlResourceParser parser = null;
//如果音效资源文件列表不为空直接返回
if (!SOUND_EFFECT_FILES.isEmpty()) {
return;
}
//加载按键默认的音效资源
loadTouchSoundAssetDefaults();
...代码省略...
}
private void loadTouchSoundAssetDefaults() {
//在类型为List<String>的SOUND_EFFECT_FILES中添加默认的按键音效资源Effect_Tick.ogg
SOUND_EFFECT_FILES.add("Effect_Tick.ogg");
for (int i = 0; i < AudioManager.NUM_SOUND_EFFECTS; i++) {
SOUND_EFFECT_FILES_MAP[i][0] = 0;
SOUND_EFFECT_FILES_MAP[i][1] = -1;
}
}
}onLoadSoundEffects方法首先判断系统是否已经启动完毕，如果未启动直接返回；然后判断mSoundPool是否空，如果不为空则直接返回；
然后首先会调用一个关键方法loadTouchSoundAssets，该方法首先判断音效资源文件列表SOUND_EFFECT_FILES是否为空，不为空直接返回。如果以上判断都不成立，则会调用loadTouchSoundAssetDefaults方法加载按键默认的音效资源，该方法首先在SOUND_EFFECT_FILES的添加音效资源Effect_Tick.ogg。
5、继续往下看AudioHandler的onLoadSoundEffects方法。
java
public class AudioService extends IAudioService.Stub
implements AccessibilityManager.TouchExplorationStateChangeListener,
AccessibilityManager.AccessibilityServicesStateChangeListener {
//音效资源文件名称
private static final List<String> SOUND_EFFECT_FILES = new ArrayList<String>();
private class AudioHandler extends Handler {
//加载音效
private boolean onLoadSoundEffects() {
int status;
synchronized (mSoundEffectsLock) {
if (!mSystemReady) {
Log.w(TAG, "onLoadSoundEffects() called before boot complete");
return false;
}
if (mSoundPool != null) {
return true;
}
//加载触摸音效
loadTouchSoundAssets();
//创建SoundPool对象
mSoundPool = new SoundPool.Builder()
.setMaxStreams(NUM_SOUNDPOOL_CHANNELS)
.setAudioAttributes(new AudioAttributes.Builder()
.setUsage(AudioAttributes.USAGE_ASSISTANCE_SONIFICATION)
.setContentType(AudioAttributes.CONTENT_TYPE_SONIFICATION)
.build())
.build();
...代码省略...
int numSamples = 0;
for (int effect = 0; effect < AudioManager.NUM_SOUND_EFFECTS; effect++) {
// Do not load sample if this effect uses the MediaPlayer
if (SOUND_EFFECT_FILES_MAP[effect][1] == 0) {
continue;
}
if (poolId[SOUND_EFFECT_FILES_MAP[effect][0]] == -1) {
//获取音效资源文件路径
String filePath = getSoundEffectFilePath(effect);
//使用SoundPool加载音效资源文件
int sampleId = mSoundPool.load(filePath, 0);
if (sampleId <= 0) {
Log.w(TAG, "Soundpool could not load file: "+filePath);
} else {
SOUND_EFFECT_FILES_MAP[effect][1] = sampleId;
poolId[SOUND_EFFECT_FILES_MAP[effect][0]] = sampleId;
numSamples++;
}
} else {
SOUND_EFFECT_FILES_MAP[effect][1] =
poolId[SOUND_EFFECT_FILES_MAP[effect][0]];
}
}
...代码省略...
