Example use of dart.io websocket

dart-websocket.dart

WebSocket _wss; // class level field, so you can use it in methods

//method level
await WebSocket.connect(uri).timeout((Duration(seconds: 5))).then(
  (WebSocket ws) {
    _wss = ws;
    if (_wss?.readyState == WebSocket.open) {

      // you can tell server who you are here, right after connecting

      _wss.listen(
        (data) {
          // you get data from server here
          final msg = YourClass.fromJson(json.decode(data));
          // process message here
        },
        onDone: () => {
          log.fine(' WS closed'),
        },
        onError: (err) => {
          log.info('Error -- ${err.toString()}'),
        },
        cancelOnError: true,
      );

    } else {
      log.info('Connection Failed');
      // in case, if server is not running now
    }
  },
  onError: (err) => {
    log.severe('${err.toString()}'),
  },
);

// send to websocket:
if (_wss?.readyState == WebSocket.open) {
  _wss.add(
    json.encode(
      // your json object here
    ),
  );
}