注册耳机事件监听器

private BroadcastReceiver headSetReceiver;
private AudioManager mAudioManager;
private ComponentName mComponentName;

public void initHeadSetReceiver() {
        Log.i(TAG, "initHeadSetReceiver: ");
        headSetReceiver = new BroadcastReceiver() {
            @Override
            public void onReceive(Context context, Intent intent) {
                String action = intent.getAction();
                if (action.equals(Intent.ACTION_HEADSET_PLUG)) {
                    // phone headset plugged
                    if (intent.getIntExtra("state", 0) == 1) {
                        // do something
                        Log.d(TAG, "耳机检测：插入");
                        ChatHelper.getTnstance().isHeadset = true;
//                  Toast.makeText(context, "耳机检测：插入", Toast.LENGTH_SHORT) .show();
                        mAudioManager.registerMediaButtonEventReceiver(mComponentName);
                        // phone head unplugged
                    } else {
                        // do something
                        Log.d(TAG, "耳机检测：没有插入");
                        ChatHelper.getTnstance().isHeadset = false;
//                  Toast.makeText(context, "耳机检测：没有插入", Toast.LENGTH_SHORT).show();
                        mAudioManager.unregisterMediaButtonEventReceiver(mComponentName);
                    }
                }
            }
        };
    }

public void initMediaButtonReceiver() {
        initHeadSetReceiver();

        if (mAudioManager == null) {
            mAudioManager = (AudioManager) getContext().getSystemService(Context.AUDIO_SERVICE);
        }
        // AudioManager注册一个MediaButton对象
       mComponentName = new ComponentName(getPackageName(), MediaButtonReceiver.class.getName());

       mAudioManager.registerMediaButtonEventReceiver(mComponentName);
       getContext().registerReceiver(headSetReceiver, new IntentFilter(Intent.ACTION_HEADSET_PLUG));
    }

其中关键语句是

mAudioManager.registerMediaButtonEventReceiver(mComponentName);

当其他软件注册耳机监听事件后，系统就不会向该监听发送事件；
解决方法：
经测试，在小米5和华为mete9上使用MediaPlayer播放音频可以重新使系统向该监听发送事件，
但是在OPPO和Vivo手机上无法起作用；

查看registerMediaButtonEventReceiver源码

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holder.mSession.setMediaButtonReceiver(pi);
holder.update();

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由此可知只有一个MediaButtonReceiver能收到事件回调

解决方法：

mAudioManager.unregisterMediaButtonEventReceiver(mComponentName);
mAudioManager.registerMediaButtonEventReceiver(mComponentName);

至于为何需要调用unregisterMediaButtonEventReceiver方法，猜测某些ROM厂商修改了源码，对于已经注册过的不能重复注册

更新：在小米MAX2 和华为mete9手机上，当APP处于后台时，无法抢占耳机控制权
解决方法：使用MediaSession

@TargetApi(Build.VERSION_CODES.LOLLIPOP)//注意申明 API 21
private void setMediaButtonEvent() {
        MediaSessionCompat mSession = new MediaSessionCompat(getContext(), "MediaSession");
        mSession.setCallback(
                new MediaSessionCompat.Callback() {
                    @Override
                    public boolean onMediaButtonEvent(Intent intent) {
                        Log.i(TAG, "onMediaButtonEvent: ");
                        // 获得Action
                        String intentAction = intent.getAction();
                        // 获得KeyEvent对象
                        KeyEvent keyEvent = intent
                                .getParcelableExtra(Intent.EXTRA_KEY_EVENT);

                        if (keyEvent == null) {
                            return false;
                        }

                        Log.i(TAG, "Action ---->" + intentAction + "  KeyEvent----->"
                                + keyEvent.toString());
                        // 按下 / 松开 按钮
                        int keyAction = keyEvent.getAction();
                        Log.i(TAG, "onMediaButtonEvent: " + keyAction);

                        disposeKeyEvent(keyEvent, intentAction);

                        return true;
                    }
                }

        );
        mSession.setFlags(MediaSessionCompat.FLAG_HANDLES_MEDIA_BUTTONS |
                MediaSessionCompat.FLAG_HANDLES_TRANSPORT_CONTROLS);
//FLAG_HANDLES_MEDIA_BUTTONS 控制媒体按钮
//FLAG_HANDLES_TRANSPORT_CONTROLS 控制传输命令
        mSession.setActive(true); //激活
//      mSession.release(); //退出时需要销毁
        //更新播放状态和歌曲信息  //最重要

        PlaybackState.Builder stateBuilder = new PlaybackState.Builder().setActions(
                PlaybackState.ACTION_PLAY
                        | PlaybackState.ACTION_PLAY_PAUSE
                        | PlaybackState.ACTION_PLAY_FROM_MEDIA_ID
                        | PlaybackState.ACTION_PAUSE
                        | PlaybackState.ACTION_SKIP_TO_NEXT
                        | PlaybackState.ACTION_SKIP_TO_PREVIOUS);

        stateBuilder.setState(PlaybackState.STATE_PLAYING,
                PlaybackState.PLAYBACK_POSITION_UNKNOWN,
                SystemClock.elapsedRealtime());

        if (mSession.getMediaSession() instanceof MediaSession) {
            Log.i(TAG, "setMediaButtonEvent: session.setPlaybackState");
            MediaSession session = (MediaSession) mSession.getMediaSession();
            session.setPlaybackState(stateBuilder.build());
        }
}

private void disposeKeyEvent(KeyEvent keyEvent, String intentAction) {
        int keyAction = keyEvent.getAction();

        if (Intent.ACTION_MEDIA_BUTTON.equals(intentAction)) {
            //按下的是耳机中间按键
            // 获得按键字节码
            int keyCode = keyEvent.getKeyCode();

            if (KeyEvent.ACTION_UP == keyAction) {
                Log.d(TAG, "action=ACTION_UP");
                if (keyEvent.getKeyCode() == KeyEvent.KEYCODE_HEADSETHOOK) {
                    Log.d(TAG, "action=ACTION_UP, keyCode=KEYCODE_HEADSETHOOK");

                   //松开了耳机按键
                }
            }
        }
    }