}
//获取音效资源文件路径，默认返回的音效资源文件路径为/system/media/audio/ui/Effect_Tick.ogg
private String getSoundEffectFilePath(int effectType) {
// /product + /media/audio/ui/ + Effect_Tick.ogg
String filePath = Environment.getProductDirectory() + SOUND_EFFECTS_PATH
+ SOUND_EFFECT_FILES.get(SOUND_EFFECT_FILES_MAP[effectType][0]);
if (!new File(filePath).isFile()) {
// /system + /media/audio/ui/ + Effect_Tick.ogg
filePath = Environment.getRootDirectory() + SOUND_EFFECTS_PATH
+ SOUND_EFFECT_FILES.get(SOUND_EFFECT_FILES_MAP[effectType][0]);
}
return filePath;
}
}
}onLoadSoundEffects先是调用loadTouchSoundAssets方法加载默认的音效资源文件名称，然后构建SoundPool实例对象，随后调用getSoundEffectFilePath获取按键音效资源文件路径，默认返回的音效资源文件路径为/system/media/audio/ui/Effect_Tick.ogg，并调用SoundPool加载该音效资源。
6、简单回顾一下以上步骤。
二、点击控件，播放音效资源
在系统开启按键音效之后，当我们点击任意控件之后，都会触发按键音效。接下来我们将会结合View的系统源码来梳理该流程。
1、当我们点击一个控件的时候，首先会触发该View的performClick方法。
frameworks/base/core/java/android/view/View.java
java
public class View implements Drawable.Callback, KeyEvent.Callback,
AccessibilityEventSource {
public boolean performClick() {
// We still need to call this method to handle the cases where performClick() was called
// externally, instead of through performClickInternal()
notifyAutofillManagerOnClick();
final boolean result;
final ListenerInfo li = mListenerInfo;
if (li != null && li.mOnClickListener != null) {
playSoundEffect(SoundEffectConstants.CLICK);//播放按键点击音效
li.mOnClickListener.onClick(this);
result = true;
} else {
result = false;
}
sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);
notifyEnterOrExitForAutoFillIfNeeded(true);
return result;
}
public void playSoundEffect(int soundConstant) {
//判断mAttachInfo.mRootCallbacks是否为空，以及系统是否开启了按键音效
if (mAttachInfo == null || mAttachInfo.mRootCallbacks == null || !isSoundEffectsEnabled()) {
return;
}
//调用mAttachInfo.mRootCallbacks的playSoundEffect方法
mAttachInfo.mRootCallbacks.playSoundEffect(soundConstant);
}
}View的performClick方法会调用playSoundEffect方法，playSoundEffect方法首先判断mAttachInfo.mRootCallbacks是否为空，以及系统是否开启了按键音效，然后调用mAttachInfo.mRootCallbacks的playSoundEffect方法。我们知道WindowManager在将View添加到窗口的过程中，都需要用到ViewRootImpl这个类，具体请参考Android 9.0系统源码_窗口管理（二）WindowManager对窗口的管理过程这篇文章。
2、mAttachInfo最初是在ViewRootImpl的构造方法中被创建的。
frameworks/base/core/java/android/view/ViewRootImpl.java
java
public final class ViewRootImpl implements ViewParent,
View.AttachInfo.Callbacks, ThreadedRenderer.DrawCallbacks {
public ViewRootImpl(Context context, Display display) {
mContext = context;
mWindowSession = WindowManagerGlobal.getWindowSession();
mDisplay = display;
mBasePackageName = context.getBasePackageName();
mThread = Thread.currentThread();
mLocation = new WindowLeaked(null);
mLocation.fillInStackTrace();
mWidth = -1;
mHeight = -1;
mDirty = new Rect();
mTempRect = new Rect();
mVisRect = new Rect();
mWinFrame = new Rect();
mWindow = new W(this);
mTargetSdkVersion = context.getApplicationInfo().targetSdkVersion;
mViewVisibility = View.GONE;
mTransparentRegion = new Region();
mPreviousTransparentRegion = new Region();
mFirst = true; // true for the first time the view is added
mAdded = false;
//创建AttachInfo对象，倒数第二个参数就是View的playSoundEffect方法所用到的回调对象
mAttachInfo = new View.AttachInfo(mWindowSession, mWindow, display, this, mHandler, this, context);
...代码省略...
