今天在研究Android中实现Android 4.2.2源码中的Music应用的源码，关于通过耳机按键控制音乐播放的实现，有点好奇，就仔细分析了一下源码，
主要由 MediaButtonIntentReceiver 这个类来实现。
在AndroidManifest.xml中有如下Receiver的注册：
<receiver android:name="com.huawei.imax.music.MediaButtonIntentReceiver" >
<intent-filter>
<action android:name="android.intent.action.MEDIA_BUTTON" />
<action android:name="android.media.AUDIO_BECOMING_NOISY" />
</intent-filter>
</receiver>
其实关键是对这两个ACTION的监控：
android.intent.action.MEDIA_BUTTON的说明如下，从注释看，就是媒体按键被按下后，通过 Intent.EXTRA_KEY_EVENT 中带上触发此事件的具体按钮事件：
/**
* Broadcast Action: The "Media Button" was pressed. Includes a single
* extra field, {@link #EXTRA_KEY_EVENT}, containing the key event that
* caused the broadcast.
*/
@SdkConstant(SdkConstantType.BROADCAST_INTENT_ACTION)
public static final String ACTION_MEDIA_BUTTON = "android.intent.action.MEDIA_BUTTON";
android.intent.action.AUDIO_BECOMING_NOISY的说明如下（自己翻译的，大概这个意思），
从注释看，当音频变得“吵闹”，比如耳机拔出，或者A2DP音频通道（比如通过蓝牙音箱播放音乐就是一种A2DP的应用场景）断开，
音频系统将会自动将音频转到自带的：
/**
* Broadcast intent, a hint for applications that audio is about to become
* 'noisy' due to a change in audio outputs. For example, this intent may
* be sent when a wired headset is unplugged, or when an A2DP audio
* sink is disconnected, and the audio system is about to automatically
* switch audio route to the speaker. Applications that are controlling
* audio streams may consider pausing, reducing volume or some other action
* on receipt of this intent so as not to surprise the user with audio
* from the speaker.
*/
@SdkConstant(SdkConstantType.BROADCAST_INTENT_ACTION)
public static final String ACTION_AUDIO_BECOMING_NOISY = "android.media.AUDIO_BECOMING_NOISY";
MediaButtonIntentReceiver 的关键实现如下：
public class MediaButtonIntentReceiver extends BroadcastReceiver
{
...
@Override
public void onReceive(Context context, Intent intent)
{
String intentAction = intent.getAction();
if (AudioManager.ACTION_AUDIO_BECOMING_NOISY.equals(intentAction))
{
Intent i = new Intent(context, MediaPlaybackService.class);
i.setAction(MediaPlaybackService.SERVICECMD);
i.putExtra(MediaPlaybackService.CMDNAME,
MediaPlaybackService.CMDPAUSE);
context.startService(i);
}
else if (Intent.ACTION_MEDIA_BUTTON.equals(intentAction))
{
KeyEvent event = (KeyEvent) intent.getParcelableExtra(Intent.EXTRA_KEY_EVENT);
if (event == null)
{
return;
}
int keycode = event.getKeyCode();
int action = event.getAction();
long eventtime = event.getEventTime();
// single quick press: pause/resume.
// double press: next track
// long press: start auto-shuffle mode.
String command = null;
switch (keycode)
{
case KeyEvent.KEYCODE_MEDIA_STOP:
command = MediaPlaybackService.CMDSTOP;
break;
case KeyEvent.KEYCODE_HEADSETHOOK:
case KeyEvent.KEYCODE_MEDIA_PLAY_PAUSE:
command = MediaPlaybackService.CMDTOGGLEPAUSE;
break;
case KeyEvent.KEYCODE_MEDIA_NEXT:
command = MediaPlaybackService.CMDNEXT;
break;
case KeyEvent.KEYCODE_MEDIA_PREVIOUS:
command = MediaPlaybackService.CMDPREVIOUS;
break;
case KeyEvent.KEYCODE_MEDIA_PAUSE:
command = MediaPlaybackService.CMDPAUSE;
break;
case KeyEvent.KEYCODE_MEDIA_PLAY:
command = MediaPlaybackService.CMDPLAY;
break;
}
通过接收此事件的广播，想进行什么操作，完全看你的业务需求了。
一开始以为耳机插拔，是通过ACTION_HEADSET_PLUG这个Action来控制，但Music并没有监听此事件，
/**
* Broadcast Action: Wired Headset plugged in or unplugged.
*
* <p>The intent will have the following extra values:
* <ul>
* <li><em>state</em> - 0 for unplugged, 1 for plugged. </li>
* <li><em>name</em> - Headset type, human readable string </li>
* <li><em>microphone</em> - 1 if headset has a microphone, 0 otherwise </li>
* </ul>
* </ul>
*/
@SdkConstant(SdkConstantType.BROADCAST_INTENT_ACTION)
public static final String ACTION_HEADSET_PLUG =
"android.intent.action.HEADSET_PLUG";
自己测试了一下，在AndroidManifest.xml中静态注册，是不会生效的，为啥？
一个很简单的解释，如果你的应用还没有运行，这时插入耳机，系统如何把这个消息给这个应用？
因此，"android.intent.action.HEADSET_PLUG" 只能通过动态注册来接收此广播消息。
如果让 MediaButtonIntentReceiver 还接收"android.intent.action.HEADSET_PLUG"的广播消息，
则MediaButtonIntentReceiver会先收到 "android.media.AUDIO_BECOMING_NOISY" 这个消息，然后才会收到"android.intent.action.HEADSET_PLUG"这个消息？
为什么这样，代码层面还没有分析，后面抽空再研究下Android源码。