原生主动调用flutter

class MainActivity: FlutterActivity() {
    //通道名称为samples.flutter.io/battery，和flutter中的一样
    private val NET_CHANGE_CHANNEL = "samples.flutter.io/netChanged"
    private lateinit var context: Context

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        GeneratedPluginRegistrant.registerWith(this)
        context = this
        //通过通道监听对应名字的方法

        EventChannel(flutterView,NET_CHANGE_CHANNEL).setStreamHandler(object : EventChannel.StreamHandler{
            private  var netStateChangeReceiver: BroadcastReceiver? = null

            override fun onListen(arguments: Any?, events: EventChannel.EventSink) {
                netStateChangeReceiver = createNetStateChangeReceiver(events)
                registerReceiver(netStateChangeReceiver,  IntentFilter(ConnectivityManager.CONNECTIVITY_ACTION))
            }

            override fun onCancel(p0: Any?) {
                unregisterReceiver(netStateChangeReceiver)
                netStateChangeReceiver = null
            }
        })
    }

    private fun createNetStateChangeReceiver(events: EventChannel.EventSink): BroadcastReceiver {
        return object : BroadcastReceiver() {
            override fun onReceive(context: Context, intent: Intent) {
                val networkConnected = isConnectedToNetwork(context)
                events.success(networkConnected)
            }
        }
    }

    fun isConnectedToNetwork(context: Context): Boolean {
        val cm = context.getSystemService(Context.CONNECTIVITY_SERVICE) as ConnectivityManager
        val netInfo = cm.activeNetworkInfo
        return netInfo != null && netInfo.isConnected
    }
}

class _MyHomePageState extends State<MyHomePage> {
//通道名称为samples.flutter.io/battery，和android原生中接收的一样

  static const netPlatform =
      const EventChannel('samples.flutter.io/netChanged');

  var netState = true;

  @override
  void initState() {
    super.initState();
    netPlatform.receiveBroadcastStream().listen(_onEvent, onError: _onError);
  }

  void _onEvent(Object event) {
    setState(() {
      netState = event;
    });
  }

  void _onError(Object error) {
    setState(() {
      netState = false;
    });
  }

  @override
  Widget build(BuildContext context) {
    return new Material(
      child: new Center(
        child: new Column(
          mainAxisAlignment: MainAxisAlignment.spaceEvenly,
          children: [
            new Text('netState=' + netState.toString()),
          ],
        ),
      ),
    );
  }
}