Flutter与Native通信（二）EventChannel

朱雀 2023-07-04 11:28 172阅读 0赞

flutter可以native之间可以通过Platform Channels APIs进行通信，API主要有以下三种：

*  [MethodChanel][]：用于传递方法调用（method invocation）
 *  [EventChannel][]：用于事件流的发送（event streams）
 *  [MessageChannel][]：用于传递字符串和半结构化的消息

其中EventChannel用于从native向flutter发送通知事件，例如flutter通过其监听Android的重力感应变化等。与MethodChannel不同，EventChannel是native到flutter的单向调用，调用是多播（一对多）的，可以类比成Android的Brodcast。

## 1. EventChannel的基本流程 ##

我们照例先看一下API使用的基本流程：

1.  \[native\]EventChannel\#setStreamHandler注册Handler实现
2.  \[native\]EventChannel初始化结束后，在StreamHandler\#onLister回调中获取EventSink引用并保存
3.  \[flutter\]EventChannel\#receiveBroadcastStream注册listener，建立监听
4.  \[native\]使用EventSink\#sucess发送通知事件
5.  \[flutter\]接受到事件通知
6.  \[native\]通知结束时调用endOfStream结束

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ZpdGF2aXZh_size_16_color_FFFFFF_t_70][]

## 2.代码实现 ##

### flutter端 ###

*  创建EventChannel，注册“包名/标识符”的channel名
 *  通过StreamSubscription\#listen注册listener，其中cancelOnError参数表示遇到错误时是否自动结束监听

class _MyHomePageState extends State<MyHomePage> {
      static const EventChannel _channel = const EventChannel('com.example.eventchannel/interop');
    
      StreamSubscription _streamSubscription;
      String _platformMessage;
    
      void _enableEventReceiver() {
        _streamSubscription = _channel.receiveBroadcastStream().listen(
            (dynamic event) {
              print('Received event: $event');
              setState(() {
                _platformMessage = event;
              });
            },
            onError: (dynamic error) {
              print('Received error: ${error.message}');
            },
            cancelOnError: true);
      }
    
      void _disableEventReceiver() {
        if (_streamSubscription != null) {
          _streamSubscription.cancel();
          _streamSubscription = null;
        }
      }
    
      @override
      initState() {
        super.initState();
        _enableEventReceiver();
      }
    
      @override
      void dispose() {
        super.dispose();
        _disableEventReceiver();
      }

调用StreamSubscriptoin\#cancel时，监听被取消。

### native(android)端 ###

android需要完成以下功能

*  通过EventChannel\#setStreamHandler注册Handler实现
 *  初始化完成后，获取eventSink引用并保存
 *  eventSink发送事件通知
 *  通知结束时调用event\#endOfStream，此时onCancel会被调用
 *  必要时，可通过evnetSink\#error发送错误通知，flutter的StreamSubscription\#onError会收到通知

class MainActivity: FlutterActivity() {
        private lateinit var channel: EventChannel
        var eventSink: EventSink? = null
    
        override fun configureFlutterEngine(@NonNull flutterEngine: FlutterEngine) {
            GeneratedPluginRegistrant.registerWith(flutterEngine)
    
            channel = EventChannel(flutterEngine.dartExecutor.binaryMessenger, "com.example.eventchannel/interop")
            channel.setStreamHandler(
                    object : StreamHandler {
                        override fun onListen(arguments: Any?, events: EventSink) {
                            eventSink = events
                            Log.d("Android", "EventChannel onListen called")
                            Handler().postDelayed({
                                eventSink?.success("Android")
                                //eventSink?.endOfStream()
                                //eventSink?.error("error code", "error message","error details")
                            }, 500)
                        }
                        override fun onCancel(arguments: Any?) {
                            Log.w("Android", "EventChannel onCancel called")
                        }
                    })
        }
    }

[MethodChanel]: https://blog.csdn.net/vitaviva/article/details/104246211
[EventChannel]: https://blog.csdn.net/vitaviva/article/details/104253660
[MessageChannel]: https://blog.csdn.net/vitaviva/article/details/104260820
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ZpdGF2aXZh_size_16_color_FFFFFF_t_70]: https://img-blog.csdnimg.cn/20200210203415309.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ZpdGF2aXZh,size_16,color_FFFFFF,t_70