前言

有个需求要监听HDMI，然后把应用放到第二屏的操作。第一步：百度，Google看看有没有实现好的，按照百度的监听广播，在AndroidManifest.xml注册，然后插着HDMI开机的话要知道是否连着就要去读一个sys的文件。我按照操作来一遍，无卵用！！
所以下面是我的实现方法：

插拔HDMI

广播监听，肯定是要的

V/ActivityManager(  947): Broadcast sticky: Intent { act=android.intent.action.HDMI_PLUGGED flg=0x4000010 (has extras) } ordered=false userid=-1

但是我们监听即可

    private static final String ACTION_HDMI_PLUGGED = "android.intent.action.HDMI_PLUGGED";
    private static final String EXTRA_HDMI_PLUGGED_STATE = "state";

    private BroadcastReceiver mReceiver = new BroadcastReceiver() {
        @Override
        public void onReceive(Context context, Intent intent) {
            String action = intent.getAction();
            if (ACTION_HDMI_PLUGGED.equals(action)) {
                mIsHDMIPlugged = intent.getBooleanExtra(EXTRA_HDMI_PLUGGED_STATE, false);
                Log.d(TAG, "onReceive:mIsHDMIPlugged =  " + mIsHDMIPlugged);
            }
        }
    };

注意事项

1、需要在代码注册广播，在AndroidManifest.xml中注册无效，要在代码中注册
2、从log中可以得出，该广播为粘性广播，连接hdmi之后开机也可以收到广播的。

知其然

为什么只能在代码中注册呢？看下发广播的代码吧。
ps：源码为Android 7.1

   void initializeHdmiState() {
        boolean plugged = false;
        // watch for HDMI plug messages if the hdmi switch exists
        if (new File("/sys/devices/virtual/switch/hdmi/state").exists()) {
            mHDMIObserver.startObserving("DEVPATH=/devices/virtual/switch/hdmi");
            //也是去读文件的数据
            final String filename = "/sys/class/switch/hdmi/state";
            FileReader reader = null;
            try {
                reader = new FileReader(filename);
                char[] buf = new char[15];
                int n = reader.read(buf);
                if (n > 1) {
                    plugged = 0 != Integer.parseInt(new String(buf, 0, n-1));
                }
            } catch (IOException ex) {
                Slog.w(TAG, "Couldn't read hdmi state from " + filename + ": " + ex);
            } catch (NumberFormatException ex) {
                Slog.w(TAG, "Couldn't read hdmi state from " + filename + ": " + ex);
            } finally {
                if (reader != null) {
                    try {
                        reader.close();
                    } catch (IOException ex) {
                    }
                }
            }
        }
        // This dance forces the code in setHdmiPlugged to run.
        // Always do this so the sticky intent is stuck (to false) if there is no hdmi.
        mHdmiPlugged = !plugged;
        setHdmiPlugged(!mHdmiPlugged);
    }
  //由kernel上报的监听事件
    private UEventObserver mHDMIObserver = new UEventObserver() {
        @Override
        public void onUEvent(UEventObserver.UEvent event) {
            setHdmiPlugged("1".equals(event.get("SWITCH_STATE")));
        }
    };

//发广播出来
    void setHdmiPlugged(boolean plugged) {
        if (mHdmiPlugged != plugged) {
            mHdmiPlugged = plugged;
            updateRotation(true, true);
            Intent intent = new Intent(ACTION_HDMI_PLUGGED);
            //这个flags：启动检查时只能接受在代码中注册的Receiver
            intent.addFlags(Intent.FLAG_RECEIVER_REGISTERED_ONLY_BEFORE_BOOT);
            intent.putExtra(EXTRA_HDMI_PLUGGED_STATE, plugged);
            //粘性广播没有限制，即便后来注册的intent也可以收到这个广播
            mContext.sendStickyBroadcastAsUser(intent, UserHandle.ALL);
        }
    }