WebSocket类的静态方法connect返回一个Future，该Future在建立连接时解析为Web Socket。
如果你想在连接建立时发送一条消息，应该可以这样做：

import 'dart:io';
  import 'package:web_socket_channel/io.dart';

  WebSocket.connect("ws://a.b.c.d").then((ws) {

    // create the stream channel 
    var channel = IOWebSocketChannel(ws);

    channel.sink.add("hello");
  })

必须将web_socket_channel包添加到pubspec.yaml依赖项中。

在WebSocketChannel 2.4.0的最新版本中，有ready Future。

import 'package:web_socket_channel/io.dart';

final channel = IOWebSocketChannel.connect('ws://localhost:3000',
      connectTimeout: Duration(seconds: 20),
      pingInterval: Duration(seconds: 15));

channel.stream.listen((event) {
  print('got message');
});

await channel.ready; // ready is a future

print('WS connected');

channel.sink.add('hi'); // write data to websocket