摘要：Flutter版本号：1.12.13，Kotlin版本号：1.3.72 

版本号不同，实现的方式不一样，所以特此标注下。

具体的实现原理，自行查阅。这里只写代码实现过程，主要原因是网上Kotlin的实现方式实在太少，特此记录。

Flutter:

import 'package:base_library/base_library.dart';
import 'package:flutter/material.dart';
import 'package:flutter/services.dart';

class ChannelDemoPage extends StatefulWidget {
  @override
  _ChannelDemoPageState createState() => _ChannelDemoPageState();
}

class _ChannelDemoPageState extends State<ChannelDemoPage> {
  static const _channel =
      const MethodChannel('plugins.flutter.io/FlutterMessagePlugin');

  static const eventChannel =
      EventChannel('plugins.flutter.io/FlutterEventPlugin');

  static const _basicMessageChannel = BasicMessageChannel(
      'plugins.flutter.io/FlutterBasicMessageChannel', StringCodec());

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

    // ignore: missing_return
    _basicMessageChannel.setMessageHandler((String message) {
      LogUtil.e("Flutter收到Android发来Basic消息->, $message");
    });
  }

  void _onEvent(Object event) {
    LogUtil.e("Flutter收到Android发来的消息-> " + event.toString());
  }

  void _onError(Object error) {
    LogUtil.e("错误：" + error.toString());
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      body: new Row(),
      floatingActionButtonLocation: FloatingActionButtonLocation.endFloat,
      floatingActionButton: new FloatingActionButton(
        backgroundColor: Colors.blueAccent,
        onPressed: () {
          this.getResult();
        },
        child: new Icon(Icons.adb),
      ),
    );
  }

  void getResult() async {
    try {
      Map<String, Object> map = {"shareUrl": "https://www.baidu.com"};
      var result = await _channel.invokeMethod("getRealUrl", map);
      LogUtil.e("result->$result");
    } catch (e) {
      LogUtil.e("e->, $e");
    }
  }
}

Kotlin:

import android.content.Context
import android.util.Log
import io.flutter.embedding.engine.plugins.FlutterPlugin
import io.flutter.plugin.common.BasicMessageChannel
import io.flutter.plugin.common.BasicMessageChannel.MessageHandler
import io.flutter.plugin.common.MethodChannel
import io.flutter.plugin.common.PluginRegistry
import io.flutter.plugin.common.StringCodec

class FlutterBasicMessageChannel : FlutterPlugin, MessageHandler<String> {


    private var channelName = "plugins.flutter.io/FlutterBasicMessageChannel"

    private var context: Context? = null

    companion object {

        private var basicMessageChannel: BasicMessageChannel<String>? = null

        fun sendContent(content: String) {
            basicMessageChannel?.send(content)
        }
    }

    override fun onAttachedToEngine(binding: FlutterPlugin.FlutterPluginBinding) {
        basicMessageChannel = BasicMessageChannel(binding.binaryMessenger, channelName, StringCodec.INSTANCE)
        basicMessageChannel?.setMessageHandler(this)
        context = binding.applicationContext
    }

    override fun onDetachedFromEngine(binding: FlutterPlugin.FlutterPluginBinding) {
        basicMessageChannel?.setMessageHandler(null)
        basicMessageChannel = null;
    }

    override fun onMessage(message: String?, reply: BasicMessageChannel.Reply<String>) {
        Log.i("Android", "Received message = $message")
        reply.reply("Reply from Android")
    }

}

import androidx.annotation.NonNull;
import io.flutter.embedding.android.FlutterActivity
import io.flutter.embedding.engine.FlutterEngine
import io.flutter.plugins.GeneratedPluginRegistrant

class MainActivity: FlutterActivity () {

        override fun configureFlutterEngine(@NonNull flutterEngine: FlutterEngine) {
        flutterEngine.getPlugins().add(FlutterMessagePlugin());
        flutterEngine.getPlugins().add(FlutterEventPlugin());
        flutterEngine.getPlugins().add(FlutterBasicMessageChannel());
        GeneratedPluginRegistrant.registerWith(flutterEngine);
    }

}

显示结果：

I/flutter (19721): common_utils e  result->处理完成
I/flutter (19721): common_utils e  Flutter收到Android发来的消息-> Android发送消息到Flutter
I/flutter (19721): common_utils e  Flutter收到Android发来Basic消息->, Android发送Basic消息到Flutter