Flutter - EventChannel回调问题

onAttachedToEngine(binding: FlutterPluginBinding): void { 
  let context = binding.getApplicationContext() 
  this.context = context 
 
  this.stateChannel = new EventChannel(binding.getBinaryMessenger(), NAMESPACE + '/state') 
  this.stateChannel.setStreamHandler(this) 
 
  this.channel = new MethodChannel(binding.getBinaryMessenger(), NAMESPACE + "/methods"); 
  this.channel.setMethodCallHandler(this) 
} 
 
onListen(_: Any, events: EventSink): void { 
  this.eventSink = events 
} 
 
onCancel(_: Any): void { 
  this.eventSink = null 
}