本文探讨了Android中音量键的处理流程，从Activity的onKeyDown()方法到PhoneWindow的onKeyDown()，说明如何重写以实现自定义功能。同时介绍了每个StreamType的独立音量值和设置方法。当音量键被按下时，系统会通过AudioService发送广播通知音量变化。
根据按键派发策略，派发顺序是Activity，然后是PhoneWindow。
如果activity需要处理音量键，可以在应用中重写Activity 的onKeyDown（）方法以截获音量键将其用作其他功能，如相机的音量快捷键。Activity#onKeyDown()方法源码
/**
* Called when a key was pressed down and not handled by any of the views
* inside of the activity. So, for example, key presses while the cursor
* is inside a TextView will not trigger the event (unless it is a navigation
* to another object) because TextView handles its own key presses.
*
* <p>If the focused view didn't want this event, this method is called.
*
* <p>The default implementation takes care of {@link KeyEvent#KEYCODE_BACK}
* by calling {@link #onBackPressed()}, though the behavior varies based
* on the application compatibility mode: for
* {@link android.os.Build.VERSION_CODES#ECLAIR} or later applications,
* it will set up the dispatch to call {@link #onKeyUp} where the action
* will be performed; for earlier applications, it will perform the
* action immediately in on-down, as those versions of the platform
* behaved.
*
* <p>Other additional default key handling may be performed
* if configured with {@link #setDefaultKeyMode}.
*
* @return Return <code>true</code> to prevent this event from being propagated
* further, or <code>false</code> to indicate that you have not handled
* this event and it should continue to be propagated.
* @see #onKeyUp
* @see android.view.KeyEvent
*/
public boolean onKeyDown(int keyCode, KeyEvent event) {
if (keyCode == KeyEvent.KEYCODE_BACK) {
if (getApplicationInfo().targetSdkVersion
>= Build.VERSION_CODES.ECLAIR) {
event.startTracking();
} else {
onBackPressed();
}
return true;
}
if (mDefaultKeyMode == DEFAULT_KEYS_DISABLE) {
return false;
} else if (mDefaultKeyMode == DEFAULT_KEYS_SHORTCUT) {
Window w = getWindow();
if (w.hasFeature(Window.FEATURE_OPTIONS_PANEL) &&
w.performPanelShortcut(Window.FEATURE_OPTIONS_PANEL, keyCode, event,
Menu.FLAG_ALWAYS_PERFORM_CLOSE)) {
return true;
}
return false;
} else {
// Common code for DEFAULT_KEYS_DIALER & DEFAULT_KEYS_SEARCH_*
boolean clearSpannable = false;
boolean handled;
if ((event.getRepeatCount() != 0) || event.isSystem()) {
clearSpannable = true;
handled = false;
} else {
handled = TextKeyListener.getInstance().onKeyDown(
null, mDefaultKeySsb, keyCode, event);
if (handled && mDefaultKeySsb.length() > 0) {
// something useable has been typed - dispatch it now.
final String str = mDefaultKeySsb.toString();
clearSpannable = true;
switch (mDefaultKeyMode) {
case DEFAULT_KEYS_DIALER:
Intent intent = new Intent(Intent.ACTION_DIAL, Uri.parse("tel:" + str));
intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
startActivity(intent);
break;
case DEFAULT_KEYS_SEARCH_LOCAL:
startSearch(str, false, null, false);
break;
case DEFAULT_KEYS_SEARCH_GLOBAL:
startSearch(str, false, null, true);
break;
}
}
}
if (clearSpannable) {
mDefaultKeySsb.clear();
mDefaultKeySsb.clearSpans();
Selection.setSelection(mDefaultKeySsb,0);
}
return handled;
}
}Activity.onKeyDown()对一般按键有默认的一些处理，但是我们可以复写这个方法以处理我们想要处理的按键。如果不做处理，会让PhoneWindow的onKeyDown处理，下面是PhoneWindow#onKeyDown()方法源码：
protected boolean onKeyDown(int featureId, int keyCode, KeyEvent event) {
/* ****************************************************************************
* HOW TO DECIDE WHERE YOUR KEY HANDLING GOES.
*
* If your key handling must happen before the app gets a crack at the event,
* it goes in PhoneWindowManager.
*
* If your key handling should happen in all windows, and does not depend on
* the state of the current application, other than that the current
* application can override the behavior by handling the event itself, it
* should go in PhoneFallbackEventHandler.
*
* Only if your handling depends on the window, and the fact that it has
* a DecorView, should it go here.
