如何管理网络套接字连接拒绝错误？

Question

在将消息添加到接收器之前，我无法理解如何检查IOWebSocketChannel的状态，或者在未发送消息的情况下如何通知错误。

假设Web套接字服务器未监听localhost:8080，如何检查channel.sink.add错误或捕获有关失败的套接字发送的异常？

import 'package:web_socket_channel/io.dart';

void main() async {
  IOWebSocketChannel channel;

  channel = IOWebSocketChannel.connect('ws://localhost:8080/');
  channel.stream.listen((data) {}, onError: (err) {
    print("err: $err");
  }, onDone: () {
    print(
        "onDone. reason:[${channel.closeReason}], code:[${channel.closeCode}]");
  });

  print("sending a message");

  // how to check that channel's socket is in a connected state?
  channel.sink.add("hello");

  print("done");
}