Dart: Wait for WebSocket reponse

in my code I am establishing a server connection via a WebSocket. Now there is the case that the server responds to my message and I want to process this response. However, I can't find a good approach on how to solve this.

My approach was to write the response from the WebSocket into a list via the listen method and to query this list after sending (see code). I call the sendAndWaitForAnswer method several times in a loop, because I have to send several requests to the server for initialisation. The problem is that the WebSocket writes the answers to rxData only after sendAndWaitForAnswer has already been terminated by the timeout (see output). Does anyone have an idea how to wait for the answer from the WebSocket?

I know that normal HTTP requests would probably be better, since you have a response directly, but unfortunately I can't change the server.

Its not for a Flutter app.

Print output:

send data and wait for response
send data and wait for response
send data and wait for response
send data and wait for response
send data and wait for response
new webSocket Message: ...     // event message simplified with "..."
new webSocket Message: ...
new webSocket Message: ...
new webSocket Message: ...
new webSocket Message: ...

What i want:

send data and wait for response
new webSocket Message: ...
data found
send data and wait for response
new webSocket Message: ...
data found
send data and wait for response
new webSocket Message: ...
data found
send data and wait for response
new webSocket Message: ...
data found

Simplified Code:

class Connection {

	late IOWebSocketChannel webSocketChannel;
	var rxData = [];

	connect() {    
		webSocketChannel = IOWebSocketChannel.connect(  
			Uri.parse("ipAdress"),  
			pingInterval: Duration(seconds: 30),  
		);  
	  
		webSocketChannel.stream.listen((event) {  
			print("new webSocket Message: $event");  
			rxData.add(event);  
		});   
	}

	send(opcode, parameters) {  
		Map<String, dynamic> dataSendMap = {  
			"opcode": opcode,  
			"parameters": parameters,  
		};  
		String jsonData = json.encode(dataSendMap);  
		webSocketChannel.sink.add(jsonData);  
	}

	sendAndWaitForAnswer(dataOpt, dataParam) {
		print("send data and wait for response");
		send(dataOpt, dataParam);
        timeout = 5;
        int startTime = DateTime.now().millisecondsSinceEpoch ~/ Duration.millisecondsPerSecond;
		while((DateTime.now().millisecondsSinceEpoch ~/ Duration.millisecondsPerSecond) - startTime < timeout) {
			for (var data in rxData) {  
				if (data['opcode'] == dataOpt) {  
                    print("data found");
					answer = data;  
					rxData.remove(data);  
				}  
			}
		}
		return answer;
	}
}

To process WebSocket responses in Dart, you can use the web_socket_channel package, which provides a convenient way to establish WebSocket connections and handle incoming messages. Here's an example of how you can handle WebSocket responses:

import 'package:flutter/material.dart';
import 'package:web_socket_channel/web_socket_channel.dart';
import 'package:web_socket_channel/io.dart'; 
class MyWebSocketWidget extends StatefulWidget {
  @override
  _MyWebSocketWidgetState createState() => _MyWebSocketWidgetState();
}

class _MyWebSocketWidgetState extends State<MyWebSocketWidget> {
  final channel = IOWebSocketChannel.connect('wss://your-websocket-url.com');

  @override
  void dispose() {
    channel.sink.close();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('WebSocket Demo'),
      ),
      body: StreamBuilder(
        stream: channel.stream,
        builder: (context, snapshot) {
          if (snapshot.hasData) {
            // Process the received data
            final response = snapshot.data as String;
            // Do something with the response
            // ...
          }
          return Center(
            child: CircularProgressIndicator(),
          );
        },
      ),
    );
  }
}

I found a solution with async/await and a Completer. Here is my simplified Code:

  Completer completer = Completer();
  late IOWebSocketChannel webSocketChannel;

  void connect(String ip, {int port = 1234}) {
    String address = "ws://$ip:$port/test/";
    webSocketChannel = IOWebSocketChannel.connect(
      Uri.parse(address),
    );
    webSocketListen();
  }

  void webSocketListen() {
    webSocketChannel.stream.listen(
      (event) {
        completer.complete(event);

      },
    );
  }
  
  sendAndWaitForAnswer(data) async {
    _send(data);
    var answer = await completer.future;
    completer = Completer();
    answer = jsonDecode(answer);
    return answer;
  }

  _send(data) {
    Map<String, dynamic> dataSendMap = {
      "data": opcode,
    };
    String jsonData = json.encode(dataSendMap);
    webSocketChannel.sink.add(jsonData);
  }