}
}3、看完ViewRootImpl的构造方法，再来看下AttachInfo的构造方法。
java
public class View implements Drawable.Callback, KeyEvent.Callback,
AccessibilityEventSource {
final static class AttachInfo {
//关键回调接口
interface Callbacks {
//播放音效
void playSoundEffect(int effectId);
boolean performHapticFeedback(int effectId, boolean always);
}
final Callbacks mRootCallbacks;
AttachInfo(IWindowSession session, IWindow window, Display display,
ViewRootImpl viewRootImpl, Handler handler, Callbacks effectPlayer,
Context context) {
mSession = session;
mWindow = window;
mWindowToken = window.asBinder();
mDisplay = display;
mViewRootImpl = viewRootImpl;
mHandler = handler;
mRootCallbacks = effectPlayer;//View的playSoundEffect方法所用到的回调对象就是这个
mTreeObserver = new ViewTreeObserver(context);
}
}
}AttachInfo构造方法的最后一个参数很关键，因为View的playSoundEffect方法所调用的对象就是这个，结合ViewRootImpl的代码我们可以知道是ViewRootImpl实现了这个回调。
4、ViewRootImpl的playSoundEffect方法如下所示。
java
public final class ViewRootImpl implements ViewParent, View.AttachInfo.Callbacks, ThreadedRenderer.DrawCallbacks {
public void playSoundEffect(int effectId) {
checkThread();//检测是否是UI线程
try {
final AudioManager audioManager = getAudioManager();
switch (effectId) {
case SoundEffectConstants.CLICK://播放按键点击音效
audioManager.playSoundEffect(AudioManager.FX_KEY_CLICK);
return;
case SoundEffectConstants.NAVIGATION_DOWN:
audioManager.playSoundEffect(AudioManager.FX_FOCUS_NAVIGATION_DOWN);
return;
case SoundEffectConstants.NAVIGATION_LEFT:
audioManager.playSoundEffect(AudioManager.FX_FOCUS_NAVIGATION_LEFT);
return;
case SoundEffectConstants.NAVIGATION_RIGHT:
audioManager.playSoundEffect(AudioManager.FX_FOCUS_NAVIGATION_RIGHT);
return;
case SoundEffectConstants.NAVIGATION_UP:
audioManager.playSoundEffect(AudioManager.FX_FOCUS_NAVIGATION_UP);
return;
default:
throw new IllegalArgumentException("unknown effect id " + effectId +
" not defined in " + SoundEffectConstants.class.getCanonicalName());
}
} catch (IllegalStateException e) {
// Exception thrown by getAudioManager() when mView is null
Log.e(mTag, "FATAL EXCEPTION when attempting to play sound effect: " + e);
e.printStackTrace();
}
}
}ViewRootImpl的playSoundEffect方法首先会检测一下当前线程是不是UI线程，然后会根据传入的effectId类型来判断要播放那种音效。因为View的performClick方法传入的是SoundEffectConstants.CLICK，所以会触发audioManager.playSoundEffect(AudioManager.FX_KEY_CLICK)。
4、AudioManager的playSoundEffect方法如下所示。
java
public class AudioManager {
public void playSoundEffect(int effectType) {
//检测音效类型是否合规
if (effectType < 0 || effectType >= NUM_SOUND_EFFECTS) {
return;
}
//确定音效是否可用
if (!querySoundEffectsEnabled(Process.myUserHandle().getIdentifier())) {
return;
}
//获取AudioService服务
final IAudioService service = getService();
try {
//调用服务的playSoundEffect方法
service.playSoundEffect(effectType);
} catch (RemoteException e) {
throw e.rethrowFromSystemServer();
}
}
/**
* Settings has an in memory cache, so this is fast.
