Day17 开发套件 - 实作BasicMessageChannel

用於双向的单次讯息传递，包括发送讯息、接收讯息两个功能

经过以上的实作，其实两端的通讯步骤都差不多，首先在两端建立想要Channel并指定名称，且在两端通道名称需对应，传送端对接收端发讯息来执行Channel的对应的动作（执行Native 端方法，或监听Native 端讯息，亦或发送讯息至Native 端讯息），接收端建立完对应的Channel後要建立handler处理从传送端来的讯息执行对应动作，并回传结果

Flutter 端

新增BasicMessageChannel并指定名称，且新增要从Flutter 端传讯息给Native 端的方法，并回传Native 端要回覆的讯息

lib/batterylevel.dart：

	static const BasicMessageChannel _messageChannel =
      const BasicMessageChannel('chat_message', StringCodec());

  static Future<String> messageResponse(String message) async {
    final String response = await _messageChannel.send(message);
    return response;
  }

Native 端（Android）：

android/.../BatterylevelPlugin.kt：

    private lateinit var messageChannel: BasicMessageChannel<String>

    override fun onAttachedToEngine(@NonNull flutterPluginBinding: FlutterPlugin.FlutterPluginBinding) {
        //Added
        messageChannel = BasicMessageChannel(
            flutterPluginBinding.binaryMessenger,
            "chat_message",
            StringCodec.INSTANCE
        )
        messageChannel.setMessageHandler { message, reply ->
            val response = if(message.isNullOrEmpty()) "" else "Android 收到：$message"
            reply.reply(response)
        }
    }

    override fun onDetachedFromEngine(@NonNull binding: FlutterPlugin.FlutterPluginBinding) {
        //Added
        messageChannel.setMessageHandler(null)
    }

Native 端（iOS）：

ios/Classes/RandomNumberStreamHandler.swift

public class SwiftBatterylevelPlugin: NSObject, FlutterPlugin {
    public static func register(with registrar: FlutterPluginRegistrar) {
				//Added
        let messageChannel = FlutterBasicMessageChannel(name: "chat_message", binaryMessenger: 			registrar.messenger(),codec: FlutterStringCodec.sharedInstance())
        
        messageChannel.setMessageHandler {
            (message: Any?, reply: FlutterReply) -> Void in
            reply((message as! String) != "" ? "iOS 收到：\(message as! String)" : "");
        }
    }
}

以上就完成运用Platform Channel来实作平台两端的通讯了，[范例程序都在这](