在Flutter中，standard_message_codec 是默认的消息编码插件，用于在 Flutter 和原生平台（如 Android 和 iOS）之间传递消息。它支持基本的数据类型，如字符串、数字、布尔值、列表、映射（字典）等。默认情况下，Flutter 的 MethodChannel 和 BasicMessageChannel 已经使用 standard_message_codec，因此你通常不需要显式地配置它。

以下是一个简单的示例，展示了如何在 Flutter 和原生平台之间使用默认的消息编码（即 standard_message_codec）传递消息。

Flutter 端代码

首先，创建一个 Flutter 插件通道，用于与原生平台通信。

import 'package:flutter/services.dart';

class MyAppChannel {
  static const MethodChannel _channel = MethodChannel('com.example.myapp/channel');

  // 从原生平台接收消息
  static Future<dynamic> receiveMessage() async {
    return _channel.invokeMethod('receiveMessage');
  }

  // 发送消息到原生平台
  static Future<void> sendMessage(String message) async {
    try {
      await _channel.invokeMethod('sendMessage', {'message': message});
    } on PlatformException catch (e) {
      print("Failed to send message: '${e.message}'.");
    }
  }
}

Android 端代码

在 Android 的 MainActivity 中设置 MethodChannel，以响应 Flutter 的消息。

import io.flutter.embedding.android.FlutterActivity;
import io.flutter.embedding.engine.FlutterEngine;
import io.flutter.plugin.common.MethodChannel;
import android.os.Bundle;

public class MainActivity extends FlutterActivity {
    private static final String CHANNEL = "com.example.myapp/channel";

    @Override
    public void configureFlutterEngine(FlutterEngine flutterEngine) {
        super.configureFlutterEngine(flutterEngine);
        new MethodChannel(flutterEngine.getDartExecutor().getBinaryMessenger(), CHANNEL)
                .setMethodCallHandler(
                        (call, result) -> {
                            if (call.method.equals("sendMessage")) {
                                // 接收来自 Flutter 的消息
                                String message = call.argument("message");
                                // 在这里处理消息，比如打印到日志
                                android.util.Log.d("MyApp", "Received message from Flutter: " + message);

                                // 发送回复消息给 Flutter
                                result.success("Message received on Android");
                            } else if (call.method.equals("receiveMessage")) {
                                // 发送消息到 Flutter
                                result.success("Hello from Android");
                            } else {
                                result.notImplemented();
                            }
                        }
                );
    }
}

iOS 端代码

在 iOS 的 AppDelegate 中设置 MethodChannel，以响应 Flutter 的消息。

import UIKit
import Flutter

@UIApplicationMain
@objc class AppDelegate: FlutterAppDelegate {
  override func application(
    _ application: UIApplication,
    didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
  ) -> Bool {
    GeneratedPluginRegistrant.register(with: self)
    
    let controller : FlutterViewController = window?.rootViewController as! FlutterViewController
    let channel = FlutterMethodChannel(name: "com.example.myapp/channel", binaryMessenger: controller)
    
    channel.setMethodCallHandler({
      (call: FlutterMethodCall, result: @escaping FlutterResult) in
      if call.method == "sendMessage" {
        if let message = call.arguments as? Dictionary<String, String>, let msg = message["message"] {
          // 接收来自 Flutter 的消息
          print("Received message from Flutter: \(msg)")
          
          // 发送回复消息给 Flutter
          result(success: "Message received on iOS")
        } else {
          result(FlutterError(code: "INVALID_ARGUMENT", message: "Invalid argument", details: nil))
        }
      } else if call.method == "receiveMessage" {
        // 发送消息到 Flutter
        result(success: "Hello from iOS")
      } else {
        result(FlutterMethodNotImplemented)
      }
    })
    
    return super.application(application, didFinishLaunchingWithOptions: launchOptions)
  }
}

使用示例

在 Flutter 端调用这些方法来测试通信：

void _sendMessage() async {
  await MyAppChannel.sendMessage("Hello from Flutter");
  final result = await MyAppChannel.receiveMessage();
  print("Received from native: $result");
}

在 Flutter 的 build 方法中添加一个按钮来触发消息发送：

@override
Widget build(BuildContext context) {
  return MaterialApp(
    home: Scaffold(
      appBar: AppBar(
        title: Text('Flutter Message Codec Example'),
      ),
      body: Center(
        child: ElevatedButton(
          onPressed: _sendMessage,
          child: Text('Send Message'),
        ),
      ),
    ),
  );
}

这个示例展示了如何在 Flutter 和原生平台之间使用默认的 standard_message_codec 传递消息。注意，默认情况下，Flutter 的 MethodChannel 已经使用 standard_message_codec，因此你不需要显式地配置它。