*/
private boolean querySoundEffectsEnabled(int user) {
return Settings.System.getIntForUser(getContext().getContentResolver(),
Settings.System.SOUND_EFFECTS_ENABLED, 0, user) != 0;
}
}AudioManager的playSoundEffect会做一些校验，如果校验通过则会获取AudioService服务对象，并调用该对象的playSoundEffect方法进行音效播放。
5、AudioService和playSoundEffect相关的代码如下所示。
java
public class AudioService extends IAudioService.Stub
implements AccessibilityManager.TouchExplorationStateChangeListener,
AccessibilityManager.AccessibilityServicesStateChangeListener {
/**
* 播放音效
* @param effectType
*/
public void playSoundEffect(int effectType) {
playSoundEffectVolume(effectType, -1.0f);
}
public void playSoundEffectVolume(int effectType, float volume) {
// do not try to play the sound effect if the system stream is muted
if (isStreamMutedByRingerOrZenMode(STREAM_SYSTEM)) {
return;
}
if (effectType >= AudioManager.NUM_SOUND_EFFECTS || effectType < 0) {
Log.w(TAG, "AudioService effectType value " + effectType + " out of range");
return;
}
//发送播放音效的消息给mAudioHandler
sendMsg(mAudioHandler, MSG_PLAY_SOUND_EFFECT, SENDMSG_QUEUE,
effectType, (int) (volume * 1000), null, 0);
}
private class AudioHandler extends Handler {
//加载音效
private boolean onLoadSoundEffects() {
...代码暂时省略...
}
@Override
public void handleMessage(Message msg) {
switch (msg.what) {
...代码省略...
case MSG_UNLOAD_SOUND_EFFECTS:
onUnloadSoundEffects();//不加载音效
break;
case MSG_LOAD_SOUND_EFFECTS://加载音效
boolean loaded = onLoadSoundEffects();//调用onLoadSoundEffects加载音效，并将加载结果赋值给loaded
...代码省略...
break;
case MSG_PLAY_SOUND_EFFECT://播放音效
onPlaySoundEffect(msg.arg1, msg.arg2);
break;
...代码省略...
}
}
}AudioService的playSoundEffect方法进一步调用playSoundEffectVolume，该方法会发送播放音效的消息MSG_PLAY_SOUND_EFFECT给mAudioHandler，最终会触发onPlaySoundEffect方法。
6、AudioService的onPlaySoundEffec方法如下所示。
java
public class AudioService extends IAudioService.Stub
implements AccessibilityManager.TouchExplorationStateChangeListener,
AccessibilityManager.AccessibilityServicesStateChangeListener {
private void onPlaySoundEffect(int effectType, int volume) {
synchronized (mSoundEffectsLock) {
onLoadSoundEffects();//加载音效
if (mSoundPool == null) {
return;
}
float volFloat;
// use default if volume is not specified by caller
if (volume < 0) {
volFloat = (float)Math.pow(10, (float)sSoundEffectVolumeDb/20);
} else {
volFloat = volume / 1000.0f;
}
if (SOUND_EFFECT_FILES_MAP[effectType][1] > 0) {
//通过SoundPool播放音效
mSoundPool.play(SOUND_EFFECT_FILES_MAP[effectType][1],
volFloat, volFloat, 0, 0, 1.0f);
} else {
//通过MediaPlayer播放音效
MediaPlayer mediaPlayer = new MediaPlayer();
try {
String filePath = getSoundEffectFilePath(effectType);
mediaPlayer.setDataSource(filePath);
mediaPlayer.setAudioStreamType(AudioSystem.STREAM_SYSTEM);
mediaPlayer.prepare();
mediaPlayer.setVolume(volFloat);
mediaPlayer.setOnCompletionListener(new OnCompletionListener() {
public void onCompletion(MediaPlayer mp) {
cleanupPlayer(mp);
}
});
mediaPlayer.setOnErrorListener(new OnErrorListener() {
public boolean onError(MediaPlayer mp, int what, int extra) {
cleanupPlayer(mp);
return true;
}
});
mediaPlayer.start();
} catch (IOException ex) {
Log.w(TAG, "MediaPlayer IOException: "+ex);
} catch (IllegalArgumentException ex) {
Log.w(TAG, "MediaPlayer IllegalArgumentException: "+ex);
} catch (IllegalStateException ex) {
Log.w(TAG, "MediaPlayer IllegalStateException: "+ex);
}
}
}
}
}7、简单回顾一下以上步骤。