* ****************************************************************************/
final KeyEvent.DispatcherState dispatcher =
mDecor != null ? mDecor.getKeyDispatcherState() : null;
//Log.i(TAG, "Key down: repeat=" + event.getRepeatCount()
// + " flags=0x" + Integer.toHexString(event.getFlags()));
switch (keyCode) {
case KeyEvent.KEYCODE_VOLUME_UP:
case KeyEvent.KEYCODE_VOLUME_DOWN:
case KeyEvent.KEYCODE_VOLUME_MUTE: {
int direction = 0;
switch (keyCode) {
case KeyEvent.KEYCODE_VOLUME_UP:
direction = AudioManager.ADJUST_RAISE;
break;
case KeyEvent.KEYCODE_VOLUME_DOWN:
direction = AudioManager.ADJUST_LOWER;
break;
case KeyEvent.KEYCODE_VOLUME_MUTE:
direction = AudioManager.ADJUST_TOGGLE_MUTE;
break;
}
// If we have a session send it the volume command, otherwise
// use the suggested stream.
if (mMediaController != null) {
mMediaController.adjustVolume(direction, AudioManager.FLAG_SHOW_UI);
} else {
MediaSessionLegacyHelper.getHelper(getContext()).sendAdjustVolumeBy(
mVolumeControlStreamType, direction,
AudioManager.FLAG_SHOW_UI | AudioManager.FLAG_VIBRATE
| AudioManager.FLAG_FROM_KEY);
}
return true;
}
// These are all the recognized media key codes in
// KeyEvent.isMediaKey()
case KeyEvent.KEYCODE_MEDIA_PLAY:
case KeyEvent.KEYCODE_MEDIA_PAUSE:
case KeyEvent.KEYCODE_MEDIA_PLAY_PAUSE:
case KeyEvent.KEYCODE_MUTE:
case KeyEvent.KEYCODE_HEADSETHOOK:
case KeyEvent.KEYCODE_MEDIA_STOP:
case KeyEvent.KEYCODE_MEDIA_NEXT:
case KeyEvent.KEYCODE_MEDIA_PREVIOUS:
case KeyEvent.KEYCODE_MEDIA_REWIND:
case KeyEvent.KEYCODE_MEDIA_RECORD:
case KeyEvent.KEYCODE_MEDIA_FAST_FORWARD: {
if (mMediaController != null) {
if (mMediaController.dispatchMediaButtonEvent(event)) {
return true;
}
}
return false;
}
case KeyEvent.KEYCODE_MENU: {
onKeyDownPanel((featureId < 0) ? FEATURE_OPTIONS_PANEL : featureId, event);
return true;
}
case KeyEvent.KEYCODE_BACK: {
if (event.getRepeatCount() > 0) break;
if (featureId < 0) break;
// Currently don't do anything with long press.
if (dispatcher != null) {
dispatcher.startTracking(event, this);
}
return true;
}
}
return false;
}
在PhoneWindow#onKeyDown（）中最终调用mMediaController.adjustVolume(direction, AudioManager.FLAG_SHOW_UI);调节音量，源码如下：
/**
* Adjust the volume of the output this session is playing on. The direction
* must be one of {@link AudioManager#ADJUST_LOWER},
* {@link AudioManager#ADJUST_RAISE}, or {@link AudioManager#ADJUST_SAME}.
* The command will be ignored if the session does not support
* {@link VolumeProvider#VOLUME_CONTROL_RELATIVE} or
* {@link VolumeProvider#VOLUME_CONTROL_ABSOLUTE}. The flags in
* {@link AudioManager} may be used to affect the handling.
*
* @see #getPlaybackInfo()
* @param direction The direction to adjust the volume in.
* @param flags Any flags to pass with the command.
*/
public void adjustVolume(int direction, int flags) {
try {
mSessionBinder.adjustVolume(direction, flags, mContext.getPackageName());
} catch (RemoteException e) {
Log.wtf(TAG, "Error calling adjustVolumeBy.", e);
}
}在PhoneWindow#onKeyDown（）的flag有几个，其中AudioManager.FLAG_SHOW_UI告诉AudioService需要弹出一个音量控制板。而PhoneWindow#onKeyUp（）中对应的有AudioManager.FLAG_PLAY_SOUND,这是告诉AudioService松开音量键后有提示音，还有更多的flag自己去看。
每个流类型（StreamType）都有独立的音量值。而PhoneWindow最终设置的音量所属类型由PhoneWindow#mVolumeControlStream决定，在Activity#setVolumeControlStream（）可以设置绑定的PhoneWindow的mVolumeControlStream。
音量按键有三个值，分别是KeyEvent.KEYCODE_VOLUME_UP(加), KeyEvent.KEYCODE_VOLUME_DOWN（减）, KeyEvent.KEYCODE_VOLUME_MUTE（静音）
音量改变后，会发广播，下面是AudioService#setIndex（）源码：
mVolumeChanged = new Intent(AudioManager.VOLUME_CHANGED_ACTION);
mVolumeChanged.putExtra(AudioManager.EXTRA_VOLUME_STREAM_VALUE, index);
mVolumeChanged.putExtra(AudioManager.EXTRA_PREV_VOLUME_STREAM_VALUE, oldIndex);
mVolumeChanged.putExtra(AudioManager.EXTRA_VOLUME_STREAM_TYPE_ALIAS,
mStreamVolumeAlias[mStreamType]);
sendBroadcastToAll(mVolumeChanged);sendVolumnUpdate( )用于发送广播通知外界音量发生了变化.在使用setVolumeStream()设置音量的时候也会了发送广播.
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