How to pass a message from Flutter to Native?

This is a simple implementation showcasing :

1. Passing a string Value from flutter to Android code
2. Getting back response from Android code to flutter

code is based on example from :https://flutter.io/platform-channels/#codec

1.Passing string value "text" :

String text = "whatever";

Future<Null> _getBatteryLevel(text) async {
String batteryLevel;
try {
  final String result = await platform.invokeMethod('getBatteryLevel',{"text":text}); 
  batteryLevel = 'Battery level at $result % .';
} on PlatformException catch (e) {
  batteryLevel = "Failed to get battery level: '${e.message}'.";
}

setState(() {
  _batteryLevel = batteryLevel;
});

}

2.Getting back response "batterylevel" after RandomFunction();

 public void onMethodCall(MethodCall call, MethodChannel.Result result) {
                    if (call.method.equals("getBatteryLevel")) {

                        text = call.argument("text");
                        String batteryLevel = RandomFunction(text);

                        if (batteryLevel != null) {
                            result.success(batteryLevel);
                        } else {
                            result.error("UNAVAILABLE", "Battery level not available.", null);
                        }
                    } else {
                        result.notImplemented();
                    }
                }

Hope this helps!

Objective C

call.arguments[@"parameter"]

Android

call.argument("parameter");

Yes, flutter does has an EventChannel class which is what you are looking for exactly.

Here is an example of that demonstrates how MethodChannel and EventChannel can be implemented. And this medium article shows how an EventChannel can be implemented in flutter.

Hope that helped!