1.??前言

在文章Flutter框架分析（八）-Platform Channel中，我们分析了EventChannel的原理和结构，并详细讲解了与其相关的一些核心类，例如StreamHandler和MethodCodec等，本文主要讲解使用EventChannel的示例。

2.??使用流程

EventChannel一般用于事件流的传递，用于将Platform的点击等一系列事件等传递至Platform，仅支持native到Flutter的单向传递，接下来将分析其使用流程。

流程如下：

1）native端创建某channel name的EventChannel。

2）native端使用setStreamHandler函数，设置该EventChannel的StreamHandler。

3）Flutter端创建该channel name的EventChannel。

4）Flutter端创建该EventChannel的广播流，通知native端channel创建完毕，并注册广播接收函数。

5）native端刚刚注册的StreamHandler收到发送的消息，在onListen中处理消息，拿到其中的EventSink接口，通过该接口给Flutter端传递广播。

6）Flutter端处理该广播。

其流程图如下：

Flutter端关键代码如下：

class?_EventChannelState?extends?State<EventChannelWidget> {
??EventChannel?_eventChannelPlugin?=?EventChannel('flutter2/testeventchannel');
??StreamSubscription?_streamSubscription;

??@override
??void?initState() {
????_streamSubscription?=?_eventChannelPlugin
????????.receiveBroadcastStream(["abc",?123,?"你好"]).listen(_onToDart,
????????????onError: _onToDartError, onDone: _onDone);
????super.initState();
??}

??@override
??void?dispose() {
????if?(_streamSubscription?!=?null) {
??????_streamSubscription.cancel();
??????_streamSubscription?=?null;
????}
????super.dispose();
??}
   void?_onToDart(message) {
????print("eventchanneltest _onToDart?$message");
??}
   void?_onToDartError(error) {
????print("eventchanneltest _onToDartError?$error");
??}
   void?_onDone() {
????print("消息传递完毕");
??}

??@override
??Widget?build(BuildContext?context) {
????return?Scaffold(
??????appBar:?AppBar(
????????title:?Text("event channel test"),
??????),
????);
??}
}

native端关键代码如下：

class EventChannelActivity : FlutterActivity(), EventChannel.StreamHandler {
    private var eventSink: EventSink? = null

    private var mHandler: Handler? = null
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        mHandler = Handler()
    }

    override fun configureFlutterEngine(flutterEngine: FlutterEngine) {
        Log.d("FirstNativeActivity", "configureFlutterEngine")
        initChannel(flutterEngine)
    }

    private fun initChannel(flutter2Engine: FlutterEngine) {
        EventChannel(flutter2Engine.dartExecutor, STREAM).setStreamHandler(this)
    }

    override fun onListen(arguments: Any, events: EventSink) {
        eventSink = events
        Log.i(TAG, "onListen Object：$arguments")
        Toast.makeText(this, "onListen——obj：$arguments", Toast.LENGTH_SHORT).show()
        mHandler!!.postDelayed( {
      eventSink!!.success("hello")
            eventSink!!.endOfStream()
           } , 3000)
    }

    override fun onCancel(arguments: Any) {
        Log.i(TAG, "onCancel：$arguments")
        Toast.makeText(this, "onCancel——obj：$arguments", Toast.LENGTH_SHORT).show()
        eventSink = null
    }

    companion object {
        private const val TAG = "eventchanneltest"
        const val STREAM = "flutter2/testeventchannel"
        fun startActivity(activity: Activity) {
            val intent = Intent(activity, EventChannelActivity::class.java)
            activity.startActivity(intent)
        }
    }
}