耳机拔掉或者断开，音乐播放停止

标题所描述的场景大家都遇到过，有线耳机或者蓝牙耳机断开，此时音乐就自动暂停，那这块代码逻辑在哪呢？
这里我们使用Android 13的代码看下：http://aospxref.com/android-13.0.0_r3/
看下这样的一个广播：android.media.AUDIO_BECOMING_NOISY
http://aospxref.com/android-13.0.0_r3/xref/frameworks/base/media/java/android/media/AudioManager.java#123

/**
 * 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";

看下什么地方发送这个广播的 （ACTION_AUDIO_BECOMING_NOISY ），先看下这里：
http://aospxref.com/android-13.0.0_r3/xref/frameworks/base/services/core/java/com/android/server/audio/AudioService.java#8471

http://aospxref.com/android-13.0.0_r3/xref/frameworks/base/services/core/java/com/android/server/audio/
AudioDeviceBroker.java#postBroadcastBecomingNoisy

/*package*/ void postBroadcastBecomingNoisy() {
    sendMsgNoDelay(MSG_BROADCAST_AUDIO_BECOMING_NOISY, SENDMSG_REPLACE);
}
// 处理这个消息然后调用
private void onSendBecomingNoisyIntent() {
    AudioService.sDeviceLogger.log((new AudioEventLogger.StringEvent(
            "broadcast ACTION_AUDIO_BECOMING_NOISY")).printLog(TAG));
    mSystemServer.sendDeviceBecomingNoisyIntent();    ---> 看这个函数，发送ACTION_AUDIO_BECOMING_NOISY
}

// http://aospxref.com/android-13.0.0_r3/xref/frameworks/base/services/core/java/com/android/server/audio/
// SystemServerAdapter.java#80
78       * Broadcast ACTION_AUDIO_BECOMING_NOISY
79       */
80      public void sendDeviceBecomingNoisyIntent() {
81          if (mContext == null) {
82              return;
83          }
84          final Intent intent = new Intent(AudioManager.ACTION_AUDIO_BECOMING_NOISY);
85          intent.addFlags(Intent.FLAG_RECEIVER_REGISTERED_ONLY_BEFORE_BOOT);
86          intent.addFlags(Intent.FLAG_RECEIVER_FOREGROUND);
87          final long ident = Binder.clearCallingIdentity();
88          try {
89              mContext.sendBroadcastAsUser(intent, UserHandle.ALL);
90          } finally {
91              Binder.restoreCallingIdentity(ident);
92          }
93      }

至此，可以看到 ACTION_AUDIO_BECOMING_NOISY 广播发出，监听此广播的地方就可以暂停音乐的播放，代码里可以搜下，哪里有监听的地方，比如这个地方：

当然，也可以不暂停，这个处理逻辑自己可以更改。